CHAPTER X 

 DISEASES OF THE NERVOUS SYSTEM 



General Discussion. — The nervous system mav be divided 

 into central and peripheral portions. The central portion com- 

 prises the brain or encephalon and the spinal cord. These 

 organs are lodged in the cranial cavity and spinal canal. The 

 nerves and ganglia comprise the peripheral portion. The nerves 

 form white cords that are made up of nerve fibres. The ganglia 

 are grayish enlargements formed by nerve cells and supporting 

 tissue, situated at the origin of the nerve trunk or along its course. 



The brain is an oval mass of nerve tissue elongated from 

 before to behind, and slightly depressed from above to below. 

 It terminates posteriorly in the spinal cord. It is divided into 

 three portions: cerebrum, isthmus and cerebellum (Fig. 25). 



The cerebrum forms the anterior portion. It is divided into 

 two lateral lobes or hemispheres by a deep longitudinal fissure. 

 The surface of the cerebral hemispheres is gray and roughened by 

 pleats or folds separated by grooves or fissures. The gray or 

 cortical layer is distinct from the white or connecting structure. 

 The cortical layer is made up of nerve cells or areas which con- 

 trol the voluntary muscles of the body. It is connected with the 

 special senses of touch, temperature and muscle-sense. The 

 gray layer is connected with the posterior portion of the brain, 

 the isthmus or medulla oblongata, by the white nerve tissue. 



The isthmus or medulla oblongata is elongated from before 

 to behind and connects the cerebral hemispheres with the spinal 

 cord, anteriorly and posteriorly. It is divided into several 

 different portions, and is made up largely of white connecting 

 fibres with nuclei of gray matter scattered through them. The 

 isthmus is hollowed out by a system of small ventricles that 

 extend from the cerebral hemispheres to the spinal cord, where 



109 



