The gluteus maximus, which is the largest muscle of the human body, is so small and insignificant in animals, that it may almost be said not to exist. This muscle, which forms the great bulk of the human buttock, extends the pelvis on the thighs in standing ; and, assisted by the other two glutei, maintains that part in a state of equilibrium on the lower extre mity which rests on the ground, while the other is carried forwards in progression. The true office of these important mus cles does not therefore consist, as it is usually represented, in the common ana tomical works, in moving the thigh on the pelvis, but in that of fixing the pelvis on the thighs, and of maintaining it in the erect position.
Such then are the supports, by which the trunk of the human body is firmly maintainedin the erect position. The pro perties of the trunk, which contribute to the same end, do not so immediately belong to this article ; but may be slightly mentioned, to complete the view of the subject. The breadth of the human pelvis affords a firm basis on which all the supe rior parts rest securely ; the same part is so narrow, in other animals, that the trunk represents an inverted pyramid, and there must consequently be great difficulty in maintaining it in a state of equilibrium„if it were possible for the animal to assume the erect position. In those instances where the pelvis is broader, the other con ditions of the upright stature are absent : the bear, however, forms an exception to this observation, and consequently admits of being taught to stand and walk erect, although the posture is manifestly incon venient and irksome to the animal.
The perpendicular position of the ver tebral column under the centre of the basis cranii, and the direction of the eyes and mouth forwards, would be as incon venient to man; if he went on all-fours, as they are well adapted to his erect stature. In the former case, he would not be able to look before him ; and the greatmeight of the head, with the comparative weak ness of the extensor muscles, and the want of ligamentum nuchm, would render the elevation of that organ almost impos sible.
When quadrupeds endeavour to sup port themselves on the hind extremities ; as, for for the purpose of seiz ing any objects with the fore feet, they rather sit down than assume the erect po sition. For they rest on the thighs as well as on the feet, and this can only be done where the fore part of the body is small, as in the simix, the squirrel, &c. : in other cases, the animal is obliged also to sup port itself by the fore feet, as in the dog, cat, &c. The large and strong tail in some instances forms as it were a third foot, and thereby increases the surface for supporting the body, as in the kangu roo and the jerboa.
Various gradations may be observed in the mammalia, connecting man to those animals which are strictly quadrupds. The simim, which are by no means calculated for the erect position, are not, on the other hand, destined, like the pro per quadrupeds, to go on all•fours. They live in trees, where their front and hind extremities are both employed in climb ing, &c.
The true quadrupeds have the front of the trunk supported by the anterior ex tremities, which are consequently much larger and stronger than in man ; as the hind feet of the same animals yield in these respects to those of the human sub ject. The chest is in a manner suspend
ed between the scapula, and the serrati magni muscles, which support it in this position, are consequently of great bulk and strength. When viewed together, they represent a kind of girth surround ing the chest.
The chief agents in flying are the muscles, which more the anterior extre mities of the bird, and which constitute what in common language is termed the breast of the animal.
Birds possess three pectoral muscles, arising chiefly from their enormous ster num, and acting on the head of the hu merus. The first, or great pectoral, weighs of itself more than all the other muscles of the bird together. The keel of the sternum, the fork, and the last ribs, give origin to it ; and it is inserted in a rough projecting line of the hume rus. By depressing that bone, it produces the strong and violent motions of the wing, which carry the body forwards in flying. The middle pectoral lies under this, and sends its tendon over the junction of the fork, with the clavicle and scapula, as in a pulley, to be inserted in the upper part of the humerus, which bone it elevates. By this contrivance of the pulley, the elevator of the wing is placed at the un der surface of the body. The third, or lesser pectoral muscle, has the same effect with the great pectoral, in depressing the wing.
One of the flexor tendons of the toes of birds, (produced from a muscle which comes from the pubis) runs in front of the knee ; and all these tendons go behind the heel : hence the flexion of the knee and heel produces mechanically a bent state of the toes, which may be seen in the dead bird ; and it is by means of this structure that the bird is supported, when roosting, without any muscular action.
This circumstance of the flexion of the toes accompanying that of the other joints of the lower extremity of birds was long ago observed by Borelli, and attributed by him to the connection which the flex ors of the toes have with the upper parts of the limb, by which they are mechani cally stretched when the knee is bent. This explanation has been controverted by Vicq d'Azyr, and others, who have referred the effect to the irritability of the muscles. The opinion of Borelli ap pears, notwithstanding, to be well found ed ; for not only the tendon of the ac cessory flexor passing round the knee, but the course of the flexor tendons over the heel, and along the metatarsus, must necessarily cause the contraction of the toes when either of these joints is bent ; and if the phenomenon was not produced on mechanic principles, it would be im possible for birds to exhibit it during sleep, which they do, or to prove the ef fect on the limb of a dead bird, than which nothing is more easy. The utility of this contrivance is great in all birds, but particularly so in the rapacious tribe, which, by this means, grasp their prey in the very act of pouncing on it ; and it is still more necessary to those birds which perch or roost during their sleep, as they could not otherwise preserve their position, when all their voluntary powers are suspended.