134 HISTORY OF THE HUMAN BODY 



within the skull, leaving the axis with its pivot to serve as 

 the first free vertebra. This bone secondarily acquires artic- 

 ular surfaces to articulate with the lateral condyles. 



In animals whose limbs are strong enough to sustain the 

 body above the ground the weight of the head and the 

 -necessity of holding it in place beyond the anterior center 

 of support causes considerable modification of the vertebrae, 

 the influences sometimes reaching beyond the middle of the 

 body. In these cases the head is held up in part by muscles, 

 but in mammals there is also an important auxiliary appa- 

 ratus in the form of a strong ligament, the ligamentum 

 nuchce, which extends between the occipital region of the skull 

 and the spinous processes of the cervical and dorsal vertebrae 

 on a principle similiar to that of a check rein. 



In mammals with large and heavy heads, especially when 

 the weight is augmented by voluminous horns or large tusks, 

 the weight sustained by this ligament becomes enormous, and 

 not only is the ligament developed in proportion, but so, also, 

 are the occipital crests and the spinal processes of the 

 vertebrae, which serve it as points of attachment, the pro- 

 cesses especially involved being those of the anterior dorsal 

 region opposite the shoulders. This correlation between a 

 heavy head and exaggerated spinous processes is such that 

 from a slight indication of the one in a fossil the other may 

 be assumed. In the Cetacea, which have the buoyancy of 

 the water to assist them, and in Man, through the erect po- 

 sition of whom the head becomes almost perfectly balanced 

 upon the summit of the vertebral column, this entire appara- 

 tus, including the ligament and its points of attachment, be- 

 comes much reduced, but from a totally different cause in the 

 two instances. 



The tail, or post-sacral region of the vertebral column, is 

 developed strictly in correlation with the needs of the animal 

 and varies in development from a voluminous portion of the 

 body, containing a large number of vertebrae and furnished 

 with metameric muscles, to a mere rudiment, invisible ex- 



