CHAPTER XXXIY. 



FIG. 237. 



CENTEAL NERVOUS OEGANS. 



THE more important properties of the peripheral nerves and 

 their terminals have been discussed in the previous pages. The 

 central part of the nervous system, which remains to be consid- 

 ered, consists of the spinal marrow and the brain. These parts 

 are composed of a soft texture, the elements of which are held 

 ogether by a peculiar and very deli- 

 cate form of connective tissue, known 

 as Neuroglia. With the naked eye 

 the central nervous organs can be seen 

 to be made up of two distinct kinds 

 of substance : (1) a white substance, 

 which is found by the microscope to 

 be composed of nerve fibres, with a 

 medullary sheath, and (2) a gray sub- 

 stance, consisting of a dense feltwork 

 of naked axis cylinders, with numer- 

 ous ganglion cells interspersed be- 

 tween them in various quantities and 

 relationships. 



In the brain the gray substance is 

 distributed chiefly on the surface, 

 forming a kind of gray cortex, which 

 follows all the irregularities of the 

 convolutions. In the spinal cord the 



gray matter is situated inside, and the white outside. The gray 

 substance of the cord forms separate columns on either side, 

 which run its entire length, but is thicker in the cervical and 

 lumbar regions. These gray columns, together with their con- 

 nection with the roots of the spinal nerves, divide the white sub- 

 stance of the cord into separate columns. 



Transverse section of nerve 

 fibres, showing the axis cylin- 

 ders cut across, and looking 

 like dots surrounded by a clear 

 zone, which is the medullary 

 sheath. Fine connective tis- 

 sue separates the fibres into 

 bundles. 



