466 STRUCTURE OF VERTEBRATA. 



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 

 (<r) 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 

 internal cavity of the brain. 



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 

 of growth and constriction, these three form the five regions 

 of the adult brain. 



When first formed the brain vesicles lie in a straight line, but as a 

 consequence, probably, of their rapid and unequal growth, this condition 

 is soon lost, and a marked cranial flexure is produced. In the lower 

 forms, e.g. Cyclostomata, the flexure is slight, and is corrected later, 

 but in the higher types it is very distinct, and causes the marked over- 

 lapping of parts so obvious in the adult. 



We must now follow the metamorphosis of the primary 

 brain vesicles. 



The first vesicle gives rise anteriorly to the cerebral hemi- 

 spheres, while the remainder forms the region of the optic 

 thalami or thalamencephalon. 



The cerebral hemispheres (prosencephalon or fore-brain) 

 are exceedingly important. They predominate more and 



