482 STRUCTURE OF VERTEBRATA. 



A vertebra generally consists of several more or less 

 independent parts: the substantial centrum; the neural 

 arches which form a tube for the spinal cord, and are 

 crowned by a neural spine ; the transverse processes which 

 project laterally, and the articular processes. 



The ribs which support the body wall usually articulate 

 with the transverse processes, or with the transverse pro- 

 cesses and centra. 



Amphibians are the first to show a breast-bone or sternum. It 

 arises from two cartilaginous rods in a tendinous region on the ventral 

 wall of the thorax. The sternum of some Reptiles, and of all Birds 

 and Mammals, arises from a cartilaginous tract uniting the ventral 

 ends of a number of ribs. 



Limbs and girdles. The pectoral girdle consists of a 

 dorsal scapula, a ventral coracoid, and a forward growing 

 membrane-bone, the clavicle or collar-bone. 



According to Broom, frogs and some primitive Reptiles show a 

 coracoid and a pre-coracoid ; lizards and birds only a pre-coracoid ; 

 the Monotremes a coracoid and a pre-coracoid ; other mammals a 

 coracoid only. 



The pelvic or hip girdle consists of a dorsal iliac portion, 

 a ventral posterior ischiac portion, with the articulation for 

 the limb between them, and of a ventral, usually anterior, 

 pubic portion. 



The fore limb from Amphibians onwards consists of a 

 humerus articulating with the girdle, a lower arm composed 

 of radius and ulna lying side by side, a wrist or carpus of 

 several elements, a " hand " with metacarpal bones in the 

 "palm," and with fingers composed of several phalanges. 



The hind limb from Amphibians onwards consists 

 of a femur articulating with the girdle, a lower leg com- 

 posed of a tibia and fibula lying side by side, an " ankle " 

 region or tarsus of several elements, a foot with metatarsal 

 bones in the "sole," and with toes composed of several 

 phalanges. 



In Fishes the limbs are fins, i.e. without digits. 



Distinct from the other bones are a few little sesamoids 

 of occasional occurrence, e.g. the knee-pan or patella. They 

 develop in connection with the tendons of muscles. 



Nervous system. This includes (a) the central nervous 

 system, consisting of brain and spinal cord; (b) the peri- 



