III.] DIVISION INTO REGIONS. 23 



logous with the ribs of the thoracic vertebrae (though not 

 developed autogenously) is often called " costal " or " pleura- 

 pophysial " plate. This is usually largest on the sixth, and 

 altogether wanting on the seventh vertebra. 



The first and second cervical vertebrce, called respectively 

 atlas and axis, are specially modified for the function of 

 supporting, and permitting the free movements of the head. 

 They are not united together by an "intervertebral sub- 

 stance," but connected only by ordinary ligaments and 

 synovial joints. 



The cervical region in Mammals presents the remarkable 

 peculiarity that, whatever the length or flexibility of the 

 neck, the number of vertebrae is the same, viz. seven, with 

 very few exceptions, which will be particularized fur- 

 ther on. 



2. The Thoracic or Dorsal itgxon consists of the vertebrae 

 which succeed those of the neck, having ribs moveably 

 articulated to them. These ribs arch round the thorax, the 

 anterior one, and most usually some of the others, being 

 attached below to the sternum. 



The characters of the ribs and their mode of articulation 

 with the vertebrae will be considered further on, but it may 

 now be stated that in the anterior part of the thorax the 

 vertebral extremity of each rib is divided into " head " and 

 " tubercle ;" that the former is attached to the side of the 

 body of the vertebra, the latter to its transverse process ; 

 and that the former (capitular) attachment corresponds to 

 the interspace between the vertebrae, the head of the rib 

 commonly articulating pardy with the hinder edge of the 

 body of the vertebra antecedent to that which bears its 

 tubercle. Hence the body of the last cervical vertebra 

 usually supports part of the head of the first rib. In the 

 posterior part of the series the capitular and tubercular 



