G74 



NERVOUS SYSTEM. (NERVOUS CENTRES. THE ENCEPHALON.) 



wards, and a little backwards. As the brain 

 rests on the upper surface of its hemispheres 

 with its base upwards, the medulla oblongata 

 is seen to occupy a notch or depression be- 

 tween the hemispheres of the cerebellum. The 

 fibres of the pons Varolii are seen passing out- 

 wards and backwards into each hemisphere of 

 the cerebellum, forming the inferior layer of 

 each crus cerebelli. On each side of the me- 

 dulla oblongata is the inferior convex surface 

 of each hemisphere of the cerebellum marked 

 by its fissures and laminae. The basilar artery 

 passes in a groove along the middle of the pons 

 from before backwards. The fifth nerve emerges 

 from the crus cerebelli, the sixth immediately 

 below the posterior margin of the pons, and 

 the seventh, eighth, and ninth nerves are seen 

 springing from each side of the medulla by a 

 series of fascicles similar to those which form 

 the roots of the spinal nerves. 



Of the dissection of the brain from above 

 downwards. It will facilitate our subsequent 

 descriptions, if, previous to examining the se- 

 veral segments of the encephalon in detail, 

 I give a rapid sketch of the dissection of the 

 brain according to the topographical method, 

 proceeding from above downwards. 



This dissection is commenced by making a 

 horizontal section of one hemisphere, a little 

 above the level of the corpus callosum. The 

 surface, which is thus exposed, has in shape 

 the character of a demi-oval. It is chiefly com- 

 posed of white substance, which occupies the 

 centre of the space, bounded by a wavy border 

 of grey matter. Anatomists designate it cen- 

 trum ovale minus. We find this is a convenient 

 section on which to study the anatomy of the 

 convolutions, and to give some idea of the 

 composition of that portion of the hemispheres 

 of the brain which is situate above the ventricles. 

 On making a similar section of the other hemi- 

 sphere at the same level, a similar surface 

 is exposed, and the conjunction of both con- 

 stitutes what Vieussens denominated the cen- 

 trum ovale majus. 



By separating the hemispheres slightly, after 

 this section, the horizontal portion of the cor- 

 pus callosum is well displayed. The con- 

 tinuity of its transverse fibres with the white 

 substance of the hemispheres may be traced ; 

 and by following its anterior and posterior 

 reflections they will be found to connect the 

 hemispheres at their inferior as well as their 

 superior parts. The corpus callosum, when 

 examined in its full extent, exhibits somewhat 

 of a vaulted shape, and is found to enter 

 largely into the formation of the roof of the 

 lateral ventricles. 



\\'e notice some remarkable longitudinal 

 fibres, passing along the middle of the corpus 

 callosum, varying greatly in developement in 

 different brains. These consist of two bundles 

 placed in juxta-position, but easily separable. 

 We may trace them throughout the whole length 

 of the corpus callosum. They cut the trans- 

 verse fibres at right angles, and may be readily 

 dissected up from them. They seem to tie the 

 transverse fibres together, and are probably com- 

 missural. They form what has been improperly 



called the raphc of the corpus callosum, more 

 correctly the longitudinal tracts (Vicq d'Azyr). 



By scraping away the white substance, on 

 each side of the corpus callosum, the lateral 

 ventricles may be opened. If this be done 

 with great care, a considerable portion of the 

 membrane that lines the interior of each ven- 

 tricle may be exposed, but such is its great 

 delicacy that a very slight force ruptures it. 

 When there is fluid in the ventricles, this 

 membrane may be more easily demonstrated 

 from its floating upon the fluid. The place at 

 which the ventricles may be most certainly 

 opened without the risk of injuring any of the 

 parts contained within them, is about the eighth 

 of an inch external to the blending of the fibres 

 of the corpus callosum with the white substance 

 of the centrum ovale. With the handle of a knife 

 the fibrous matter which forms the roof of the 

 ventricle may be torn through in the antero-pos- 

 terior direction, and the cavity thereby exposed. 



Each lateral ventricle consists of a horizontal 

 and a descending portion. The former re- 

 sembles in shape an inverted italic S. Its an- 

 terior extremity, or cornu, is directed outwards ; 

 the posterior turns inwards towards that of the 

 opposite side. The descending cornu passes 

 downwards, forwards, and inwards in a curved 

 course with the concavity forwards and inwards, 

 and terminates at the fissure of Sylvius. The 

 first has been appropriately designated the 

 frontal ventricle, the second the occipital, and 

 the third the sphenoidal, from their relations to 

 the bones after which they have been respec- 

 tively named. The posterior cornu is also named 

 the digital, or anci/roid cavity. 



The anterior cornua of the lateral ventricles 

 are separated from each other by a vertical sep- 

 tum situated on the median plane, very thin 

 and transparent, the septum lucidum. This 

 may be easily demonstrated on a vertical sec- 

 tion of the brain, made a little to one side 

 of the mesial plane, or if both lateral ven- 

 tricles have been opened, by supporting the 

 corpus callosnm on each side with the handle 

 of a knife, by which means the septum is 

 stretched, and its extent and connections may 

 be more readily determined. The septum is 

 of a triangular form with curvilinear base, which 

 is directed forwards, and fits into the anterior 

 reflection of the corpus callosum. Posteriorly 

 it fits in between the corpus callosum and 

 the anterior extremity of the horizontal portion 

 of the fornix. 



The septum lucidum, although so extremely 

 delicate and transparent, is very obviously com- 

 posed of two layers, which enclose a space or 

 cavity called the fifth ventricle. This may be 

 shewn by dividing the septum horizontally from 

 behind forwards. Each of these lamiiiEe con- 

 sists, as may be easily observed by examining 

 the margin of the section, of four layers : the 

 outer one is derived from the lining membrane 

 of the ventricles ; immediately within this is a 

 layer of a pale grayish matter continuous with 

 a similar layer which covers the optic thalami 

 and the internal surface of the third ventricle, 

 consisting of clear nucleus-like particles homo- 

 geneous in texture; a third layer is composed 



