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Connective Tissues

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In anatomy. Very widely distributed throughout the tissues and organs of the animal body, there occur tissues characterized by the presence of a high proportion of intercellular substance. This intercellular substance may be homogeneous in structure, or, as is more commonly the case, it may consist, in whole or in part, of a number of fibrous elements. All these tissues are grouped together under the name Connective Tissues. They comprise the following types:- areolar tissue, adipose tissue, reticular or lymphoid tissue, white fibrous tissue, elastic tissue, cartilage and bone. They are all developed from the same layer of embryonic cells and all perform a somewhat similar function, viz, to connect and support the other tissues and organs. According to the nature of their work the ground substance varies in its texture, being fibrous in some, calcareous and rigid in others. As forming the most typical of these tissues, we will first consider the structure of areolar connective tissue.

Areolar Tissue

This tissue is found in its most typical form uniting the skin to the deeper lying parts. It varies greatly in its density according to the animal and the position of the body from which it is taken. A piece of the looser variety may be spread out as a thin sheet and then examined microscopically. It is then seen to consist chiefly of bundles of extremely fine fibres running in all directions and interlacing with one another to form a meshwork. The spaces, or areolae thus formed give the name to this tissue (see fig. 1). The constituent fibres of each bundle are termed White Fibres. The bundles vary very much in size, but the fibres of which they are composed are of wonderfully constant size. A bundle may branch by sending off its fibres to unite with similar branches from neighbouring bundles, but the individual fibres do not branch nor do they at any time fuse with one another. They form bundles of greater or less size by being arranged parallel to one another, and in these bundles are bound together by some kind of cement substance. The meshwork formed by these fibres is filled up by a ground substance in the composition of which mucin takes some part. In this ground substance lie the cells of the tissue. In addition to the white fibres a second variety of fibres is also present in this tissue. They can be readily distinguished from the white fibres by their larger and variable size, by their more distinct outline, and by the fact that they,for the most part, run as straight lines through the preparation. Moreover they frequently branch, and the branches unite with those of neighbouring fibres. They are known as Yellow Elastic Fibres. Several of these will be found torn across in any preparation especially at the edges, and the torn ends will be found to be curled up in a very characteristic manner. The two types of fibre further differ from one another both chemically and physically. Thus the white fibre swells up and dissolves in boiling water, yielding a solution of gelatin, whereas the yellow elastic fibre is quite insoluble under these conditions. The white fibres swell when treated with weak acetic acid, and are readily dissolved by peptic digestion but not by pancreatic. The yellow elastic fibres, on the other hand, are unaffected by acetic acid and resist the action of gastric juice for a long time, but are dissolved by pancreatic juice. In FIG. I. - Connective tissue, showing cells, fibres and groundsubstance. X 350. (Szymnowicz.) c, Cell; e, elastic fibril; f, white fibril.

physical properties the white fibres are inextensible and extraordinarily strong, even being able, weight for weight, to carry a greater strain than steel wire. The yellow elastic fibres, on the other hand, are easily extensible arid very elastic, but are far less strong than the white fibres. Their elasticity is exhibited by their straight course when viewed in a stretched preparation of areolar tissue, and this contrasts markedly with the wavy course of the bundles of white fibres seen in the same preparation.

The Cells of Areolar Tissue

Several types of cells are found in the spaces of this tissue and are usually classified as follows. (I) Lamellar cells. These are flattened branching cells which usually lie attached to the bundles of white fibres or at the junction of two or more bundles. The branches commonly unite with similar branches of neighbouring cells. (2) Plasma cells. These are composed of a highly vacuolated plasma, are not flattened but otherwise vary greatly in shape. ( 3) Granular cells. These are spherical cells densely packed with granules which stain deeply with basic dyes. (4) Leucocytes. These are typical blood corpuscles which have left the blood capillaries and gained the tissue spaces. They vary much in amount and in variety.

