The muscles, like the nerves, are divided into two classes or orders, the one under the influence of the will, the other independent of it. The texture is different in each of these two orders : in the voluntary muscles, the fibres and bundles of which the peculiar tissue con sists are very regularly disposed, and generally in straight and parallel lines relatively to one another; in the involuntary muscles again, the fibres appear of different degrees of density, run parallel or obliquely with regard to one another, are superposed in layers, intermingled and entangled like a kind of felt, &c.
The fifth tissue which prevails among ani mals is the fibrous. This is or may be divided into the tendinous and ligamentous. These are alike subservient to the muscular tissue and to the function of voluntary motion. They con sist of fibrous, parallel bundles, of a white colour and pearly lustre, of great strength, and possessing little elasticity.
The sixth tissue which is peculiar to animals (the first of those less universally distributed) is the osseous. This forms the frame-work or skeleton which gives form and fixity to all the other parts entering into the constitution of the higher animals. The essential organic element of bone is a cellular net-work consisting of gelatine, within the meshes of which certain calcareous salts, the phosphate and a little carbonate of lime especially, are deposited in order to give them greater solidity.
The cartilaginous is generally reckoned as the seventh among the elementary tissues of animals ; it may and has been very properly assimilated to the osseous : the bones are car tilaginous at first, and with the progress of years many of the cartilages show a tendency to, or do actually become, converted into bone. The cartilages that cover the articular heads of the bones are almost the only ones that show no disposition to undergo this change. The organic element of cartilage is gelatine.
Thefibro-cartilaginous is a mere modification, although an interesting one, of the cartilaginous or rather of the fibrous tissue. The fibro-car tilages are very strong, and particularly elastic.
The horny and calcareous coverings of in sects, and the crustacea have uses corresponding to those of the bones. The calcareous shells of the mollusca, too, bear a certain, though a very remote analogy to the skeletons of the higher animals.
The horny or eighth tissue peculiar to ani mals might with propriety be reckoned among the number of those that are very widely dis tributed. We meet with it in the epidermis of man, and as low in the scale at least as the molluscs and annelides ; it is the most universal clothing provided by nature for the bodies of animals.
So much for the simple tissues entering into the composition of animals, to many of which nothing analogous can be discovered among vegetables. But these are by no means the only solid elements that make up the aggregate of animal bodies. The organs, as we entitle them, for the performance of certain functions so generally encountered among animals,—the lungs, liver, stomach, kidneys, testes, ovaries, &c., &c., are so many peculiar compounds of the more simple tissues, occasionally with ad ditions denominated parenchyma, nothing con responding to which has ever been discovered among vegetables. These various organs are associated in animals into groups, denominated systems, which severally tend to the accom plishment of the individual functions mani fested by the creature examined,—the teeth, tongue, salivary glands, (esophagus, stomach, liver, pancreas, and intestinal canal, constitute one great and important system, subservient to the conversion of food into nourishment, and the preservation of the individual ; the testes, penis, vagina, uterus, and ovaries, in the two sexes, compose another great system by which the species is continued, and so on.
Besides these solids we have a great variety of fluids, which in animal bodies subserve various and important purposes : we have, for instance, the general nutrient fluid distributed to all parts of their bodies, denominated blood. We have a variety of fluids prepared for aiding or accomplishing the act of digestion,—the saliva, gastric juice, pancreatic juice, and bile ; we have various fluids as emunctories of the worn-out parts and particles of the system,— the perspiration and the urine ; and we have a peculiar fluid prepared as a means of con tinuing the species—the spermatic fluid. Fluids corresponding in their destination to one or two of these are also met with among vege tables, but there they are greatly modified.