678 



NERVOUS SYSTEM. (NERVOUS CENTRES. THE ENCEPHALON.) 



The interval between the processus cerebelli 

 ad testes is occupied by a horizontal stratum of 

 nervous matter composed of a thin layer of grey 

 and of white matter. This is called the valve of 

 Vieussens, although there is evidently nothing 

 valvular in its nature or office. Its surface is 

 marked by slight transverse depressions and 

 eminences. The median lobe of the cerebel- 

 lum overlaps and conceals it from view. 



The valve of Vieussens* must be regarded 

 as a portion of the median lobe of the cerebel- 

 lum, which is extended forwards between the 

 processus cerebelli ad testes. Its constitu- 

 tion is precisely the same as the laminae of that 

 body, and the transverse markings upon its 

 superior surface are indications of imperfectly 

 developed fissures between the laminae. 



The corpora quadrigemina form the anterior 

 superior part of the mesocephale. They lie above 

 the crura cerebri, upon those columns of nervous 

 matter by which the latter bodies are connected 

 with the medulla oblongata. These columns 

 are continuous above with the optic thalami, 

 and below with the central portion of the me- 

 dulla oblongata, the olivary tracts, or fasciculi 

 innominati of Cruveilhier. They are distin- 

 guished by their reddish grey colour and their 

 close resemblance in point of structure to the 

 optic thalami. In transverse section they appear 

 as two columns, circular in outline, quite dis- 

 tinct from the surrounding greyish matter in 

 which they seem imbedded (jig. 388, i). 



The lower half of the thickness of the me- 

 socephale is formed by transverse curved fibres 

 with anterior convexity, which extend between 

 the lateral lobes of the cerebellum, and of 

 longitudinal fibres which interlace with the 

 superior layers of those transverse fibres and cross 

 them at right angles. The former constitute 

 the pons Varolii, a great commissure between 

 the hemispheres of the cerebellum ; the latter 

 are, in greater part at least, the fibres of the 

 anterior pyramids of the medulla oblongata, 

 which ascend through the pons, and enter into 

 the formation of the inferior layer of each crus 

 cerebri. 



In examining the inferior surface of the me- 

 socephale, the pons Varolii, we observe that a 

 longitudinal groove extends along its middle 

 from above downwards. In this lies the ba- 

 silar artery. Above the anterior edge of the 

 pons, the crura cerebri are seen emerging, and 

 diverging from each other as they pass, to enter, 

 stalk-like, into the inferior surface of the cere- 

 bral hemispheres. Beneath its posterior edge, 

 the medulla oblongata is seen, its anterior and 



- Valnda cerebri major is the name which Vi- 

 eussens applied to this process. He describes it as 

 " membrana quam transversus medullaris tractus 

 circa anteriora subit, processui vermiformi anteriori, 

 processibus a cerebello ad testes et posticse pontis 

 Varolii parti adhan-et etunitur." He further adds, 

 " illam valvubc vices gerere asserimus. Ex quo fit, 

 ut habitfi officii et inagnitudinis illius ratione, 

 ipsam valvulam cerebri majorem nominemus, ut 

 earn a membranaceis liganientis distinguamus, qua; 

 intra longitudinalis et latcraliura sinuum cavitates 

 valvularum minorum vices supplent et munia prae- 

 stant." Newrogra/phia Univer salts, p. 7b'. Ed. Lugd. 

 1716. 



middle columns passing through the mesoce- 

 phale to the crura cerebri. On each side the 

 fibres of the pons pass off into each hemi- 

 sphere of the cerebellum and form the inferior 

 lamina of each crus of that organ. 



The cerebellum. Some account of the ge- 

 neral disposition of the cerebellum will serve 

 to conclude this brief review of the topography 

 of the brain. The superior surface of this 

 organ is a little above the level of the qua- 

 drigeminal bodies. It is smooth and slightly 

 convex. The lamella? of the cerebellum are 

 visible upon it, but cannot be separated with- 

 out removing the arachnoid and pia mater. 

 A notch is seen, dividing the posterior edge 

 into two equal portions, and a larger notch 

 exists in front, at which the cerebellum forms 

 its connection with the mesocephale. These 

 notches denote a subdivision of the organ into 

 two lateral portions, or hemispheres, and a me- 

 dian portion. The superior surface of the 

 median portion is called the superior ver- 

 miform process; its anterior terminal laminae 

 form the valve of Vieussens. On the inferior 

 surface the hemispheres of the cerebellum are 

 much more convex than on the superior. The 

 median portion too is somewhat differently 

 arranged on its inferior surface ; it consists of a 

 series of laminae, following a transverse direc- 

 tion ; those in its centre are of greater trans- 

 verse extent than those at either extremity, 

 whence the appearance of a crucial figure re- 

 sults. This is the inferior vermiform process. 

 The posterior margin of the cerebellum is 

 convex, and corresponds to the concave surface 

 of the occipital bone, the falx cerebelli occupy- 

 ing the notch in its middle. Along the line of 

 this margin, the pia mater sinks into a deep 

 fissure, which takes a horizontal direction from 

 behind forwards, and divides the cerebellum 

 into a superior and inferior portion. 



As the brain, removed from the cranium, 

 lies with its base upwards, the medulla ob- 

 longata is seen between the lateral hemispheres 

 of the cerebellum occupying a portion of the 

 depression between them, in which is the in- 

 ferior vermiform process (fig. 382). 



The fourth ventricle is a lozenge-shaped 

 cavity situated in the upper and posterior part 

 of the medulla oblongata, and formed by the 

 separation of its postero-lateral columns (cor- 

 pora restiformia). The cerebellum contributes 

 to inclose it above by means of the anterior 

 laminae of the superior vermiform process and 

 the valve of Vieussens, and below and behind 

 by the inferior vermiform process (Jig. 386). 



We now proceed to the examination of the 

 various segments of the encephalon, with a 

 more special reference to the structure and 

 physiological bearing of each. It may be here 

 remarked that, while all the segments are in- 

 timately connected with each other and are 

 therefore mutually dependent, there is much 

 in their structure to justify the assumption that 

 each is capable of exercising an independent 

 function, which is, however, liable to be mo- 

 dified by the influence which any one, or all of 

 the other segments may have upon it. 



Or THE MEDULLA OBLONGATA. (Fl\ 