Adipose or Fatty Tissue

This consists of rounded vesicles closely packed together to form a dense tissue, found for instance around an organ, along the course of the smaller blood vessels, or in the areolar tissue beneath the skin. This tissue is formed from areolar tissue by an accumulation of fat within certain of the cells of the tissue. These are especially the granular cells, though some regard the fat cells as specific in character, and to be found in large numbers only in certain parts of the body. The fat is either taken in as such by the cell, or, as is more commonly the case, manufactured by the cell from other chemical material (carbohydrate chiefly) and deposited within it in the form of small granules. As these accumulate they run together to form larger granules and this process continuing, the cell at last becomes converted into a thin layer of living material surrounding a single large fat globule. The use of fatty tissue is to serve as a storehouse of food material for future use. In conformity with this it is packed away in parts of the body where it will not interfere with the working of the different tissues and organs, and in several positions is made use of as packing to fill up irregular spaces, e.g. between the eyeball and the bony socket of the eye.

Reticular Tissue

This is a variety of connective tissue in which the reticulum of white fibres is built up of very fine strands leaving large interspaces in which the cells typical of the tissue are enclosed. The ground substance of the tissue is reduced to a minimum. Many connective tissue cells lie on the fibres which may in places be completely covered by them. This tissue therefore forms a groundwork holding together the main parts of an organ to form a compact whole. It may thus be demonstrated in lymphatic glands, the spleen, the liver, in mucous membranes and many other cellular organs.

White Fibrous Tissue

This is the form of tissue in which the white fibres largely preponderate.

The fibrous bundles may Vi= _:? = r __ FIG. 2. - Tendon of rat's tail, stained with gold chloride and showing cells arranged in rows between the bundles of fibres.

be all arranged parallel to one another to form a dense compact structure as in a tendon. It is found wherever great strength combined with flexibility is required and the fibres are arranged in the direction in which the stress has to be transmitted. In other instances the bundles may be united to form membranes, and in such cases the main number of bundles run in one direction only, which a is again that in which the main stress has to be conducted. Such are the ligaments around the joints or the fasciae covering the muscles of the limbs, &c. In other positions, e.g. the dura mater, the fibrous bundles course in all directions, thus forming a very tough membrane. The cells of such tissues lie in the spaces between the bundles and are found flattened out in two or three directions where they are compressed by the oval fibrous bundles surrounding them (see figs. 2 and 3). The cells thus lie in linear groups running parallel to the bundles, presenting a very characteristic appearance.

Yellow Elastic Tissue

This is the form of connective tissue mainly composed of elastic fibres. It is found in those positions where a continuous but varying stress has to be supported. In some positions the elastic tissue is in the form of branching FIG. 4. - Isolated elastic fibres of ligamentum nuchae. Branching fibres of very definite outline with irregularly placed transverse markings.

fibres arranged parallel to one another and bound together by white fibres, e.g. ligamentum nuchae (fig. 4). In other cases it may be in the form of thin plates perforated in many directions to form a fenestrated membrane. In this type a series of such plates are arranged round the larger arteries forming a large proportion of the artery wall.

All the connective tissues are vascular structures though as the number of cells present is not great, and further as those cells are not as a rule the seat of a very active metabolism, the number FIG. 3. - Transverse section of portion of a tendon showing arrangement of white fibres in large bundles bounded by connective tissue, with tendon cells between the fibres. a, tendon cells; b, tendon bundles.

of blood vessels is quite small. The tissues are also supplied with lymphatics and nerves.

