172 SPECIAL PHYSIOLOGY. 



and diminish the effects of concussion, the lines of force passing out 

 through various parts of a curve, instead of being continued throughout 

 the whole length of a straight line. The several curves of the spine 

 are, moreover, so adjusted, that, in the erect posture, a perpendicular 

 line from the summit of the movable part of the vertebral column, 

 would fall through the centre of its base. 



The peculiar mode of adjustment of the head upon the. trunk, Fig. 

 12, also affords proof of the special adaptability of man to the erect 

 position ; for the foramen magnum of the occipital bone, is placed 

 farther forward in the base of the skull than in the vertebrate animals 

 generally, showing a special fitness for the support of the head upon 

 a vertical or upright vertebral column, instead of at the end of a verte- 

 bral column more or less inclined towards the horizon. In man, the 

 foramen magnum is placed a little behind the centre of the skull, 

 which is, therefore, not quite exactly balanced, but exhibits a slight 

 tendency to incline forwards, when muscular effort is relaxed from 

 fatigue, fainting, or sleep. The absence of prominent ridges on the 

 cranial surface, also indicates that its supporting muscles do not re- 

 quire the advantages of leverage, which they usually possess in ani- 

 mals ; the shortness of the cervical spines, and the want of a ligamen- 

 tum nuchae, are facts having a like bearing. The position of the mouth, 

 and of the several organs of the senses, especially of the eyes, in re- 

 lation to the wants of man, also demonstrates his fitness for the up- 

 right posture. 



Finally, the absolute unfitness of the upper limb, if examined from 

 its single point of bony support at the inner end of the collar-bone to 

 the tips of the fingers, whether considered as regards its position, 

 length, size, shape, or the structure of its several parts, for bearing 

 any share of the weight of the body, affords a negative argument in 

 favor of the intended erect attitude of man. 



Thus constructed and adapted for the erect position, the entire 

 body may be regarded as composed of five chief segments, the lowest 

 being formed by the arched foot, the next above that by the legs, and 

 the others in succession by the thighs, the trunk, and the head. Now, 

 the foot affords a base of support to the rest, and, when naked, can 

 grasp the ground by aid of the flexor muscles of the toes. The weight 

 is supported chiefly on three points, viz., the heel, and the anterior 

 ends, or heads, of the first and fifth metatarsal bones. In standing on 

 the toes, with the heel raised, the weight is borne on the ends of all 

 the metatarsal bones generally. Opera-dancers, however, by practice, 

 are able to sustain the weight on the end of the great toe. The next 

 segment of the body, the leg, is balanced on the foot, partly by the 

 extensor and flexor muscles of the leg (see Figs. 4, 5), in front and 

 behind, acting as opponents to each other, but, mainly, by an instinc- 

 tive adjustment of the centre of gravity over the vertical axis of the 

 tibia. The thigh is similarly balanced upon the leg, also by extensor 

 and flexor muscles ; and the pelvis and trunk upon the thigh, by very 

 numerous and large muscles, such as the glutseal, adductor, iliac, and 

 psoas muscles. The spine itself is kept erect bj the powerful erectores 

 spinse muscles, aided, however, by many others. Lastly, the head is 



