They are made to contract by the influence of cold and terror, which thus give rise to "horripilation" or "goose-skin," and the "standing of the hair on end."

6. The crystalline lens is composed of fibres, which are the modified cells of the epidermis of that inverted portion of the integument, from which the whole anterior chamber of the eye and the lens are primitively formed.

7. Cartilage. While epithelium and epidermis are found only on the free surfaces of the organs, gristle, or cartilage, is a deep-seated structure (see Lesson VII.). It is composed of a semi-transparent, resisting, elastic matter, which yields the substance called chondrine by boiling, and contains a great number of minute cavities, in which lie single nucleated cells, or groups of such cells (Fig. 78). These cells increase in number by division. Cartilage contains no vessels, or only such as extend into it from adjacent parts.

A section of cartilage, showing the matrix (a), with the groups of cells (b) containing nuclei (c) and fat globules (d). Magnified about 350 diameters.

8. Connective Tissue (also called fibrous, or areolar, or sometimes cellular tissue).

This tissue, the most extensively diffused of all in the body, consists of bands or cords, or sheets of whitish substance, having a wavy fibrous appearance, and capable of being split up mechanically into innumerable fine filaments. It swells up and yields gelatine when it is boiled in water. A

B

FIG. 79.

Connective tissue.-A, unchanged-a, connective tissue; b, fat cells: B, acted upon by acetic acid, and showing (a) the swollen and transparent gelatine-yielding matter, and (b) the elastic fibres. Magnified about 300 diameters.

The addition of strong acetic acid also causes it to swell up and become transparent, entirely losing its fibrous aspect; and, further, reveals the presence

of two elements which acetic acid does not affect, viz. nuclei and elastic fibres of different degrees of fineness. If the acid be now neutralized by a weak alkali, the connective tissue assumes its former partial opacity and fibrillated aspect. The nuclei are the descendants of those which existed in the indifferent tissue from which the connective tissue has proceeded while the elastic fibres, like the gelatine-yielding fibres, proceed from the metamorphosis of the matrix. The proportion of elastic fibre to the gelatine-yielding constituents of connective tissue varies, in different parts of the body. Sometimes it is so great that elasticity is the most marked character of the resulting tissue.

Ligaments and tendons are simply cords, or bands, of very dense connective tissue. In some parts of the body the connective tissue is more or less mixed with, or passes into, cartilage, and such tissues are called fibro-cartilages (see Lesson VII.).

9. Fat Cells are scattered through the connective tissue, in which they sometimes accumulate in great quantities. They are spheroidal sacs, composed of a delicate membrane, on one side of which is a nucleus, and distended by fatty matter, from which the more solid fats sometimes crystallize out after death. Ether will dissolve out the fat, and leave the sacs empty and collapsed (Fig. 80).

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.

Fat cells.-A, having their natural aspect; B, collapsed, the fat being exhausted; C, with fatty crystals. Magnified about 350 diameters.

10. Pigment Cells are either epidermic or epithelial cells, in which coloured granules are deposited, or they are particular cellular elements of the deeper parts of the body, in which a like deposit occurs. Thus the colour of the choroid and of the iris arises from the presence of a layer of such cells.

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 lacunæ, 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 is constantly

found in each lacuna; and, not unfrequently, the intermediate substance appears minutely fibrillated. 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. 81, 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. 81, 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 of 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,