28 THE VERTEBRAL COLUMN. [CHAP. 



they have two transverse processes, superior and inferior, 

 which, meeting at their extremities, enclose a canal. (See 

 Fig. 7 ; Fig. 8, p. 33 ; and Figs. 17 and 18, p. 45, 46.) This, 

 however, rarely applies to the last vertebra of the re- 

 gion, in which only the upper transverse process is usually 

 developed. 



The transverse process moreover very often sends down 

 near its extremity a more or less compressed plate (inferior 

 lamella, Fig. 7, /'), which being considered to be serially homo- 

 logus 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 vertebrae, 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 particularised further on. 



2. The Thoracic or Dorsal region consists of the vertebrae 

 which succeed those of the neck, having ribs movably 

 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 



