Immediately below the skin, and connected with the sub jacent parts, is a series of membranous cells, which have been called the cellular texture, and which is extended through nearly every part of the body. It not only unites the cutis with the muscles over which it is extended, but it fills up the interstices between the muscles, and occupies all the spaces that intervene between the different viscera. The cells of which this texture is composed appear to be of all shapes and sizes, adapted to the peculiar parts in which they are situated. They have a communication with each other, so that air introduced into them, either intentionally or accidentally, is soon diffused over every part of the body. This occurrence sometimes takes place in wounds of the chest, where a puncture having been made in the lungs, and a communication formed with the neighbouring cells, the air received in respiration is ad mitted into the membranous texture, and becomes diffus ed through the whole body, puffing it up in an extraordi nary degree. These destined for the reception of a serous fluid, which, in the healthy state, exists in small quantity only, but in dropsical affections is much increas ed, so that the body sometimes becomes distended to a great size.
It is in this system of cells that the fat is deposited ; but it appears that the fat and the serous fluid are not actually in contact, in consequence of each globule of fat being pro vided with a distinct vesicle, in which it is contained. Ou this account fat cannot pass from one part to another, as we observe to be the case with the water of dropsy, or with air introduced into the cellular texture, but the same particles of fat always remain stationary in the same cells.
The tendons and ligaments bear a close relation to the fibrous membranes, and indeed are conceived to differ from them solely in their external form, for, like them, they con sist entirely of strong fibres, closely united together, with out nerves, and possessing very few blood-vessels. Car tilages are of a more uniform texture than tendons and li garnents, so that no fibres can be perceived in them. They consist principally of albumen, with only a small propor tion of jelly or mucus, and at the same time a small quan tity of the earth of bones enters into their composition. They are harder than tendons, but generally more elastic. Their principal use is to supply the place of bones, where strength and elasticity are both necessary, particularly in the chest, the wind-pipe, the gullet, and about the joints. They may be regarded as forming a kind of intermediate link between membrane and bone, and accordingly it is found, that many parts of the body which are cartilaginous in the young 'animal, become converted into perfect bone as it advances to maturity.
As the fibrous membranes, the tendons, ligaments, and cartilages, are without nerves, are very scantily supplied with vessels of all kinds, and possess neither contractility nor scnsihility, it may be asked, How are they connected with the vital system, or in what sense are we.to regard them as possessing life ? The question is one which it is not very easy to answer, and which, perhaps, must de pend more upon the definitions which we give to certain words and expressions, than upon any facts which we have it in our power to advance. Some eminent physiologists conceive life to be always connected with vascularity and sensibility, and, in conformity with their views, they do not hesitate to style the dense membranous matter dead or inanimate, seeming to regard it as only mechanically attached to the more vital parts. But to this it may be
objected, that there is no portion of the animal body which has not a regularly organized structure, and that this can only be produced by some vascular action, analogous to secretion, by which the matter that composes them may be deposited in its proper position. There are many facts, which would seem to prove, that all these parts are under the influence of the absorbents, by which their substance is gradually removed, particle by particle, in the same gradual manner in which it was deposited, and there are likewise other facts which show that these extravascular parts are subject to various diseased actions, which prove their connexion with, and dependence upon, the arterial system. We may therefore conclude from analogy, that all these structures are, in some manner, under the influ ence of the secretory and absorbent vessels, or of vessels which possess similar powers, although it is admitted that we cannot demonstrate their existence, or bring forward any arguments in favour of it, except the effects which, arc produced.
There is a class of bodies, connected with the external surface of all animals, which, although very different in their shape and appearance, are analogous to each other in their origin and uses. They may be divided into two varieties, the first consisting of nails, claws, hoofs, scales, Stc., the second consisting of hair, bristles, wool, quills, and feathers. The fot variety may be considered as weapons of defence oilrotection ; they are either produc tions of the skin, or at least are so intimately united to it, that it is often difficult to trace the exact line of demarcation between them. We may generally perceive in them a kind of fibrous or laminated texture, although this cannot be detected in those that are the most dense, which are nearly homogeneous. They are composed of coagulated albumen, united to different proportions of jelly and MUCUS.
Hair and feathers differ materially from the bodies just described, both in their origin and in their organization ; they proceed from a kind of root or bulb, that is situated below the skin, and they pass out through its pores. They consist essentially of an external tube, and an internal pulp ; in hair the tube is very delicate, and is entirely filled with the pulp ; in the quill the tube is denser, and the internal pulp is much smaller in quantity. Although hair is so smooth to the touch, it is stated to possess an imbricated or bristled texture, the processes all pointing in one direction, from the root to the point, analogous to the feather part of the quill. It is upon this texture that the operation of felting has been supposed to depend, in which the hairs are entangled together, and are retained in this state by the inequalities on their surface. The basis of all these bodies is coagulated albumen, generally mixed with small quantities of jelly and mucus. The pulpy part of hair contains an oily substance, which pro duces its peculiar colour, and generally corresponds to the shade of the complexion, as depending upon the rete mucosum of the skin. In their natural state all these bodies are without sensation, and possess no visible blood vessels, but, under certain circumstances, many of them are subject to a species of inflammation, when vessels may be detected in them, and they become acutely painful.