II.] THE SPIXAL SKELETON. 55 



and forms a conspicuous plate, as e.g. in the Ox and Dog ; 

 and even in the Dolphin, where the cervical vertebrae are 

 such thin plates of bone, the same root is suddenly enlarged. 



In no animal but man does the spine of this vertebra ever 

 bifurcate. 



The SEVENTH vertebra sometimes in man has its transverse 

 process imperforate, by the non-development of its capitular 

 root. This condition, abnormal in him, is normal but not 

 universal in the rest of his class. In other Mammals, as also 

 in man, a short free rib may be attached to the transverse 

 process of this vertebra ; and where, as in the Three-toed Sloth, 

 the number of vertebrae is nine, it is the eighth and ninth 

 vertebrae which resemble in structure the sixth and seventh of 

 man thus showing that in this animal it really is the cervical 

 vertebrae which are increased in number, and not that the 

 condition has been produced by the first two dorsal ribs 

 being imperfectly developed. 



20. The coalescence and degradation which characterize 

 the SACRAL VERTEBRA generally occur more or less in Verte- 

 brates above the Ichthyopsida, which possess fully developed 

 limbs. 



The coalescence is generally less extensive than in man, 

 though sometimes (as in Birds, some Edentates, and some 

 Reptiles) it is much greater. 



The sacrum of man is indeed a peculiar structure as regards 

 the coexistence of characters each of which, taken separately, 

 is shared by some or other members of his own class and order. 

 The characters of which the coexistence is peculiar to him 

 are : (i) The very marked sacro- vertebral angle. (2) The trans- 

 verse and axial concavity of the ventral surface of the sacrum. 

 (3) The concurrence of as many as five, or even six verte- 

 brae in its formation three of them generally contributing to 

 form the auricular surface. (4) Its relative breadth, and the 

 gradual way in which it narrows post-axially without any 

 sudden contraction. (5) The large size of the foramina. 

 (6) The small development of the spinous and other processes. 



This region may, however, be more exceptional and pecu- 

 liar than in man, as e.g. in the Rhea, where the vertebral inter- 

 space between the haunch bones is filled by a very narrow 

 continuous ossification without a trace of division into 

 vertebrae. 



The sacrum of man may be looked at from two points of 

 view, and it is very difficult to determine what parts really 

 answer to it in many animals. 



