THE FROG 71 



XIII. THE NERVOUS SYSTEM. 



A. General Anatomy.- -The nervous system is composed 

 of two great divisions, the central and the peripheral. 

 The central nervous system includes the brain and the spinal 

 cord; the peripheral system includes all the nervous struc- 

 tures outside the central system : the cranial, spinal and sym- 

 pathetic nerves, and various small ganglia. The system of 

 nerves and ganglia which supplies the viscera is sometimes 

 considered separately as the sympathetic nervous system, 

 and in distinction from it all the rest of the nervous system 

 is called the cerebro-spinal nervous system. 



i. The central nervous system. The method of mak- 

 ing the following dissection will be demonstrated at each 

 table, but each student should do the work on his own speci- 

 men. Expose the brain and spinal cord as follows : Remove 

 the skin from the dorsal surface of the body, and dissect 

 away the muscles which cover the vertebrae ; then cut through 

 the tough membrane roofing over that portion of the brain 

 between the skull and the first vertebra, taking care not to 

 injure the brain. Through the opening thus made introduce 

 one blade of a pair of strong scissors and cut away the roof 

 of the cranium. This is best done by making a forward in- 

 cision through the roof of the cranium on each side of the 

 brain, then with forceps removing the piece between these 

 two cuts. Working backward from the same starting-point, 

 in like manner remove the roof of the spinal canal, in which 

 the cord lies, by cutting through the neural arches of the 

 vertebrae. 



Observe that the cord possesses two slight enlargements, 

 the anterior or brachial and the posterior or lumbar en- 



