THE FROG AND VERTEBRATES IN GENERAL 179 



The vertebrae composing the vertebral column articulate with 

 each other by means of articulating processes, or zygapophy- 

 ses, which are developed both anteriorly and posteriorly on the 

 neural arches. The anterior pair of processes of each vertebra, 

 except the first, articulates with the posterior pair of the vertebra 

 just anterior. Thus the vertebral column is made up of a series 

 of definite articulating units, the vertebrae, each of which, by means 

 of the neural arch, contributes a portion of the common neural canal 

 containing the spinal cord. The vertebral column, while giving 

 firm axial support, also permits a considerable amount of bending. 

 The first vertebra possesses two concave articulating surfaces into 

 which the convex prominences of the occipital condyles at the pos- 

 terior end of the skull fit. 



The structure of the vertebral column in the various Vertebrates 

 is essentially the same as that just described for the Frog, but the 

 number of vertebrae present is subject to considerable variation. 

 In the Mammals, including Man, five regions are typically present, 

 which, beginning at the anterior end, are known as the cervical 

 (neck region), thoracic (chest region), lumbar (abdominal re- 

 gion), sacral (pelvic region), and caudal (tail region). In the 

 adult condition of Man there are 26 vertebrae in the vertebral 

 column divided as follows : 7 cervical, 12 thoracic with a pair of ribs 

 attached to each, 5 lumbar, 1 sacral, and 1 caudal. In early life 

 there are 33 vertebrae present. The reduction is due to a fusion 

 of 5 sacral vertebrae to form the sacrum of the adult, and a fusion 

 of the 4 caudal vertebrae to form the coccyx. In most Mam- 

 mals, except Man and the Anthropoid Apes, the number of caudal 

 vertebrae is quite large, thus in the Cat there are 22 caudal verte- 

 brae. (W. fs. 104-108.) 



II. APPENDICULAR SKELETON 



The appendicular skeleton of the Vertebrate, as typically found, 

 consists of the fore limbs and the hind limbs, both of which are 

 of the five-fingered, or pentadactyl type, and the girdles by 

 which they are connected with the axial skeleton. The fore limbs 

 are attached and supported by the shoulder, or pectoral girdle, 

 and by the breast bone, or sternum. The latter is the common 

 ventral uniting structure for the two halves of the girdle, and also 

 serves as a protection for various internal organs which lie in that 

 part of the body. Each half of the pectoral girdle is made up 



