132 HISTORY OF THE HUMAN BODY 



type of articulation is retained. In birds even this restricted 

 motion is undesirable except in neck and tail, owing to the, use 

 of the entire body as an air-ship which must be held in a rigid 

 position, and the trunk-vertebrse ossify into two completely 

 anchylosed pieces, the first including the thoracic, and the sec- 

 ond the lumbar, sacral, and a part of the caudal, vertebrae. 



The head is responsible for many modifications of the verte- 

 bral column, developed in part in response to the necessity of 

 turning it in all directions, and in part to the need of lifting 

 it from the ground, or even sustaining it above the level of the 

 rest of the body. Like many others, these problems are as- 

 sociated with a terrestrial environment and are not experienced 

 by fishes, in which the main endeavor is to retain the head in 

 a rigid state as the direct anterior extension of the body axis, 

 since a pliant head would render a change of direction while 

 swimming of almost momentary occurrence and would entirely 

 forbid those quick, arrow-like propulsions upon which most 

 fishes depend for safety and for the successful pursuit of their 

 prey. In the first experience of a terrestrial life, however, all 

 this becomes changed. The turning of the head not only gives 

 an increased power of observation, but is necessary in attack 

 and defense, and thus the vertebrae lying between the skull 

 and the place of support for the anterior limbs become differ- 

 entiated to form a cervical or neck region, the main en- 

 deavor of which is to gain flexibility and increase the mobility 

 of the head. 



Although in some of the higher terrestrial' vertebrates this 

 power is but little used, in others it develops to an extraordi- 

 nary extent, notably among the birds, in which this is the 

 only part of the vertebral column, except the tail, to which 

 motion is allowed. Here, in some instances, the neck not only 

 becomes extremely flexible, but greatly elongated, accompanied 

 by extraordinary modifications of the trachea and the blood- 

 vessels, in order to accommodate themselves to the rapid 

 changes of shape and position of which the neck becomes 

 capable. 



On the other hand, certain mammals, like the whales and 