Cartilage

Cartilage or gristle is a tough and dense tissue possessing a certain degree of flexibility and high elasticity. It is found where a certain amount of flexibility is required but where a fixed shape must be retained, e.g. in the trachea which must always be kept open or in the external ear or pinna which owes its typical and permanent shape to the presence of cartilage. It is largely associated with the bones in the formation of the skeleton. The tissue consists of a number of cells embedded in a solid matrix or ground substance. Three varieties are distinguished according to the nature of the matrix. Thus if the matrix is homogeneous in structure the cartilage is termed hyaline. Two other forms occur in which fibrous tissue is embedded in the cartilage matrix. They are therefore termed fibro-cartilages and if the fibres are of the white variety the cartilage is called white fibro-cartilage, if of the yellow elastic form, elastic cartilage. Hyaline Cartilage (fig. 5). - This consists of a number of rounded cells enclosed within a homogeneous matrix. The cells possess an oval nucleus and a granular, often vacuolated cellbody. The number of cells present varies considerably in different specimens. In freshly formed cartilage the cells are numerous, the amount of matrix separating them being small. Cartilage grows by a deposition of new matrix by the cartilage cells which thus become more and more separated from one another. After a time the cells divide and subsequently become parted from one another by deposition of fresh matrix between them. The cells are often to be seen in groups of two, three or four cells, indicating the common origin of each group from a parent cell. Towards the surface of the cartilage the cells are often modified in shape tenaing to become flattened in a direction parallel to the surface. Some of the cells near the surface of a piece of cartilage may be branched, appearing as a transition form between connective tissue corpuscles and typical cartilage cells. This is particularly the case at points where tendon or ligaments are attached. There may often be a deposit of lime salts in the matrix of hyaline cartilage especially in old animals or in the deeper layers of articular cartilage where it is attached to bone. A similar deposit of lime salts is well marked in the superficial parts of the skeleton of the cartilaginous fishes. In the development of animals possessing a bony skeleton, the skeleton is first laid down as hyaline cartilage which subsequently becomes gradually removed, bone being deposited in its place. In the adult, hyaline cartilage is found at the ends of the long bones (articular cartilage), uniting the bony ribs to the sternum (costal cartilage), and forming the cartilages of the nose, trachea and bronchi, &c. This as well as the other forms of cartilage are non-vascular so that the cells must gain their food-stuffs and get rid of their waste products by a process of diffusion through the matrix, a process which must of necessity be slow.

White Fibro-Cartilage.--This is a variety of cartilage in which numerous white fibres ramify in all directions through the matrix (fig. 6). The cells lie separate and not in groups, and the amount of matrix between is commonly small. The white fibres may run in all directions or may chiefly run in one direction only. Under the microscope the tissue closely resembles a dense white fibrous tissue, only the cells enclosed in it are cartilage cells and not connective tissue cells. Owing to the presence of so much fibrous tissue this variety of cartilage is very much tougher than hyaline cartilage and less flexible. It is found in places which have to withstand a considerable amount of compression but where a less rigid structure than bone is demanded. Thus it is found forming the intervertebral disks, the interarticular cartilages, or at the edges of joint surfaces to deepen the surface.

FIG. 6. - White fibro-cartilage of intervertebral disk, with typical cartilage cells, matrix characterized by presence of many white fibres.

Elastic Fibro-Cartilage

In this variety the matrix is permeated by a complex and well-defined meshwork of elastic fibres (fig. 7). The size of the fibres varies considerably in different specimens. It is found in parts which have to retain a .x permanent shape but where a considerable amount of flexibility is requisite, as in the pinna of the ear, the epiglottis, the cartilage of the Eustachian tube, &c.

Bone

Bone is a connective tissue in which a considerable amount of mineral matter is deposited in the intercellular matrix whereby it acquires a dense and rigid consistency. If bone be incinerated so that the organic matter is burnt away, a residue of mineral matter is left. This consists chiefly of calcium phosphate, and amounts to as much as two-thirds of the weight of the original bone. If, on the other hand, bone be macerated in hydrochloric or nitric acid for a time the calcium phosphate is dissolved, leaving the organic matter practically unaffected and still showing the microscopic structure of bone. Hence it follows that the organic matrix is uniformly impregnated with the calcium salts.

According to its naked-eye appearance bone is distinguished as being either compact or cancellated. The former is dense like ivory and forms the outer surface of all bones. The whole of the shaft of a long bone is composed of this compact form. Cancellated bone has a spongy structure and contains large interspaces filled with a fatty tissue rich in blood vessels. This form of bone tissue is found forming the interior of most bones, especially the heads of the long bones, the interior of the ribs, &c. The cavity of the shaft of a long bone is filled, just as in the case of the smaller cavities in cancellated bone, with a fatty tissue, the Bone Marrow (see below).

