Two sebaceous glands commonly open into the hairsac near its opening, and supply the hair with a kind of natural pomatum; and delicate unstriped muscular fibres are so connected with the hair-sac as to cause it to pass from its ordinary oblique position into one perpendicular to the skin, when they contract (Fig. 31, B).

They are made to contract by the influence of cold and terror, which thus give rise to "horripilation or "gooseskin," 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.). Like them it is essentially cellular in nature, but differs from them widely in appearance on account of the development of a large quantity of the so-called intercellular substance. That is to say, the several cells do not lie closely packed together and touching each other, but are separated from each other by a quantity of material of a different nature from themselves. Just as in indifferent tissue each nucleus is imbedded in a matrix of protoplasm, so in cartilage, each cell, i.e. each nucleus with its allotted quantity of protoplasm, is imbedded in a matrix of intercellular substance.

Inasmuch as during the growth of cartilage the cells remain soft and protoplasmic, while the intercellular substance is converted into a solid semi-transparent hard matter, it comes to pass that the soft nucleated cells appear to lie in cavities in the harder intercellular substance or matrix.

In epithelium it is only the deepest lying cells which undergo division, and so carry on the growth of the tissue. In cartilage, cell-division is much more general; a cell lying in its cavity divides first into two, then into four, and so on, the intercellular substance meanwhile growing in between the young cells and thrusting them apart. It is by means of the repeated divisions of the cells in this way, and subsequent development of intercellular matrix in between the young cells, that cartilage grows.

Con

A section of cartilage, showing the matrix (a), with the groups of cells (6) containing nuclei (c) and fat globules (d). (Magnified about 350 diameters.)

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 (6) the elastic fibres. (Magnified about 300 diameters.)

sequently, the cells are frequently seen arranged in groups with more or less matrix between, according to their age. The cells remain during life soft and protoplasmic, but often contain a number of large oil globules. It is to the hard matrix which yields, on boiling, the substance chondrine, that the physical features of cartilage, its solidity and elasticity, are due. Cartilage contains no vessels, or only such as extend a little way into it from adjacent parts. 8. Connective tissue (also called fibrous, or areolar, or sometimes cellular tissue), the most extensively diffused

FIG. 93.

Connective tissue corpuscles (a, nucleus, b, cell substances), of various shapes, those to the right hand branching, and the branches joining.

of all in the body, at first sight seems to differ wholly from the preceding tissues. Viewed under the microscope, it is seen to consist 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 or fibrilla. The addition of acetic acid 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 certain

sharply defined fibres of different degrees of fineness, which are called elastic fibres. If the acid be now very carefully neutralized by a weak alkali, the connective tissue assumes its former partial opacity and fibrillated aspect. The nuclei thus brought to light by acetic acid are worthy of attention because careful examination shows that they belong to certain cells which exist in all connective tissue in greater or less number, though never in abundance. These cells, generally called connective tissue corpuscles, consist of a nucleus and protoplasmic cell-substance, and in fact are not unlike cartilage cells except that they are very often very irregular in form, and as a general rule very

A, having their natural aspect. B, collapsed, the fat being exhausted. C, with fatty crystals. The nuclei are not seen in this case. (Magnified about 350 diameters.)

small. Indeed we may very justly compare connective tissue with cartilage, much as they seem to differ in general appearance. The connective tissue corpuscles correspond to the cartilage-cells; both are imbedded in a matrix which, in the case of cartilage, remains structureless, but becomes solid and dense, while it, in the case of connective tissue, is altered or metamorphosed, as it is said, into a substance composed of excessively fine filaments, mingled with which are elastic fibres.

The fine fibrillated substance is not very elastic, and when boiled swells up and yields gelatine. The elastic fibres do not yield gelatine, and, as their name indicates, are highly elastic. 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, while fascia are sheets, 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.), or, in other words, the matrix of the cartilage becomes more or less fibrillated, thus indicating the analogies of the two tissues.

The name cellular applied to this tissue is apt to lead to confusion, When first used it referred to the cavities left in the meshes of the network of fibres; it has nothing whatever to do with cells technically so called.

9. Fat cells are scattered through the connective tissue, in which they sometimes accumulate in great quantities.

A, network round a group of fat cells. a, the artery; b, the vein.
B, the loops of capillaries round three individual fat cells.

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 (B, Fig. 94).