662 THE NERVOUS SYSTEM. 



head of the caudate nucleus of the corpus striatuin. Its apex reaches the poste- 

 rior surface of the genu of the corpus callosum. 



The posterior cornu curves backward into the substance of the occipital lobe, 

 its direction being backward and outward, and then inward ; its concavity is there- 

 fore directed inward. Its roof is formed by the fibres of the corpus callosum 

 passing to the temporal and occipital lobes. On its inner wall is seen a longi- 

 tudinal eminence, which is in an involution of the ventricular wall produced by the 

 calcarine sulcus ; this is called the hippocampus minor, or calcar avis. Just above 

 this the forceps major of the corpus callosum, sweeping round to enter the occipital 

 lobe, causes another projection, which is known as the bulb of the posterior horn. 

 The hippocampus minor and bulb of the posterior horn are extremely variable in 

 their degree of development, being in some cases ill defined, while in others they 

 are unusually prominent. 



Between the middle and posterior cornu is a triangular area, called the trigo- 

 num ventriculi (see Descending Horn). 



The middle or descending cornu, the largest of the three, traverses the temporal 

 lobe of the brain, forming in its course a remarkable curve round the back of the 

 optic thalamus. It passes at first backward, outward, and downward, and then 

 curves round the crus cerebri, forward and inward, to within an inch of the apex 

 of the temporal lobe, its direction being fairly well indicated on the surface of the 

 brain by that of the parallel sulcus. Its upper boundary, or roof, is formed chiefly 

 by the under surface of the tapetum of the corpus callosum, but the tail of the 

 nucleus caudatus of the corpus striatum and the tsenia semicircularis are also 

 prolonged into it, and extend forward in the roof of the descending horn to its 

 extremity, where they end in a mass of gray matter, the amygdaloid nucleus; 

 this nucleus is merely a localized thickening of the adjacent gray cortex. Its 

 lower boundary, or floor, presents for examination the following parts : the hippo- 

 campus major, pes hippocampi, eminentia collaterally or pes accessorius, corpus 

 fimbriatum, prolonged from the posterior pillar of the fornix, and the choroid 

 plexus. Along the mesial aspect of the descending cornu there is a cleft-like 

 opening, which is the lower part of the transverse fissure, through which the 

 choroid plexus of the pia mater is invaginated into the ventricle, but covered by 

 the ependyma, which is pushed in before it. 



The corpus striatum has received its name from the striped appearance which 

 its section presents, in consequence of diverging white fibres being mixed with the 

 gray matter which forms the greater part of its substance. The larger portion of 

 this body is embedded in the white substance of the hemisphere, and is therefore 

 external to the ventricle. It is termed the extra-ventricular portion or the nucleus 

 lenticularis ; a part, however, is visible in the ventricle and its anterior cornu: 

 this is the intra-ventricular portion, or the nucleus caudatus. 



The nucleus caudatus (Fig. 355) is a pear-shaped, highly arched mass of gray 

 matter ; its broad extremity is directed forward into the fore part of the body and 

 anterior cornu of the lateral ventricle ; its narrow end is directed outward and 

 backward on the outer side of the optic thalamus ; it is continued downward into 

 the roof of the descending cornu, where it terminates in the nucleus amygdala?, a 

 collection of gray matter in the apex of the temporal lobe. It is covered by the 

 lining of the ventricle, and crossed by some veins of considerable size. It is 

 separated from the extra-ventricular portion, in the greater part of its extent, by a 

 lamina of white matter, which is called the internal capsule, but the two portions 

 of the corpus striatum are united in front. 



The nucleus lenticularis, or extra-ventricular portion of the corpus striatum, is 

 only seen in sections of the hemisphere. When divided horizontally, it presents, 

 to some extent, the appearance of a biconvex lens, while a vertical transverse 

 section of it gives a somewhat triangular outline. It does not extend as far forward 

 or backward as the nucleus caudatus. It is bounded externally by a lamina of white 

 matter called the external capsule, on the outer surface of which is a thin layer of 

 gray matter termed the claustrum. The claustrum presents ridges and furrows on 