The histological structure of bone may be made out from a piece of dried bone which has been ground down between grinding stones until it is sufficiently thin for microscopic purposes. If such a section be prepared from a thin transverse slice of a long bone the appearance pictured in fig. 8 will be seen. The section comprises a number of circular units bound into a compact whole by intervening material showing in the main the same structural details. Each of these circular structures is termed an Haversian system. In the centre of each is seen a dark area, the Haversian canal, around which the bone matrix is deposited in the form of a number of concentric laminae. Enclosed between the laminae are a number of small spaces also appearing black in this preparation. These are the bone lacunae and spreading away from them in directions generally transverse to the laminae are seen a large number of fine branching lines - the canaliculi. All parts of a preparation such as this which appear dark in reality represent spaces in the bone matrix. In the course of the preparation of the FIG. 5. - Hyaline Cartilage. Homogeneous matrix interspersed with groups of cells whose arrangement shows their development by division of the mother cell.

FIG. 7. - Elastic fibro-cartilage of Epiglottis. Abundant cartilage cells in a matrix containing many branching elastic fibres.

specimen all these cavities have been filled up with finely divided debris and hence appear opaque. In the living bone these spaces are filled with a tissue or a cell or with fine protoplasmic processes. Thus the Haversian canal contains an artery and vein, some capillaries, a flattened lymph space, fine medullated nerve fibres - the whole being supported in a fine fatty tissue. Each lacuna is filled with a cell - the bone corpuscle - and the canaliculi contain fine branching processes of these cells. On comparing such a section with one taken parallel to the long axis of the shaft of a bone it is seen that the Haversian canals run some distance along the length of the bone, and that they frequently unite with one another or communicate by obliquely coursing channels. The spaces between the Haversian systems are filled in with further bony tissues which may or may not be arranged in laminae. Finally, the systems are as it were bound together by other laminae running parallel to the surface of the bone. If a piece of fresh bone be decalcified so that a thin section can be cut from it, the bone corpuscles can be seen filling up the lacunae but the section does not give so typical a picture as that already examined because it is not possible to make the protoplasmic structures filling the lacunae and canaliculi stand out in marked contrast with the surrounding matrix.

FIG. 8. - Section of Bone. Showing four Haversian systems and interlying bone material. This is a dry preparation, hence all the cavities (such as the Haversian Canals, the lacunae and canaliculi), being filled with debris from the grinding, appear dark.

Cancellous bone only differs from compact bone in the arrangement of the bone tissue. This encloses a number of irregular spaces which communicate with one another to form a kind of spongework. Commonly the framework is so constructed that a number of trabeculae running parallel to one another are produced. This is for the purpose of especially strengthening the bone in that direction. This direction is in all cases found to be that in which the bone has to support its maximum strain while in position within the body. Usually the bone trabeculae are so narrow that there is no need for Haversian systems within them, and they therefore usually consist of a few laminae arranged parallel to the surface. These laminae include bone corpuscles as in the rest of the bone tissue.

Bone Marrow

Filling the central cavity of the tubular bones and the cavities of the spongy bone tissue is a tissue largely composed of fat cells. This is the bone marrow. Two varieties are distinguished, the one being red in colour, the other yellow. Red marrow is composed of a number of fat cells lying in a tissue made up of large and small marrow cells and typical giant cells or myeloplaxes (fig. 9). The whole of these elements are supported in a delicate connective tissue. The marrow cells exhibit manifold forms. Some are typical leucocytes and lymphocytes as found in circulating blood. Others named myelocytes are slightly larger than leucocytes, with round or oval nuclei, and a protoplasm containing neutrophile granules. Yet another variety contains large eosinophile granules in the protoplasm. These different types of cell probably develop into leucocytes. The giant cells are large spherical cells with several nuclei.

In addition to fully developed red blood corpuscles there are also present numerous nucleated red blood cells (erythro blasts or haematoblasts). These are red blood corpuscles in an early stage of formation. They reach the blood after they have lost their nuclei: FIG. 9. - Section of Bone Marrow.

f, Fat vacuole. e, Eosinophile cells.

my, Myeloplaxes. r, Red corpuscles.

m, Marrow cells. h, Haematoblasts or erythro-blasts.

