Cervical Rihs in Man. 89 



In this paper it will be assumed that the evolution of the ueck 

 is controlled by impulses that are generated by the reaction of the 

 tissues to external e-hcmical and physical conditions. These im- 

 pulses determine chanoe and stability in animal tissues and under 

 their influence it will be shown that tixed-type may be destroyed and 

 new tissues arise. 



The Evolution of the Neck. 



Mammalian nei'k evolution may be studied in 'lie Sr/nidpjjcr, Ctu- 

 todus, Trarhyidnrns riit/oxii'^, Variniu.< no-'titx^ CrocodilKi, FhilDiins.^ 

 Echidna and .Uammn/ia. 



In uumy fish dorsal and ventral ribs occur ; the former extend 

 between the nuiscles and the latter stiffen the walls of the body- 

 cavity. Some seem to use the words dorsal and ventral to the parts 

 of a mammalian rib, the rib proper l)einu termed the dorsal rib and 

 the costal cartilage the ventral ; this use is still more commonly 

 adopted in desiribing' avian ribs. There also seems to be an ill- 

 defined tendency on the part of some writers to assign to vertebrae 

 the power of laying- down ribs. As all the bones of the body are 

 formed in response to impulses it follows that vertebrae and ribs 

 come alike under the influence of the impulse. As the body requires 

 ncAv bone, fresh centres of ossification undoubtedly develop ; and, as 

 movement becomes necessary, joints appear. 



In the schnapper and haddock the heart occupies a position in the 

 gill-area, and ribs are in the area that ultimately in reptiles, birds 

 and mammals becomes the neck. Fish are neckless, a state which 

 is marked by ribs extending to the head. Fish are propelled by the 

 thrust of the tail and this force is transmitted through the body 

 mainly by the vertebrae, but the ribs serve to stiffen the body and 

 thus to prevent loss of propelling power. 



The neck is formed l)y the passage of the pectoral girdle, which 

 strips the neck-area of its libs ; or. to l)e more exact, the passage of 

 the girdle is associated with the denudation nf the neck of its rib 

 tissue. From the fact that fish are neckless, and that limbed verte- 

 brates have necks, it is apparent that the evolution of the neck 

 becomes necessary as the limbs are evolved. A neck is of no use to 

 a fish and a formed neck in marine animals has the effect of diminish- 

 ing speed and making steering difficult. As the limbs were evolved 

 they descended the neck, and as they descended they became larger 

 and stronger, and as the limbs became larger and stronger the neck 

 proportionately developed. Ihe pectoral girdle provided the means 

 wherein' the fore-limbs were carried down the neck, and Avith them 

 the heart, from bhe gill-area to the thorax, these migrations becoming 

 necessary as more perfect land progression was essential to the 



