72 ELEMENTARY ANATOMY. [LESS. 



there may be two distinct series of ribs on each side of the 

 body, one series being dor'sally situate with regard to the 

 other. This condition is found in many Fishes, as e.g. the 

 Tunny and Polypterus^ and in the latter some vertebras have 

 four ribs on each side springing from the doubled transverse 

 process before noticed (Fig. 52). 



36. That mode of DEVELOPMENT of the vertebral column 

 which we have seen to take place in man, takes place also, 

 broadly speaking, in all Vertebrates ; only the process is 

 arrested at different stages in different forms. Thus the 

 notochord may, as we have seen, persist, or the ossification 

 of the vertebras break off at various stages, leaving a great 

 deal or only a rudiment of the notochord persisting. 



The process of consolidation and union may proceed only 

 so far as to leave transverse processes distinct, e.g. the Carp, 

 or neurapophyses in two or four pieces, and separate from the 

 neural spines, as before noted. 



Finally, not only may vertebrae be found denser than those 

 of man, as the vertebras of Serpents, but coalescence may 

 extend to adjacent vertebras in the several ways already 

 described. 



Our survey shows us that the backbone aloce, without the 

 help of any limb, may serve as an organ for creeping over 

 the ground or swimming through water, for climbing trees, 

 for crushing prey as in the Boa Constrictor, and even as a 

 hand to present food to the mouth as also in the Boa, or to 

 grasp and bring near small detached objects, as is done by 

 the tail of the Spider Monkey. 



FIG. 8 1. SKELETON OF HEAD AND GILLS OF LAMPREY. 



&, cartilaginous basket ; n, neural laminae. (For the parts of the skull sec 

 Lesson III.) 



An exceptional structure which probably belongs to the same 

 skeletal category as the ribs and sternum may here be 

 mentioned. This is the cartilaginous "basket" which supports 

 the gills in the Lamprey. It consists of arcs of cartilage 



