44 A COURSE ON ZOOLOGY. 



The various parts of the nervous system are usually 

 classified in two groups : 



First. The cerebro-spinal system, which includes the 

 brain, the spinal cord, and the nerves which emanate 

 from different points of the system and are distributed 

 to the different organs. 



Second. The great sympathetic system, which is com- 

 posed of series of small masses or ganglions connected 

 together by a large number of nervous ramifications, 

 one series being located on each side of the body. 



The Cerebro-spinal System. The cerebro-spinal system 

 comprises a central mass, the brain, a long, central ap- 

 pendage, called the spinal cord, and a chain of nerves, all 

 of which have their origin in the brain or in the spinal 

 cord. 



The brain is the collection of organs contained within 

 the cranium or skull : they are the cerebrum, the cere- 

 bellum, and the medulla oblongata. 



Three membranes cover the brain : beginning at the 

 outside, they are the dura mater, which is thick and 

 strong ; the arachnoid, so called because it resembles 

 a spider's web ; and the pia mater, which is full of 

 vessels. 



The cerebrum, by itself, occupies almost the entire 

 cranial cavity. Its average weight in man is somewhat 

 less than fifty ounces, .while the cerebellum weighs only 

 about six ounces, the medulla oblongata only about an 

 ounce, and the spinal cord an ounce. The cerebrum fills 

 the front and upper portion of the skull ; a very deep 

 fissure in the middle divides it into two hemispheres, 

 each of which is divided into several lobes, and contains 

 an interior cavity called a ventricle. The surface appears 

 made up of hills and valleys by folds called cerebral con- 



