These elementary tissues, combined and arranged in a great variety of modes, constitute the root, trunk, leaves, flowers, and fruit of all vascular vegetables ; and it is wonderful how nearly the whole of this tribe, however dis similar in their outward appearance, resemble one another in their intimate structure.
The tissues that enter into the composition of animals are much more numerous than those of vegetables. The most universally distributed of these in the more perfect species of animals are the cellular, the vascular, the nervous, and the muscular, to which must be added the tendinous or fibrous, the osseous, the cartila ginous, and the horny, which are less uniformly diffused among the individuals composing the animal kingdom.
The cellular is the tissue the most universally encountered among animals ; it is demonstrable from the very lowest to the very highest. Its general appearance is that of a soft, homo geneous, whitish, semi-transparent, extensible, and during life slightly contractile substance. It is permeable to air and liquids, and is easily distended by either of these, when it forms a series of continuous cavities or cells, strangers at first to its constitution, but so readily pro duced as to have given the tissue its distin guishing title. The cellular tissue is dispersed abundantly through every of the animal body ; it enters as a principal element into the composition of many other tissues; it pervades the innermost parts of almost all organs, and in a modified shape forms a covering for them externally ; it may be said to constitute the frame-work of the organs generally, supporting them in their particles as it does in their masses ; it connects them together also, includes and accompanies the bloodvessels that supply them with nourishment, fills the intervals between them, and establishes continuity between every part of individual organized beings. The cellu lar tissue consists of filaments and laminae, mingled and entangled together; the interstices it contains, and which may be blown up into cells, appear to be moistened during life by a thin vapour, or a variable quantity of serous fluid.* The cellular substance appears to constitute the element of the various membranes encoun tered in animal bodies : the fibrous membranes, the skin, the mucous membranes, the serous membranes, and the synovial membranes, are all readily resolvable into cellular tissue ; they in fact appear to consist of this tissue in dif ferent states of condensation.
The vascular is another tissue extensively distributed among animals. Three modifica tions of the vascular tissue have been reckoned by anatomists, occurring respectively in arteries, veins, and lymphatics.
The third tissue which is peculiar to animals is the nervous. This may be held the most eminently distinctive of this class of organized beings, as it is by its intermedium that they exhibit almost all the faculties which place them so immeasurably above vegetables in the scale of creation, and as, generally speaking, they may be reckoned by so much the more perfect as particular portions of this system are more fully developed. The element of the nervous tissue is a soft, whitish, and little consistent substance, composed of mi nute globules surrounded by a semifluid sub stance, and connected together by a tissue of cellular membrane of extreme tenuity. The globules are mostly disposed longitudinally, when they form the medullary fibres of the brain ; surrounded by denser sheaths, they take the form of nerves. In all the higher animals at least, two orders of nerves are distinguished, each, however, being intimately connected with the other,—the nerves of animal, or, better, of phrenic life, and the nerves of organic or vege tative life. The nerves of the first order are connected in the higher classes of animals with a brain and spinal cord ; those of the second proceed from small bodies of a reddish grey colour, and irregular shape, named ganglions. The functions of the first take place with con sciousness, those of the second without this mental phenomenon.* The fourth tissue peculiar to animals is the muscular. In several of the very lowest tribes of these, indeed, the existence of this tissue cannot be demonstrated ; yet its actions begin to be manifested at a very low grade in the scale. The element of the muscular tissue is a fibre, on the ultimate constitution of which there have been many disputes. The ultimate muscular fibre would appear to consist of a series of solid globules longitudinally disposed, and connected into larger and larger fasciculi, which at length compose the distinct bundles denominated muscles. Fibrine is the organic element of the muscular tissue. Its peculiar and distinguishing property is its capacity to contract or to become shorter, and to relax again or return in its quiescent state to its first lentgh.