434 STRUCTURE OF VERTEBRATA. 



scapular portion or shoulder blade, a ventral coracoid 

 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 ischiac portion, with the articulation for the limb 

 between them, and of a pubic region possibly homologous 

 with the clavicle. 



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. 



The fin-like limbs peculiar to Fishes, are discussed along 

 with the other characteristics of that class. 



Distinct from the other bones are a few little sesamoids of 

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

 develop in the tendons of muscles. 



Nervous System. 



This includes {a) the central nervous system, consisting 

 of brain and spinal cord ; {h) the peripheral, 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. 