Development of Bone

The formation of new bone always takes place from connective tissue, but we may distinguish two different modes. In the first the bone is preceded by cartilage (development from cartilage), in the second the bone is laid down directly from a vascular fibrous membrane (development from membrane). The development of bone from cartilage is the more complicated of the two because in it bone formation is taking place in two positions at the same time and in two rather different manners. Thus bone is being laid down from the outside (perichondral formation) from the fibrous membrane surrounding the cartilage, the perichondrium and also within the substance of the cartilage (endochondral formation). Perichondral formation takes place somewhat earlier than endochondral and in the case of a long bone is first observed around the centre of the shaft, i.e. in that portion of the bone which forms the diaphysis. Here the perichondrium is vascular and carries on the surface next to the cartilage an almost continuous layer of typical cells cuboid in shape, the osteoblasts or bone-formers. Calcium salts are deposited in the matrix of the immediately subjacent cartilage and the cell spaces of the cartilage increase in size while the cartilage cells shrink. Further growth of cartilage ceases in this region so that at one time the shaft of the cartilage may appear constricted in the middle. The formation of bone endochondrally is ushered in by the ingrowth of blood vessels from the perichondrium. A way through the calcified matrix of the cartilage is made for them by a process of erosion. This is effected by a number of polynucleated giant cells, the osteoclasts, which apply themselves to the matrix and gradually dissolve it away. The enlarged cartilage spaces are thus opened to one another, and soon the only remnants of the matrix consist of a number of irregular trabeculae of calcified matrix. In this way the primary marrow spaces are produced, the whole structure representing the future spongy portion of the bone. The next step in both perichondral and endochondral bone formation consists in the deposition of bone matrix. This is effected by the osteoblasts. In the spongy portion they deposit a layer upon the surfaces of the calcified cartilage matrix, and thus in newly formed bone we find a central framework of cartilage matrix enclosed in a layer of bone matrix (see fig. 10). In the perichondral formation the deposition is effected in the same manner but is not uniformly spread over the whole surface, but trabeculae are formed. These become confluent at places, thus leaving spaces through which blood vessels and osteogenetic tissue pass to reach the interior of the bone. As the deposition of bone matrix proceeds, some of the osteoblasts become included within the matrix. These cease to form fresh matrix and in ?

formation then ceases. are produced.

Growth of the bone proceeds by the deposition of more matrix on the exterior, but simultaneously a process of absorp tion is also taking place. This is most typically seen within the spongy portion of the bone. The absorption of the trabeculae is effected by the osteoclasts. These become applied to the trabeculae and gradually eat their way into the matrix thus coming to lie within lacunae. They possess the power of dissolving both bone and cartil age matrix. Side by side with this solution process we may often see new formation taking place by the activity of the osteoblasts (fig. io).

In this manner the whole framework of the bone may be gradually re placed. The process is most active CJ ® , in embryos and very young animals, but also continues during the whole life of an animal, thus effecting alterations in shape and structure of the whole bone. Growth in the length of a bone is effected by formation of new bone at either end of the shaft. After the ossification centre 0.1 ' has been formed in the shaft (diaphysis) of the bone subsidiary centres make their appearance in the heads of the bones. These form, by a similar process of bone formation, fresh bone masses which, o, however, are not continuous with the bone tissue of the shaft. They form the epiphyses. They are attached to the diaphysis by an intermediate piece of cartilage, and it is by a process of growth of this cartilage and its subsequent replacement by bone that growth in length of the whole bone is effected (fig. 10). This piece of intervening cartilage can be easily seen in a young bone and persists as long as the bone can increase in length. Thus in man the last junction of epiphysis to diaphysis may not take place until the 28th year.

Development of bone in membrane shows a course in all respects very similar to perichondral bone formation. A layer of osteogenetic tissue makes its appearance in the membrane from which the bone is to be formed. In this tissue a number of stiff fibres are deposited which soon become covered and impregnated with calcium salts. Around these bundles of fibres numbers of osteoblasts are deposited and by them bone matrix is deposited in irregular trabeculae. The bone increases by the deposition of fresh matrix just as in perichondral bone formation and Haversian systems are formed after precisely the same manner as in that position. The factor determining the position of one of these systems is of course the presence of a blood vessel penetrating towards the deeper part of the bone.

