438 STRUCTURE OF VERTEBRATA. 



portion, with the articulation for the limb between them, 

 and of a forward growing clavicle or collar-bone. 



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 composed 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- 

 pheral system, consisting of spinal and cranial nerves ; and 

 (c) the sympathetic nervous system. 



The central nervous system first appears as a superficial 

 groove along the mid-dorsal line of the embryo. The sides 

 of this ectodermic groove meet, and, uniting, convert the 

 medullary groove into the medullary canal. The greater 

 part of this canal forms the spinal cord ; the anterior portion 

 of it is specialised as the brain. There is at first a posterior 

 connection between the neural canal and the primitive gut 

 of the embryo ; when this is lost the cavity still persists as a 

 little ciliated canal in the centre of the cord, and as the in- 

 ternal cavity of the brain. In Cyclostomes and Bony Fishes 

 the central nervous system arises as a solid cord of cells, 

 the cavities not appearing until a later stage ; this condition 

 does not seem to be primitive. 



Brain. — At an early stage, even before the closing-in 

 process is completed, certain portions of the anterior region 

 of the medullary canal grow more rapidly than others, and 

 form the three primary brain vesicles. By further processes 



