482 STRUCTURE OF VERTEBRATA. 
A vertebra generally consists of sevzral 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 « 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, z.e. without digits. 
Distinct from the other bones are a few little sesamoids 
of occasional occurrence, ¢.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; (0) the peri- 