Muscle

Muscle is the contractile tissue of the body, that tissue by which the various parts of the body are moved. Thus it forms the main bulk of the limbs, back, neck and body wall.. Most of the viscera too possess well-developed muscular coats. When separated into its constituent parts it is seen that muscle in all instances is built up of a number of long fibres. These are of three well-defined types. Those forming the skeletal muscles are of large size, even in some instances up to 12 cms. in length, their diameter varying from 0,01 to o,i mm. When these are examined under the microscope they are found to be characterized by possessing a decided transverse marking, and they are therefore known as striated muscle fibres. From the fact that they comprise those muscles which are under the control of the will they are also called voluntary muscle fibres. The second variety of muscle is made up of much smaller fibres varying in different parts from 0,05 to 0,15 mm. in length and about 0,005 mm. in diameter. These fibres show no transverse striations nor are they directly under the control of the will. They are therefore termed smooth or involuntary muscle. Lastly, there is a third type of muscle found in the heart which lies intermediate in structure between these two varieties. In this the fibres are small and show distinct transverse striations. Longitudinal striations are also present though somewhat less marked. In most respects this form of muscle fibre resembles smooth muscle more closely than striated muscle. Voluntary or Striated Muscle. - Each muscle fibre of which this is composed is what is known as a syncytium or plasmodium, i.e. a structure containing a number of nuclei, which has been formed from a single cell by proliferation of its nucleus without subdivision of the protoplasm. It is thus an assemblage of cells. possessing a common protoplasm. Each fibre generally runs parallel to the length of the muscle and if that muscle is short extends the whole length. Thus the one end of the fibre may be attached to tendon when the end is rounded off. The other end may also terminate in tendon or in the fibrous covering of bone in which case it is again rounded. In long muscles, however, the fibre may only extend a certain distance along the muscle, and it is then found to terminate in a tapering or bevelled end. In some of the long muscles some of the fibres may both arise and terminate in the substance of the muscles. In such a case both ends are bevelled. All the fibres in a muscle are arranged parallel to one another.

The outer surface of each muscle fibre consists of a tough homogeneous membrane, the sarcolemma. The main muscle substance (see fig. II) is composed of several parts, viz. the fibrillae, the sarcoplasm and the nuclei. Under the action of reagents the muscle fibre may be split into a number of longitudinal elements. These are the fibrillae. They possess alternate bands of light and dark substance which give them a striated appearance. When viewed under polarized light the dark substance is found to be doubly refracting or anisotropic, the light band is singly refracting or isotropic. According to many observers, in the centre of each isotropic segment there is a thin transverse disk of anisotropic material and in the centre of each anisoptropic segment a thin disk of isotropic substance. The fibrillae are arranged in the muscle fibre parallel to one another and with the alternate light and dark bands at approximately the same level across the fibre, thus giving to the whole muscle fibre its typical transverse striation. The fibrillae are united to one another by interfibrillar substance to form bundles, of which there may be a considerable number in each muscle fibre. The bundles lie in a surrounding layer of sarcoplasm which apparently represents the remaining portion of unaltered protoplasm of the syncytium. This structure of muscle is best seen in the transverse sections of the fibres. A number of areas separated by a clear protoplasm are then to be seen. The areas are formed by the bundles of fibrillae seen in transverse section, fact become bone corpuscles. Increase in thickness of the new bone is effected by the deposition of fresh matrix followed again by the inclusion of further osteoblasts. The spaces within the trabeculae become in this way gradually narrowed by the deposition of matrix until at last only a narrow centre is left large enough to contain the blood vessels and their accompanying nerves, lymphatics and a small number of osteoblasts. Bone In this manner the Haversian systems FIG. Io. - A part of bone developing from cartilage showing enlarged cartilage spaces.

Osteoblasts lining a cavity and depositing bone matrix on the wall of that cavity.

