NERVOUS SYSTEM. (NERVOUS CENTRES. THE ENCEPHALON.) 



686 



these bodies in considerable quantity. The 

 pia mater which adheres to their surface 

 abounds in minute bloodvessels, and in sepa- 

 rating it these are seen to penetrate the tuber- 

 cles in vast numbers. This layer of pia mater 

 contributes to form the velum interpositum. 



The quadrigeminal bodies are the analogues 

 of the optic lobes in birds, reptiles, and fishes. 

 In these classes there is only a single pair of 

 tubercles. They are of considerable size in 

 birds, and form a conspicuous portion of their 

 encephalon. The division into four takes place 

 only in Mammalia. The anterior are the larger 

 in herbivorous animals, the posterior in the 

 Carnivora. In most quadrupeds these bodies 

 are concealed from view by the posterior lobes 

 of the brain ; but in Rodentia they are exposed 

 in consequence of the imperfect developement 

 of the brain in the backward direction. 



The quadrigeminal bodies rest upon two 

 processes of fibrous matter, which extend 

 backwards to the median lobe of the cere- 

 bellum, and forwards to the optic thalami. 

 These processes form a connection between 

 the thalami and the quadrigeminal bodies and 

 the cerebellum. They have been variously 

 designated processus cerebelli ad testes, proces- 

 sus cerebelli ad corpora quadrigemina, processus 

 cerebelli ad cerebrum. 



The valve of Vieussens intervenes between 

 these processes, and closes the fourth ventricle 

 at its upper part. 



A longitudinal groove separates the right and 

 left pair of quadrigeminal bodies. The ante- 

 rior extremity of this groove forms an expanded 

 and somewhat flattened surface on which rests 

 the pineal gland (jig. 386, S). From the pos- 

 terior extremity a small band extends to the 

 valve of Vieussens, called frenum. An inci- 

 sion made vertically downwards along the 

 course of this groove exposes the canal through 

 which the fourth ventricle and the third com- 

 municate (Her a tertio ad quartum ventricu- 

 lum). This canal communicates with the pos- 

 terior part of the third ventricle by an opening 

 which is situate beneath the posterior commis- 

 sure, and with the superior extremity of the 

 fourth ventricle beneath the valve of Vieussens. 



The fourth pair of nerves are seen upon this 

 surface, attaching themselves to the processus 

 cerebelli ad testes, or to the Vieussenian valve, 

 or to the posterior pair of quadrigeminal bodies. 



Besides the anterior pyramids, the olivary 

 columns are continued through the mesoce- 

 phale to form with the former the crura cerebri. 

 These columns are exposed along the floor of 

 the fourth ventricle ; higher up, however, they are 

 surrounded by a lightish grey matter, form the 

 superior stratum of each crus cerebri, separated 

 from the quadrigeminal tubercles by the pro- 

 cessus cerebelli, and finally merge into the 

 optic thalami. Their course is well displayed 

 in Jig. 380, where J represents the olivary 

 columns, t the processus cerebelli ad testes, 

 and v the pons penetrated by p, the pyramids. 



The olivary columns retain their greyish hue 

 in their upward course. Their cylindrical form 

 is very apparent on the floor of the fourth ven- 

 tricle; but it is still more obvious on viewing 



a transverse section, when each olivary column 

 appears as a cylinder, to be distinguished 

 from the rest by its roundness and its peculiar 

 colour. 



No other mode of dissection conveys so 

 much knowledge of the anatomy of this part 

 as a transverse section, carried from above 

 downwards through either pair of quadrige- 

 minal bodies, and inclined a little backwards, 

 so as to pass through the pons. The parts 

 which may be observed on such a section, 

 enumerated from above downwards, are, 1, 

 either pair of quadrigeminal tubercles; 2, 

 between and beneath them, the Her cut across ; 

 3, on either side of this, fibrqus matter ; 4, 

 below this on each side, the section of each 

 olivary column ; 5, planes of transverse fibres 

 interlacing with longitudinal ones, and grey 

 matter between the planes ; 6, transverse fibres 

 forming the pons Varolii. 



Fig. 388. 



Plan of a transverse vertical section of the meso- 

 cephale anterior to the putts, passing tliruuyh the 

 cms cerebri. 



p, iter a tertio ad quartum ventriculum. This is sur- 

 mounted by a pair of the quadrigeminal tubercles. 



z i, olivary columns. 



n, locus niger. 



a, the inferior plane of fibres diverging upwards, 

 which are continuous with the anterior pyramids. 



From the preceding description of the me- 

 socephale it may be concluded that two classes 

 of elements enter into its formation. These 

 are intrinsic and extrinsic. The former con- 

 sists in the masses of vesicular matter, with 

 which the fibrous matter, whatever be its course, 

 is intimately connected. Such are the grey 

 matter of the quadrigeminal bodies ; that light 

 grey matter which surrounds the olivary co- 

 lumns in their upward course ; the darker 

 matter which intervenes between the transverse 

 fibrous lamellte; and more in front, that which 

 forms the locus niger of the crus cerebri. 



The extrinsic elements are those which pass 

 through this segment, being continuous with 

 some portion of a neighbouring segment, or 

 serving to connect the grey matter of the me- 

 socephale with the hemispheres of the cere- 

 brum or cerebellum, or with the medulla ob- 

 longata. The fibres which form the inferior 

 laver of the pons are perhaps the only element 

 that does not connect itself in some way with 

 the grey matter of the mesocephale, since they 

 seem simply to pass across from one crus cere- 

 belli to the other. The deeper transverse fibres, 

 the pyramids, the olivary columns, the pro- 



