CHAPTER XXXIV. 



CENTRAL NERVOUS ORGANS. 



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 

 together 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- 



,. . , , Transverse section of nerve 



ous ganglion cells interspersed between fibres> 8howing the axis cylindera 

 them in various quantities and relation- cut across, and looking like dots 



i . surrounded by a clear zone, which 



is the medullary sheath. Fine 

 In the brain the gray Substance is connective tissue separates the 



distributed chiefly on the surface, < into bundle., 

 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 connection with the 

 roots of the spinal nerves, divide the white substance of the cord 

 into separate columns. 



As already pointed out, the nerve fibres are simply conducting 



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