972 INTERNAL CAPSULE. [BOOK in. 



ventricle. In front and to the sides of the optic thalami are seen 

 the corpora striata (G. S.) forming anteriorly the lateral walls of 

 the two lateral ventricles, and diverging behind to allow of the 

 interposition of the optic thalami. On each side of the brain 

 then these two bodies, the corpus striatum and optic thalamus, 

 appear as two masses of grey matter placed on the crus cerebri 

 as this, diverging from its fellow, begins to spread out into the 

 cerebral hemisphere, the corpas striatum being placed somewhat 

 in front of the optic thalamus. * The relations of the two bodies 

 moreover are such that while the optic thalamus alone forms the 

 wall of the third ventricle to which it properly belongs, and the 

 corpus striatum forms part of the wall of the lateral ventricle to 

 which it in turn properly belongs, the optic thalamus also projects 

 into and seems to form part of the wall of the lateral ventricle, 

 though at its origin it had nothing to do with the cerebral vesicle. 



We spoke just now of these bodies as being placed on the 

 crura cerebri, but though their dorsal surfaces thus project from 

 the dorsal surface of the diverging crura, a large portion of each 

 body is, so to speak, imbedded in the substance of the diverging 

 crus, and what is seen in the above surface view is only a part 

 of each body, and indeed, in the case of the corpus striatum, only 

 a small part. In order to understand the nature and relations of 

 these two important bodies we must study sections taken through 

 a cerebral hemisphere in various planes (Figs. 116 123). 



Each crus is made up as we have seen of a dorsal portion or 

 tegmentum consisting largely of grey matter, and a ventral portion 

 or pes consisting exclusively of longitudinally disposed fibres. 

 The tegmentum ends partly in structures lying ventral to the 

 thalamus, partly in the thalamus itself; and we may for the 

 present leave this part of the crus out of consideration. The 

 fibres of the pes, while continuing their oblique course forwards 

 and outwards, soon rise dorsally by the side of the thalamus and 

 hence, in a transverse dorso- ventral section at the level of the hind 

 part of the thalamus (Fig. 116), are seen leaving their previous 

 position ventral to the substantia nigra (Sn) and passing (Cip) by 

 the side of the thalamus on their way to the central white matter 

 of the hemisphere. In this part of their course they form a thick 

 strand separating the thalamus (In) from a large mass of grey 

 matter which, roughly triangular in section, is divided by parti- 

 tions of white matter into three parts (Gp', Gp", Pt), and of which 

 we shall speak directly as the nucleus lenticularis. 



If instead of taking a transverse we take a longitudinal dorso- 

 ventral (or as it is called sagittal) section (Fig. 122) we find that 

 the fibres forming the strand in question do not continue parallel 

 to each other as they rise dorsally but diverge in a radiating 

 manner, forming the so-called corona radiata. If again we take 

 horizontal sections at proper levels (Figs. 115, 121), we find that 

 this strand or rather thick band of dorsally directed radiating 



