178 



SPECIAL PHYSIOLOGY. 



In the Anthropoid apes, the centre of gravity is placed higher up in the 

 trunk than in man, owing to the comparative shortness of the hinder limbs, 

 and the greater proportionate length of the anterior ones, as well as to the 

 forward inclination of the trunk. In the lower Quadrumana, the centre of 

 gravit}' advances still further forwards, approaching its normal position in the 

 quadruped. 



In the Quadruped mode of standing and locomotion, the intrinsic power is, 

 as usual, obtained by muscular force, exerted upon movable levers, having 

 their points of support and resistance upon the ground. Their progressive 

 motion is compounded of the results of muscular force and gravity. Owing 

 to the near approximation, in length, between the fore and hind limbs, and to 

 the larger development and greater length of the neck and facial part of the 

 head, to which the prehensile functions are now transferred, the centre of 

 gravity of the whole body is advanced forwards, and is placed somewhere about 

 the middle of the thorax, a little behind the junction of the fore-limbs with 

 the trunk. It is mainly in consequence of this forward position of the chief 

 mass of the body, that is, of its centre of gravity, that a quadruped animal 

 experiences such difficulty in the act of rearing, and cannot long maintain that 

 position. The size of the muscles of the hinder limb, the nature of its joints, 

 the angular position of its segments, and the narrowness of its base of sup- 

 port at its extremity, also occasion this difficulty ; whilst the mechanism of 

 the vertebral column", the absence of that gradual increase in size of the bodies 

 of the vertebrae from the neck to the lumbar region, the comparative small 

 amount of intervertebral substance, and other peculiarities, such as the ab- 

 sence of the triple vertebral curve, and the relative size of the pelvic and spinal 

 muscles, display a want of adaptation for the erect posture, and a fitness of 

 the trunk for the horizontal attitude. The position of the head at the end of 

 the neck, and the development of the dorsal and cervical spinous processes, also 

 show an express adaptation to this horizontal position. The trunk of the 

 body, especially its thoracic part, is now compressed from side to side ; and 

 the anterior limbs are attached near together on the under side of the trunk, 

 the weight of the fore part of which is thus more easily supported. The 

 hinder limbs are moderately thrown out above at the pelvis, but are also inclined 

 towards each other where they reach the ground, so as to bring the base of 

 support there also under the weight to be carried. 



In standing, in the quadruped position, owing to the forward situation of 

 the centre of gravity, a greater amount of weight is carried on the anterior 

 than on the posterior limbs, the weight not being equally divided between them, 

 as might, without consideration, appear to be the case. In the most perfect 

 forms of quadruped progression, as, for example, in the active Ruminants 

 (stags, antelopes, &c.), and in the Soliped Pachydermata (horse, zebra, donkey), 

 the fore limbs are brought very near together under the body, there being no col- 

 lar-bone to thrust them outwards from the sternum ; owing to the absence of this 

 bone, moreover, the fore limbs are attached to the trunk by muscles only. The 

 chief muscle is called the great serratus (represented, in the human body, at 5, 

 Fig. 4), which, arising by many heads from most of the ribs low down, passes 

 upwards, in the quadruped, towards the scapula, into which it is inserted ; by 

 this arrangement on the two sides, the trunk of the animal may be said to be 

 suspended in a sort of muscular sling, composed of the two large serrati 

 muscles, which spring from the thorax, and are attached to the scapula, or 

 upper segment of each anterior limb. The posterior limbs, on the other hand, 

 are connected by articulations with the pelvis, and the pelvis similarly with 

 the vertebral column ; so that here the weight of the body is transmitted 

 directly from bone to bone. These differences between the modes of connec- 

 tion of the fore and hind limbs, have reference to the offices of those limbs in 

 progression. The hinder limbs, with their segments inclined more or less at 

 angles to each other, and provided with powerful muscles for unfolding those 

 angles, and so straightening and increasing the length of the limbs, are con- 

 cerned mainly in giving the forward and upward impulse to the body of the 

 animal over the ground. The anterior limbs, besides assisting in the progres- 

 sive movements, have also, and chiefly, to receive and check the downward 

 course of the centre of gravity lying in the anterior part of the body ; the 



