NKRVOUS SYSTEM. 317 



Centrum Ovale. — In the dissection of the brain I shall pursue 

 the customary method of the schools of anatomy; and in de- 

 scribing its several parts, preserve the names used in human phre- 

 nology so long as I find them applicable to correspondent ap- 

 pearances. The first step consists in making a clean horizontal 

 section of the brain, parallel to its present convexity, by laying 

 the knife flat upon the surface of the corpus callosum, and making 

 a sweeping cut (With its edge first inclined a little upward, after- 

 wards a little downward) through one hemisphere at a time. 

 There being no distinguishable parts in the abscised portions, 

 they may be cast away ; but the surface which their removal has 

 exposed being one that presents the greatest superficial extent of 

 medullary substance that can be shewn at one view, the appear- 

 ance altogether has been described as the centrum ovale* . The 

 form and disposition of the corpus callosum become now de- 

 monstrable. It is placed in the centre of this medullary con- 

 vexity, with which it is continuous in substance. It is narrower 

 below than above. It unites the medulla of the two hemispheres 

 in the middle. Inferiorly, it passes between the hemispheres, 

 turns backward and terminates at the base of the brain, in the 

 crura cerebri ; superiorly, it joins the fornix, and spreads upon 

 the hippocampi ; and anteriorly, it may be said to form the roof 

 of the lateral ventricles. Running along its surface are seen two 

 prominent lines of medullary matter, between which exists a lon- 

 gitudinal linear depression, named the raphe. 



Lateral Ventricles. — By making a longitudinal incision through 

 the corpus callosum, on either side of the raphe, we penetrate a 

 cavity occupying the innermost part of the brain, which, with its 

 fellow within the opposite hemisphere, constitute the two lateral 

 ventricles. Having freely opened it, we find a watery fluid 

 within, which serves to keep its parietal surfaces constantly wet, 

 and to prevent (it is said) adhesion of them. The figure of the 

 ventricle, both from its construction and from the inequalities of 

 its contents, to which it is adapted, is extremely irregular. Its 

 cavity extends as far forward as the front incurvation of the 

 corpus callosum, from whence a winding canal is continued for- 

 ward and downward to the base of the anterior lobe of the cere- 

 brum, where it ends in a small circular conduit that leads into 

 the ethmoidal ventricle : this canal, from its being represented to 



* The cortical matter, however, cannot be all pared away — it will intrude 

 in places, and interrupt the uniformity of the surface : indeed, the horse's 

 brain will less admit of being carved into such an aspect, in consequence 

 of the proportionably greater quantity of cortical substance in its com- 

 position. 



