CHAP. IL] THE BRAIN. 953 



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 epi- 

 thelium 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 pecu- 

 liar 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 ter- 

 minalis, 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 re- 

 maining cranial nerves are connected with their respective nuclei. 



