THE GRAY MATTER. 615 



Lastly, the auditory nerve, both from its character as a nerve of special 

 sense and from the remarkable features of its nuclei, is even more difficult. 

 Most probably it results from the fusion of more roots than one ; but it is 

 impossible at present to obtain a clear conception of the nature of the whole 

 nerve. 



2. The Superficial Gray Matter. 



538. The whole of the surface of each cerebral hemisphere for some 

 little depth inward consists of gray matter, possessing special characters; 

 this is called the cortical gray matter, or the cortex cerebri, or shortly and 

 simply the cortex. As we shall see, by its histological and still more by its 

 physiological features, it stands apart from all other kinds of gray matter. 



The whole of the surface of the cerebellum is also covered with gray 

 matter, which, while possessing features of its own, so far resemble the cere- 

 bral cortex, in its histological characters that it too has been spoken of as 

 cortex, as the cortex cerebelli. By its functional manifestations, however, 

 it differs widely from the cerebral cortex ; and since there are many advan- 

 tages in being able to use the word cortex in connection with the cerebrum 

 only, it is desirable not to speak of a cerebellar cortex but to employ the 

 term " superficial gray matter of the cerebellum." 



The third ventricle and the hinder part of the fourth ventricle are not 

 roofed in by nervous material, and possess no superficial gray matter at all. 

 In the corpora quadrigemina, which form the roof of the aqueduct or cavity 

 of the mid-brain, gray matter is present and possesses, in the case of the 

 anterior corpora quadrigemina at least, characters to a certain extent analo- 

 gous to those of the cortex and to the cerebellar superficial gray matter ; 

 but it will be best to consider the gray matter of these bodies as belonging 

 to another category. 



3. The Intermediate Gray Matter of the Crural System. 



539. We have seen ( 516) that the crura cerebri form the promi- 

 nent part of a system of longitudinal fibres stretching from each cerebral 

 hemisphere to the bulb and to the spinal cord. This system of fibres, upon 

 which we may consider the various parts of the brain to be, as it were, 

 founded, we may speak of as the crural system. It is, it is true, not one 

 continuous strand, but a number of different strands, having different begin- 

 nings and endings ; but these all contribute to the crura and are so far alike 

 as to justify us in considering them as a system. The cortical gray matter 

 of each hemisphere is, as we shall see, connected with various parts of this 

 system, and in one sense we may regard this system as beginning in the cor- 

 tex of each hemisphere, and ending in the spinal cord. But certain masses 

 of gray matter in the hemisphere not strictly cortical, and several important 

 masses and areas of gray matter lying between the hemisphere and the cord, 

 are connected with the system ; and these we may speak of as the " inter- 

 mediate gray matter of the crural system." 



Corpus striatum and optic thalamus. Of all these several collections of 

 gray matter, the largest, most conspicuous, and perhaps the most important 

 are the two masses in the front part of the system known as the corpus stria- 

 tum and optic thalamus. The former is, as we have seen ( 515), a develop- 

 ment of the wall of the cerebral vesicle, the latter a development of the wall 

 of the vesicle of the third ventricle. They are, therefore, of different origin ; 

 although in the course of the growth of the brain they become closely attached 

 to each other, they are at the outset quite separate and distinct. Moreover, 

 as we shall see, they differ from each other so essentially, in their nature and 

 relations, that they cannot be considered as homologous bodies ; and the term 