0.l, Osteoblasts which have become included in the deposited bone to form bone corpuscles.

b, Freshly laid down bone matrix.

cl, Giant cells or osteoclasts.

c, Cartilage cells arranged in rows.

a, Unaltered matrix of hyaline cartilage.

FIG. I I. - Striated or Voluntary muscle fibre, with alternate light and dark bands and many nuclei immediately beneath the sarcolemma.

the intermediate substance is the sarcoplasm. In some muscles, apparently, each fibrilla is surrounded by a considerable amount of sarcoplasm, in which case the fibrillae are easily isolated from one another and can be readily examined. This is the case in the wing muscles of insects. The nuclei of the fibre are arranged close FIG. 12. - Transverse section of FIG. 13. - Isolated a striated muscle fibre. smooth muscle fibres.

n, Nucleus. Very much contracted.

s, Sarcoplasm. Fibres tapering at each In, Bundle of fibrillae forming end, with nucleus in a muscle column. centre of cell.

under the sarcolemma. Each is surrounded by a small quantity of sarcoplasm and in shape is an elongated ellipse. In most cases the muscle fibres do not branch, though in a few instances, such as the superficial muscles of the tongue, branching is found.

Involuntary or Smooth Muscle (figs. 13 and 14). - This form of muscle tissue when separated into its single constituents is seen to consist of fibres possessing a typical long spindle shape. The central part is somewhat swollen and contains an elongated nucleus centrally placed. The ends of the fibres are drawn out and pointed sharply. There is no definite surrounding membrane to each cell. In most of the cells, especially the larger, a distinct longitudinal marking can be seen. This is due to the presence of the fibrils which run the length of the fibre and in all probability are the essential contractile elements.

In most instances the cells are arranged with one another in a tissue to form bundles or sheets of contractile substance. In each bundle or sheet the cells are cemented to one another so that they may all act in unison. The cementing material is apparently of a membranous character and is so arranged that contiguous fibres are only separated by a single layer of membrane. According to some, neighbouring fibres are connected to one another by minute offshoots, and these communications serve to explain the manner in which the contractionisobserved to pass from fibre to fibre along a sheet composed of the muscles.

Involuntary muscle is the variety of muscle tissue found in the walls of the hollow viscera, such as stomach, intestines, ureter,bladder, uterus, &c., and of the respiratory passages, in the middle coats of arteries, in the skin FIG. 14. - Preparation of Frog's Bladder and the muscular trashowing smooth muscle in situ forming a beculae of the spleen. network. The arrangement is very typical, for in stance, in the small intestine. Here the muscular coat consists of two layers of muscle. Each is in the form of a sheet which varies greatly in thickness in different animals. In the inner sheet the fibres, which are all parallel to one another, are disposed with their long axis transverse to the direction of the gut. In the outer layer, the direction of the fibres is at right angles to this. In a viscus with thick muscle walls the fibres are bound into bundles and the bundles may run in all directions. In some instances the bundles may form branching systems, thus constituting a network, as in the bladder (fig. 14). In other instances, e.g. the villi of the small intestine, the muscle fibres are separate, forming a felt-work with wide meshes.

Heart Muscle

The fibres of which the muscular walls of the heart are composed though cross striated are not voluntary, for they are not under the control of the will. Each fibre is an oblong cell possessing distinct transverse and less distinct longitudinal striations (fig. 15). There is no sarcolemma, and the nucleus of each fibre is placed in the centre. The longitudinal striation is due to the presence of fibrillae, each of which is cross striated. These lie parallel to one another in the cell, the sarcoplasm surrounding them being much more abundant in these fibres than is striated muscle. The fibrillae are arranged in rows, and when a transverse section of one of these fibres is Isolated cells. examined it is seen that the rows radiate away from the centre of the cell. A further distinctive character of cardiac muscle fibres is that they frequently branch, the branches uniting with others from neighbouring cells. Moreover, the ends of the fibres are attached to corresponding faces of other cells, and through these attached faces the fibrillae pass, so that there is an approximation to the formation of a syncytium. (T.:G. BR.)