THE GRAY MATTER. 601 



pears no less adapted to act as a middleman between parts of the spinal cord 

 below and various regions of the brain above. As we shall see, experiment 

 and observation give support to these suggestions. 



THE DISPOSITION AND CONNECTIONS OF THE GRAY AND WHITE 

 MATTER OF THE BRAIN. 



THE GRAY MATTER. 



526. As we pass up from the bulb to the higher parts of the brain, the 

 differentiation of the gray matter into more or less separate masses, which 

 we have seen begin in the bulb, becomes still more striking. We have to 

 distinguish a large number of areas or collections of gray matter, more or 

 less regular in form and more or less sharply defined, from the surrounding 

 white matter ; to such collections the several terms corpus, locus, nucleus, 

 and the like, have from time to time been given. These areas or collections 

 vary greatly in size, in form, and in histological characters ; they differ from 

 each other in the form, size, features, and arrangement of the nerve-cells, in 

 the characters of the nervous network of which the nerve-cells form a part, 

 and especially perhaps in the extent to which the more distinctly gray mat- 

 ter is traversed and broken up by bundles of white fibres. Guided by the 

 analogy of the spinal cord, as well as by the results of experiments and 

 observations directed to the brain itself, we are led to believe that the com- 

 plex functions of the brain are intimately associated with this gray matter ; 

 and a full knowledge of the working of the brain will carry with it a know- 

 ledge of the nature and meaning of the intricate arrangement of the cerebral 

 gray matter. At present, however, our ignorance as to these things is great ; 

 and, though various theoretical classifications of the several collections of 

 gray matter have been proposed, it will perhaps be wisest to content our- 

 selves here with a very broad and simple arrangement. We will divide the 

 whole gray matter of the brain into four categories only : 1. The central 

 gray matter lining the neural canal ; and this we may consider the nuclei 

 of the cranial nerves, some of which are closely associated with it. 2. The 

 superficial gray matter of the roof of some of the main divisions of the brain, 

 such as that of the cerebral hemispheres, and of the cerebellum. 3. The 

 intermediate gray matter more or less closely connected with the crura 

 cerebri. 4. Other collections and areas of gray matter. We will, more- 

 over, confine ourselves at present for the most part to their general features 

 and topography, reserving what we have to say concerning their histological 

 characters for another occasion. 



1. The Central Gray Matter and the Nuclei of the Cranial Nerves. 



527. The ventricles of the brain, like the central canal of the spinal 

 cord, of which they are a continuation, are lined by an epithelium which is 

 in general a single layer of columnar cells said to be ciliated throughout, 

 though it is often difficult to demonstrate the cilia. Beneath this epithelium 

 lies a layer of somewhat peculiar neuroglia, forming with the epithelium, as 

 we have said ( 523), the ependyma, which, well developed in the floor of the 

 fourth ventricle and in the walls of the third ventricle and of the aqueduct, 

 is thin and scanty in the lateral ventricles. Beneath, and more or less con- 

 nected with the ependyma in the sides and floor of the third ventricle, is a 

 fairly conspicuous layer of gray matter, which is well developed in the parts 

 of the floor exposed on the ventral surface of the brain, and known as the 

 lamina terminalis, the anterior and posterior perforated spaces, the tuber 



