COMPARATIVE ANATOMY. 



the hind legs must be very insecure, as 

 the thigh and leg form an angle, instead 

 of continuing in a straight line. 



The glutens 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 great weight 

 of the head, with the comparative weak- 

 ness of the extensor muscles, and the 

 want of ligamentum nuchae, would render 

 the elevation of that organ almost impos- 

 sible. 



When quadrupeds endeavour to sup- 

 port themselves on the hind extremities ; 

 as, for instance, 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 tore part of the body is small, 

 as in the simia;, 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 me body, as in the kangu- 

 rooand the jerboa. 



Various gradations may be observed in 

 the mammalia, connecting man to those 

 animals which are strictly quadrupds. 

 The simix, 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 scapulae, 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 move 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 violentmotions 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 th;. 

 wing. 



