CHAP, ii.] THE BRAIN. 953 



connected with the crura cerebri. 4. Other collections and areas 

 of grey matter. We will, moreover, 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 Grey Matter, and the Nuclei of the Cranial 



Nerves. 



614. 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 ( 610), 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 connected with the ependyma in the 

 sides and floor of the third ventricle, is a fairly conspicuous layer 

 of grey 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 cinereum &c. This layer is not continued forwards into 

 the lateral ventricles of the cerebral hemispheres, but it is well- 

 developed backwards along the aqueduct (Figs. 113, 114) and in 

 the floor of the fourth ventricle, and through the bulb becomes, 

 as we have seen ( 610), continuous with the central grey matter ^ 

 of the cord. The nerve cells of this grey matter are on the whole 

 small and in many places scant. 



615. The several roots of the cranial nerves from the third 

 nerve backwards may be traced within the brain substance to 

 special collections of grey matter, called the nuclei of the cranial 

 nerves, some of which lie close upon the central grey matter, 

 while others are placed at some distance from it. The optic 

 nerve and what is sometimes called the olfactory nerve, namely, 

 the olfactory bulb and tract, may advantageously be dealt with 

 apart, since these two nerves are not, like the other cranial 

 nerves, simple outgrowths from the walls of the original neural 

 canal, but are in reality elongated vesicles, budded off from the 

 neural canal, the cavities of which have been obliterated. We ( 

 may add that part of the retina, and of the grey matter of the 

 olfactory tract, may perhaps be considered as corresponding to the 

 nuclei of which we are speaking, the retinal and proper olfactory 

 fibres being connected with them very much as the fibres of the 

 remaining cranial nerves are connected with their respective nuclei. 



