

XXII] CERVICAL VERTEBRAE 565 



in any Fish or Amphibian. The first vertebra of the neck which is 

 termed the atlas is formed in a different way to the rest : it has the 

 form of a ring and it is constituted by the union of the first pair of 

 basiventrals with the first pair of basidorsals. The first ventral 

 intercalary forms a cylindrical centrum to which no neural arch is 

 attached, but which becomes attached to the next centrum as the 

 so-called odontoid process; this projects forwards through the 

 ring formed by the atlas. The vertebra bearing the odontoid 

 process is termed the axis vertebra, and all the rest of the 

 vertebrae of the neck are termed cervical vertebrae (Fig. 278). 

 There is great reason to believe that these vertebrae, like the neck 

 region generally, have been formed by a process of intercalary 

 growth : because of their peculiar relations to the chain of sympa- 

 thetic ganglia. In primitive Vertebrates there is one sympathetic 

 ganglion attached to each spinal nerve, but in animals with a neck 

 there are only two cervical sympathetic ganglia : an anterior one (the 

 so-called superior cervical ganglion) connected with the first cervical 

 spinal nerve, and a posterior one (the so-called inferior cervical 

 ganglion) connected with the last cervical nerve. The intervening 

 region of the neck is totally devoid of sympathetic ganglia, and 

 seems therefore to be a new formation. 



The brain of Amniota has twelve, not ten pairs of cranial 

 nerves, i.e. two additional pairs to the ten found in Pisces and 

 Amphibia. The second of these two pairs corresponds to the 

 hypoglossal nerve of the Frog, which in that animal emerges behind 

 the first vertebra. It follows that the cranium of Amniota must 

 have at least two more vertebrae amalgamated with it than are 

 comprised in the cranium of Amphibia. In all Amniota the cranium 

 is ossified in its hinder region by four bones, viz., the supra-occipital 

 bone, an exoccipital at each side and a basi-occipital below. In no 

 Amphibian are basi-occipital and supra-occipital elements developed. 

 In the buccal cavity we have shelf-like outgrowths from the sides 

 the palatal flaps which divide the cavity more or less completely 

 into an air passage above and a food passage below. In the pelvic 

 girdle, in addition to ilium and ischium, there is a third bone below 

 and in front termed the pubis. In the heart the conus has 

 completely disappeared as a division of the heart. It is cleft to the 

 base into "aortic arches," of which one conveys impure blood to 

 the lungs. In the ventricle there is a flap-like outgrowth from the 

 ventral wall which divides its cavity completely or incompletely into 

 two compartments. 



