406 THE CENTRAL NERVOUS SYSTEM. 



VII. THE CENTRAL NERVOUS SYSTEM. 



IN a study of the minute anatomy of the central nervous system 

 consideration should be given to the arrangement of the nerve-cells 

 and nerve-fibers in the various regions, and to the mutual relations 

 which the elements of the nervous system bear to one another. In 

 a text-book of this scope, however, we shall be unable to enter into 

 the consideration of these subjects in detail, but must content our- 

 selves with a very general discussion of the structure of certain 

 regions of the central nervous system and an account of a few typical 

 examples illustrating the mutual relationship of the nerve-elements 

 to one another. We shall, therefore, give a general description of 

 the structure of the spinal cord, cerebellum, cerebrum, olfactory 

 lobes, and ganglia. In this description we have drawn freely 

 from the results of the researches of Golgi (94), Ramon y Cajal 

 (93, i), von Lenhossek (9^),. Kolliker (93), and van Gehuchten 

 (96). ' 



A. THE SPINAL CORD. 



The spinal cord extends from the upper border of the atlas to 

 about the lower border of the first lumbar vertebra. It has the form 

 of a cylindric column, which at its lower end becomes quite abruptly 

 smaller, to form the conns medullaris, and terminates in an attenu- 

 ated portion the filum terminate. It presents two fusiform enlarge- 

 ments, known as the cervical and lumbar enlargements respectively. 

 The spinal cord is partly divided into two symmetric halves by an 

 anterio r median fissure and by a septum of connective tissue, extend- 

 ing into the substance of the cord from the pia mater (one of the 

 fibrous tissue membranes surrounding the cord), and known as the 

 posterior median septum. Structurally considered, the spinal cord 

 consists of white matter (mainly medullated nerve-fibers) and gray 

 matter (mainly nerve-cells and medullated nerve-fibers). The white 

 and the gray matter present essentially the same general features at 

 all levels of the spinal cord, although the relative proportion of the 

 two substances varies somewhat at different levels. The different 

 portions of the cord present also certain structural peculiarities. 



The distribution of the gray and the white substances of the 

 spinal cord is best seen in transverse sections. 



The varying shape of the spinal cord in the several regions and 

 the changing relations of the gray to the white substance are shown 

 in the illustrations of cross-sections of the adult human spinal 

 cord (see p. 407). 



The gray substance is arranged in the form of two crescents, 

 one in each half of the cord, united by a median portion extending 

 from one half of the cord to the other, the whole presenting some- 

 what the form of an H. The horizontal part contains the commis- 



