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They are, in fact, cells with a distinct cell wall, the cell contents, or cell substance, of which have been wholly, or all but wholly, converted into fat.

Considerable aggregations of fat cells are constantly present in some parts of the body, as in the orbit, and about the kidneys and heart; but elsewhere their presence, in any quantity, depends very much on the state of nutrition. Indeed, they may be regarded simply as a reserve, formed from the nutriment which has been taken into the body in excess of its average consumption.

10. Pigment cells are either epidermic, or epithelial, cells, in which coloured granules are deposited ; or they are connective tissue corpuscles of the deeper parts of the body, in which a like deposit occurs. Thus the colour of the choroid arises partly from the presence of a layer of epithelial cells (see Fig. 74), placed close to the retina, containing pigment granules, and partly from a large number of irregularly-shaped, connective tissue corpuscles crammed with pigment, which belong to the deeper connective tissue layer of the choroid. The pigment cells of the frog's web are, for the most part, connective tissue corpuscles, containing colouring matter.

11. Bone is essentially composed of an animal basis impregnated with salts of carbonate and phosphate of lime, through the substance of which are scattered minute cavities--the lacuna, which send out multitudinous ramifications, called canaliculi. The canaliculi of different lacunæ unite together, and thus establish a communication between the different lacunæ. If the earthy matter be extracted by dilute acids, a nucleus may be found in each lacuna ; and if young, fresh bone be carefully examined, a certain amount of cell substance will be found filling up the lacuna round the nucleus; and, not unfrequently, the intermediate substance appears minutely fibrillated. In fact bone, if we lay on one side the earthy matters, presents very close analogies in its fundamental structure with both cartilage and connective tissue. The corpuscles lodged in the lacunæ correspond to the corpuscles of connective tissue and to the cells of cartilage, while the matrix in which the earthy matter is deposited corresponds to the matrix of cartilage, and to the fibrillated material of connective tissue. (These three tissues

indeed are often classed together as “the connective tissue group.”) In a dry bone the lacunæ are usually filled with air. “When a thin section of such a bone is, as usual, covered with water and a thin glass, and placed under the microscope, the air in the lacunæ refracts the light which passes through them in such a manner as to prevent its reaching the eye, and they appear black. Hence the lacunæ were, at one time, supposed to be solid bodies, containing the lime salts of the bone, and were called bone corpuscles (Fig. 96, C).

All bones, except the smallest, are traversed by small canals, converted by side branches into a network, and containing vessels supported by more or less connective tissue and fatty matter. These are called Haversian canals (Fig. 96, A, B). They always open, in the long run, upon the surface of the bone, and there the vessels which they contain become connected with those of a sheet of tough connective tissue, which invests the bone, and is called periosteum.

In many long bones, such as the thigh bone, the centre of the bone is hollowed out into a considerable cavity, containing great quantities fat, supported by a delicate connective tissue, rich in blood-vessels, and called the marrow, or medulla. The inner ends of the Haversian canals communicate with this cavity, and their vessels are continuous with those of the marrow.

When a section of a bone containing Haversian canals is made, it is found that the lacunæ are dispersed in concentric zones around each Haversian canal, so that the substance of the bone appears laminated ; and, where a medullary cavity exists, more or fewer of these concentric lamellæ of osseous substance surround it.

This structure arises from the mode of growth of bones. In the place of every bone there exists, at first, either cartilage, or connective tissue hardly altered from its primitive condition of indifferent tissue. When ossification commences, the vessels from the adjacent parts extend into the ossifying tissue, and the calcareous salts are thrown down around them. These calcareous salts invade all the ossifying tissue, except the immediate neighbourhood of its nuclei, around each of which a space, the lacuna, is left. The lacunæ and canaliculi are thus, substantially,

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A. A transverse section of bone in the neighbourhood of two Haversian

canals, a a; b, lacunæ. (Magnified about 250 diameters.) B. A longitudinal section of bone with Haversian canals, a a, and lacuna,

b. (Magnified about 100 diameters.), C. Lacunæ, c, and canaliculi, d. (Magnified about 6oo diameters.)

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gaps left in the ossific matter around each nucleus, whence it is that nuclei are found in the lacunæ of fully-formed bone.1

Bone, once formed, does not remain during life, but is constantly disappearing and being replaced in all its parts. Nevertheless, the growth of a bone, as a general rule, takes place only by addition to its free ends and surfaces. Thus the bones of the skull grow in thickness, on their surfaces, and in breadth at their edges, where they unite by sutures; and when the sutures are once closed, they cease to increase in breadth.

The bones of the limbs, which are preceded by complete small cartilaginous models, grow in two ways. The cartilage of which they consist grows and enlarges at its extremities until the bones have attained their full size, and remains to the end of life as articular cartilage. But in the middle, or shaft, of the bone, the cartilage does not grow with the increase in the dimensions of the bone, but the small primary bone which results from the ossification of the cartilaginous model becomes coated by successive layers of bone, produced by the ossification of that part of the periosteum which lies nearest to it, and which really consists of indifferent tissue, that is of nuclei imbedded in a matrix. The shaft of the bone thus formed is gradually hollowed out in its interior to form the medullary cavity, so that, at last, the primitive cartilage totally disappears.

When ossification sets in, the salts of lime are not diffused uniformly through the whole mass of the preexisting cartilage, or connective tissue, but begin to be deposited at particular points called centres of ossification, and spread from them through the bone. Thus, a long bone has usually, at fewest, three centres of ossificationone for the middle or shaft, and one for each end ; and it is only in adult life that the three bony masses thus formed unite into one bone.

12. Teeth partake more of the nature of bones than of any other organ, and are, in fact, partially composed of true bony matter, here called cement; but their chief constituents are two other tissues, called dentine and enamel. Each tooth presents a crown, which is exposed to wear,

For the sake of simplicity. I purposely omit all mention of the complex secondary processes in the ossification of cartilage.

and one or more fangs, which are buried in a socket furnished by the jawbone and the dense mucous membrane of the mouth, which constitutes the gum. The line of junction between the crown and the fang is the neck of the tooth. In the interior of the tooth is a cavity, which communicates with the exterior by canals, which traverse the fangs and open at their points. This cavity is the pulp cavity. It is occupied by a highly vascular and nervous tissue, the dental pulp, which is continuous below, through the openings of the fangs, with the mucous membrane of the gum. A

B d

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Fig. 97

A, vertical, B, horizontal section of a tooth.-a, enamel of the crown; b, pulp cavity ; C, cement of the fangs; d, dentine. (Magnified about three diameters.),

The chief constituent of a tooth is dentine-a dense calcified substance containing less animal matter than bone, and further differing from it in possessing no lacunæ, or proper canaliculi. Instead of these it

presents imumerable, minute, parallel, wavy tubules, which give off lateral branches. The wider ends of these tubules open into the pulp cavity, while the narrower ultimate terminations ramify at the surface of the dentine, and may even extend into the enamel or cement (Fig. 98, C).

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