NERVOfS CENTRES. (HUMAN ANATOMY. THE ENCEPHALON.) 



C85 



pans Varolii ; and on its superior surface, which 

 looks backwards, are the corpora quadrigemina, 

 the processus cerebelli ad testes, and part of 

 the floor of the fourth ventricle (fig. 386). 



According to Chaussier, its weight is equal 

 to about the sixtieth or sixty-fifth part of the 

 entire brain. 



We shall describe separately the inferior and 

 the superior surfaces of this segment of the en- 

 cephalon, and its intimate structure as unfolded 

 by sections. , 



The inferior surface, (pans Varolii, annular 

 protuberance,) convex from side to side, is inter- 

 rupted along the median plane from behind for- 

 wards by a shallow groove in which the basilar 

 artery usually lies, giving off in its course nu- 

 merous minute capillaries to the nervous struc- 

 ture of the mesocephale. 



When the pia mater has been stripped off 

 this surface, it is seen to be very evidently 

 composed of a series of transverse fibres which 

 take an arched course. The fibres are collected 

 into large fascicles separated from each other 

 by very distinct intervals, so that there is no 

 part where the fibrous structure is more appa- 

 rent than here. They form arcs of circles, not 

 concentric, lying one behind the other in a se- 

 ries nearly parallel. Owing to this want of 

 complete parallelism the width of this surface 

 measured from before backwards is much less 

 at each extremity than in the centre. The an- 

 terior margin is convex, and forms a thick edge 

 crossing the crura cerebri like a bridge ; hence 

 the term pans was applied by Varolius to the 

 whole series of fibres. The posterior border is 

 concave, less curved than the anterior, and 

 crosses the anterior pyramids and olivary co- 

 lumns, as the latter does the crura cerebri. The 

 intervening fascicles of fibres become gradually 

 less curved as they approach the posterior 

 margin. 



These transverse fibres form a stratum of 

 considerable thickness at the inferior surface of 

 the mesocephale. Some grey matter is depo- 

 sited between the less superficial layers which 

 constitute it. The more deep-seated layers are 

 penetrated and crossed at right angles by the 

 ascending fibres of the anterior pyramids. A 

 remarkable interlacement takes place at this 

 situation between the vertical and transverse 

 fibres the latter passing alternately in front of 

 and behind adjacent bundles of the former. 

 Some of the vertical fibres seem to sink into 

 and connect themselves with the grey matter. 



A transverse vertical section of the meso- 

 cepliale gives a more complete view of the 

 exact extent of the transverse fibres. They are 

 found to occupy rather more than one-third of 

 the depth of the exposed surface. Their dispo- 

 sition in lamina? is very apparent. Those which 

 are nearest the centre of the mesocephale have 

 between them considerable intervals, which 

 are filled up by grey matter, through which 

 pass vertically the fibres of the pyramids. The 

 intervals between the laminae gradually dimi- 

 nish towards the inferior surface of the pons, 

 and the quantity of intervening grey matter 

 becomes proportionally less, and disappears 

 altogether from between those lamina; the in- 



tervals of which are not traversed by the fibres 

 of the pyramids. 



The transverse fibres pass on either side into 

 each hemisphere of the cerebellum, contributing 

 with the processus cerebelli ad testes and the 

 restiform bodies to form the crura cerebelli. 

 They are the inferior peduncles of these crura. 



The anatomy of these transverse fibres evi- 

 dently denotes that they serve to connect the 

 right and left cerebellar hemispheres, as com- 

 missures, and in a manner strikingly analogous 

 to that in which the fibres of the corpus callosum 

 connect the cerebral hemispheres. This view 

 of the office of these fibres is strongly confirmed 

 by the fact that their number is always in the 

 direct ratio of the size of the lateral hemispheres, 

 and that when the hemispheres are absent, these 

 fibres no longer exist. When, therefore, the 

 cerebellum consists only of a median lobe, there 

 is no pons Varolii. 



Some of the transverse fibres nearer the in- 

 ferior surface appear to dip in along the me- 

 dian line, and to pass upwards and backwards, 

 forming a vertical plane of fibres which divides 

 the mesocephale into two symmetrical portions, 

 and Chaussier imagined that a decussation 

 took place at this situation. The groove in 

 which the basilar artery lies is formed partly 

 by the greater condensation which is produced 

 along the median plane by this arrangement, 

 and partly by the slight bulging on either side 

 of it, caused by the ascent of the anterior py- 

 ramids. These fibres are continuous with a 

 series of similar ones in the medulla oblongata 

 ( untero-ptisterior fibres of Cruveilhier). 



The extent of the superior surface of the me- 

 socephale may be limited in front by a line 

 which passes from side to side just before the 

 anterior of the corpora quadrigemina, and pos- 

 teriorly by the base of the valve of Vieussens. 

 This occupies a much greater space than the 

 inferior surface. It is an inclined plane, and 

 passes downwards and backwards, being con- 

 cealed by the anterior lamina of the superior 

 vermiform process of the cerebellum and the 

 posterior border of the corpus callosum. 



The corpora or tubercula quadrigemina are 

 four rounded eminences gangliform bodies 

 disposed in pairs (fig. 386, D, D'). The ante- 

 rior pair are larger than the posterior. The for- 

 mer have been distinguished as the nates, tlie 

 latter the testes* These bodies are situate further 

 forwards than the pons,and are chiefly connected 

 with the superior surface of each crus cerebri. 



The nates are of a deeper grey colour than 

 the testes. In this respect they resemble the 

 optic thalami. Both pairs are similar in struc- 

 ture to those bodies. When cut into, they ap- 

 pear to consist of fibrous matter intermingled 

 with vesicular. Thin sections examined with 

 the microscope exhibit intricate interlacements 

 of tubular fibres with vesicular matter inter- 

 posed a true ganglionic structure. 



An important fact deserves special notice as 

 indicating that vesicular matter is found in 



* In reference to these absurd appellations Willis 

 has the following remark : " Prominentia oibicularis 

 quarum usus longe nobilior videtur, quam ut 

 viliora ista natium et testium noraina mercantur." 



