ii.] THE SPINAL SKELETON. 37 



transverse processes of some at least of the cervical vertebras 

 arise as distinct ossifications, as also do the lateral bony 

 pieces in the sacrum. 



In the axis vertebra not only do the transverse processes 

 arise as separate ossifications, but primitively both the body 

 and the odontoid process are distinct bones, and even an 

 epiphysis is formed between them, as well as below the cen- 

 trum and at the summit of the odontoid process. 



The anterior part of the ring of the atlas also arises as a 

 separate ossification. 



19. We will now pass on to the relations existing between 

 OTHER ANIMALS and man with regard to the spinal skeleton. 

 As most animals have their bodies horizontal, confusion in 

 descriptions is apt to arise from parts being " anterior " in 

 them which in man are " superior," and irice "versa. To avoid 

 this ambiguity, it will be well to imagine an axis drawn at 

 right angles to the general direction of the backbone. Then 

 all parts which in man are relatively superior, and in beasts 

 anterior, can be termed prc-axial in all cases ; and similarly, 

 parts relatively inferior in man, and in beasts posterior, can 

 be spoken of as post-axial : such terms referring not to the 

 long axis of the skeleton, but to the imaginary line drawn at 

 right angles to it. 



In that man's spine is made up of distinct and ossified 

 vertebras, man agrees with the vast majority of the members 

 of his sub-kingdom. Yet, in the class of Fishes, there are 

 many examples (as in the Sturgeon, Lepidosiren, and Lam- 

 prey) of the persistence throughout the whole of life, of the 

 notochord, or chorda dor satis, of the embryo. Moreover, 

 when th'e spine is fully ossified, and even in man's own class 

 (Mammalia) it may be that the greater number of the ver- 

 tebrae are anchylosed together into a solid bone, as in the 

 extinct Glyptodon. 



Instead of being connected as in man, the adjacent 

 vertebrae may be connected only by synovial sacs, as in 

 Snakes, or by intervertebral substances, perforated in the 

 middle ; and such sacs (as in Birds), or a large part of the 

 primitive notochord (as in most Fishes), may persist between 

 each pair of bony centra. 



That degree of union which exists among the different 

 parts of one vertebra in man, does not by any means obtain 

 in all cases. Thus ; in the extinct Ichthyosaurus the neural 



