THE NERVOUS SYSTEM 



The nervous system controls most of the activities of the 

 rest of the organism. It receives through the sense organs 

 the stimuli from the organs of the body and the external 

 world, transmits them as nervous impulses, coordinates 

 them, and originates impulses which it carries to the mus- 

 cles, glands, etc., stimulating these structures to action. It 

 comprises the following parts: (1) central nervous system 

 (brain and spinal cord), (2) peripheral nervous system 

 (spinal and cranial nerves, and the sympathetic nervous 

 system), and (3) the sense organs. 



THE CENTRAL NERVOUS SYSTEM 



The early embryonic central nervous system of verte- 

 brates is a comparatively simple tube (neural tube). The 

 anterior portion develops into the brain, the remainder 

 into the spinal cord. The embryonic brain at first com- 

 prises three divisions — in front the prosencephalon, then 

 the mesencephalon, and posteriorly the rhombencephalon. 

 The prosencephalon further differentiates into the telen- 

 cephalon (which includes the cerebral hemispheres) and the 

 diencephalon. The mesencephalon forms dorsally the op^ 

 tic lobes (corpora qMadrigemina) , while the rhomben- 

 cephalon is transformed into the metencephalon (cerebel- 

 lum and pons) anteriorly and the myelencephulon (me- 

 dulla oblongata) posteriorly. The cavities of the brain are 

 derivatives of the lumen of the neural tube. 



The brain fills the cranial cavity. It has already been 

 adequately preserved by chipping away a part of the roof 



103 



