Book II. MUSCULAR STRUCTURE OF ANIMALS. 287 



opposite bone. It thus forms a close bag at the joint, in which nothing from without 

 can enter, and from which nothing can escape. Into this bag the lubricating liquor 

 termed synovia is conveyed. It is secreted by a mucous membrane on the interior ; on 

 which account, as it in some cases appears like little bags, the term bursa mucosa has 

 been bestowed upon it. 



1891. Ligaments. Besides the sheath formed by the continuation of the periosteum, which is too slender 

 to retain the bones in their proper place, the joints are furnished with ligaments. These are membranes 

 of a dense fibrous texture, flexible, elastic, and possessed of great tenacity. They have their insertion in 

 the periosteum and bone, with which they are intimately united. The motions which joints of this kind 

 are capable of performing, may be reduced to three kinds, flexion, twisting, and sliding. In flexion, the 

 free extremity of the bone which is moved, approaches the bone which is fixed, describing the segment 

 of a circle, whose centre is in the joint. In twisting, the bone which is moved turns round its own axis, 

 passing through the articulation. In sliding, the free extremity of the bone moved, approaches the bone 

 which is fixed, in a straight line. 



SuBSECT. 2. Muscular Structure of Animals. 



1 892. The muscles are the organs by which motion is executed : they unfold the most 

 singular mechanism of parts, and an infinite variety of movements. The muscles 

 appear in the form of large bundles, consisting of cords. These, again, are formed of 

 smaller threads, which are capable of division into the primary filaments. Each muscle, 

 and all its component cords and filaments, are enveloped by a covering of cellular mem- 

 brane, liberally supplied with blood-vessels and nerves. At the extremities of the 

 muscular fibres, where they are attached to the more solid parts, there are usually threads 

 of a substance, differing in its appearance from the muscle, and denominated tendon 

 or sinew. The tendons are, in general, of a silvery white colour, a close, firm, fibrous 

 texture, and possessed of great tenacity. The thread of which they consist, are attached 

 on the one extremity to the surface of a bone, or other hard part ; and, on the other, they 

 are variously interspersed among the fibres or bundles of the muscle. They are consi- 

 dered as destitute of sensibility and irritability, and form a passive link between the 

 muscle and the bone, or other point of support. 



1893. Muscles are the most active meinbers of the animal frame. They alone possess the power of 

 irritability, and execute all the motions of the body. The causes which excite them to action may be 

 reduced to two kinds. In the first the will, through the medium of the nerves, excites the irritability of 

 the fibres ; and, in the second, the action is produced by the application of external objects, either directly 

 or by the medium of the nerves. The changes which take place in the tenacity of muscles after death 

 are very remarkable. The same force which they could resist with ease in a living state is sufficient to 

 tear them to pieces after the vital principle has departed. 



1894. The functions of the muscles are either those of rest or motion. Many animals 

 protect themselves against the disturbing movements of the air and water, by placing 

 their bodies in a prone posiiion. To give still greater efiicacy to this protecting attitude, 

 they retire to valleys, woods, or dens, on the earth, or to the deepest places in the waters ; 

 and are thus able, by the weight of their own bodies, and the advantage of their 

 position, to outlive the elemental war. But there are other animals, which, while they 

 are equally cautious to make choice of proper situations for their safety, employ in 

 addition, peculiar organs with which they are provided, to connect themselves more 

 securely with the basis on which they rest. 



1895. Grasping. The most simple of these expedients, grasping, is displayed by bats, birds, and insects, 

 in the employment of their toes and claws in seizing the objects of their support. In birds, the assumption 

 and continuance of this attitude is accomplished bva mechanical process; so that there is no expenditure 

 of muscular energy. In every case of this kind, the claws are so admirably adapted to the station of the 

 animal, that the detention of the body in the same spot, during this state of rest, is accompanied with little 

 exertion. 



1896. Suction. The sucker by which animals fix themselves varies greatly in its form, and even struc- 

 ture. In the limpet, and other gasteropodous MoUusca, its surface is smooth and uniform ; and the 

 adhesion appears to depend on its close application to every part of the opposing surface. In other 

 animals, as the leech and the sea-urchin, the sucker is formed at the extremity of a tube ; the muscular 

 motions of which may serve to pump out any air which may remain, after the organ has been applied to 

 the surface of the body. 



1897. Cementation. The cementation which is employed by animals to preserve themselves stationary, 

 consists in a part of their own bodies being cemented to the substance on which they rest. This takes 

 place in the common muscle, by means of strong cartilaginous filaments, termed the byssus, united in 

 the body to a secreting gland, furnished with powerful muscles, and, at the other extremity, glued to the 

 rock or other body to which it connects itself. In other cases, as in the oyster, the shell itself is cemented 

 to the rock. 



1898. The muscular motions of animals are standing, walking, leaping, flying, and 

 swimming. 



1899. In standing it is necessary that the parts of the body be so disposed, as that the centre of gravity 

 of the whole body fall within the space which they occupy, and that the muscles have sufficient power to 

 counteract those movements which might displace the body from that position. It is obvious that the 

 more numerous the limbs, and the more equally they are distributed on the inferior side of the body, the 

 more securely will the centre of gravity be retained within the space which these feet include. 



1900. Walking is defined by Cuvier to be a motion on a fixed surface, in which the centre of gravity is 

 alternately moved by one part of the extremities, and sustained by the other, the body never being at 

 any time completely suspended over the ground. It is produced by the alternate flexion and extension 

 of the limbs, aided by the motions of the trunk, advancing the position of the centre of gravity in the 

 intended direction. 



