I L] THE SPINAL SKELE TON. 43 



become distinctly solidified. Inasmuch, however, as man's 

 dorsal vertebrae form a series the first of which bears ribs 

 which join a sternum, man agrees with all Vertebrates (ex- 

 cept Serpents) above the Ichthyopsida ; but he differs from 

 the whole of the last-named vast group, as in that group 

 there is either no sternum or else no ribs which join it. 



In their number the dorsal vertebrae of man are a little 

 below the average of his class, in which the number may 

 be augmented to twenty, as in the Elephant, or doubled (i.e. 

 twenty-four) as in the Two-toed Sloth, or on the other hand 

 reduced to ten, as in Azara's Armadillo. Comparing the 

 condition existing in man with that in the Non-Mammalian 

 Vertebrates above Fishes, we find his number to be smaller 

 than that in most Reptiles, but somewhat greater than in 

 Birds, where, on account of the prodigiously enlarged sacrum, 

 but three (e.g. Ciconia alba], seven or nine, or, very rarely, 

 eleven vertebrae are reckoned as dorsal. 



In the main proportions of the centrum (the greater 

 height in proportion to breadth, smaller medullary canal, 

 elongated spinous processes, the articular process, short 

 transverse processes, and considerable lateral notches), these 

 vertebrae in man agree with thpse of other Mammals. 



Often, however, the spinous processes may be very much 

 more prolonged, as in the Ungulata (e.g. the Ox and the Horse), 

 or they may be swollen at their summits, and more or less 

 anchylosed together, as sometimes in the true Opossums. On 

 the other hand, they are sometimes almost or quite absent, 

 as in Bats. 



FIG. 56. DIAGRAM OF A SECTION OF SHELL OF A TORTOISE MADE TRANS- 

 VERSELY TO THE LONG AXIS OF THE SKELETON. 



ns, neural spine ; r, rib ; ic and hp, ventral plates not belonging to the true axial 

 skeleton. 



The most remarkable modification of dorsal vertebras is 

 that in Tortoises and Turtles, where the neural spines ex- 

 pand at their summits into wide plates which articulate by 



