300 



niL inti:rnal environment of the body 



Mesencephoion 

 Prosencephalon Rhombencephalon 



Part III 



A. 



Telenceph. 



/^ a b c ^=5aaEiiiD 



>■ -^^^ ■"^-^-^ ^^^ 



I 



Three parts 



B. 



C. 



Myelenceph. 



Optic 

 nerves 



Cerebral 

 hemisphere 

 (Telencephalon) 



Optic nerves 



Five main parts 



D. 



Outline of 

 lenceph. 



Mesenceph. 



Cerebellum 

 (Metenceph.) 



Medulla 

 (Myelenceph.) 



L Her\ie cord 



Fig. 16.17. Diagrams to illustrate some of the changes that transform the 

 smooth bulb at the anterior end of the neural tube into a highly complex brain. 

 A and B, Median vertical sections of brains showing the changes that occur in 

 given regions, e.g., the telencephalon, as they appear in certain primitive chordates 

 and (C) certain vertebrates. D, Outlines of the brain seen in a human embryo of 

 about 11 weeks' development, 50-60 mm length, about two inches. {D redrawn 

 and modified from Patten: Human Embryology, ed. 2. New York, The Blakiston 

 Co., 1953.) 



During its development the nervous system is at first on the outside of the 

 body. At one period in the life of every vertebrate, fish or human, there is an 

 open groove that extends the whole length of the central nervous system. Later 

 when the system takes its place inside the body, only the central canal in the 

 cord and the communicating ventricles of the brain are left of the former 

 open ditch. 



The working parts of the brain are composed of many types of neurons 

 associated in groups for general and specialized functions. Thus the human 

 brain is divided into districts occupied by neurons that control definite parts 



