378 STRUCTURE OF VERTEBRATES. 



of a longitudinal series of segmentally arranged outgrowths, 

 perhaps comparable to the parapodia of an Annelid. 



The pectoral or shoulder girdle consists of a dorsal 

 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 the brain, the spinal 

 cord, and the cranial and spinal nerves. Brain and spinal 

 cord develop from the outer layer or ectoderm of the 

 embryo, gradually acquiring a less superficial position ; from 

 the central nervous system thus established, the nerves grow 

 outwards to the various parts of the body. 



More precisely, the central nervous system appears along 

 the dorsal median line of the embryo, as a superficial groove 

 of ectodermic cells. The sides of this medullary groove 

 meet, and a medullary canal is formed. The cavity of this 

 canal is connected posteriorly with the primitive gut of the 

 embryo, and persists after this connection is lost as a little 

 ciliated canal in the centre of the spinal cord, and as the 

 internal cavity of the brain. For the brain is but an anterior 

 expansion of the medullary canal. 



The brain.^kx. an early stage the brain exhibits three 



