616 THE BRAIN. 



" basal ganglia " often applied to them is, therefore, unfortunate. Neverthe- 

 less, it will render the description of their topographical relations easier, if 

 for a little while we consider them together. 



When the lateral ventricle is laid open from above, part of the corpus 

 striatum is seen projecting into the cavity of the ventricle. In front the 

 projecting part is broad, forming the lateral wall and part of the floor of 

 the ventricle, and to its median side lies the cavity of the ventricle, sepa- 

 rated from its fellow by the septum lucidum. Further back the projecting 

 part, becoming gradually narrower, assumes a more lateral position and 

 passes into the descending horn. In this part of its course there lies on its 

 median side, separated from it by a narrow band called the tsenia semicir- 

 cularis or stria terminalis, the optic thalamus, a narrow strip of the sur- 

 face of which is seen projecting outside the edge of the choroid plexus. 

 If now, both lateral ventricles be laid open by removal of the corpus cal- 

 losum, and the fornix with the velum interpositum and choroid plexus be 

 taken away, so as fully to expose the third ventricle, and also, in order to 

 obtain a better view, the whole of the hinder part of the cerebrum contain- 

 ing the posterior horns of the lateral ventricle, be completely cut away, it 

 is seen (Fig. 138) that the two optic thalami (0. T.) present themselves as 

 two large oval bodies, placed obliquely athwart the diverging crura cerebri 

 and converging in front to form the immediate walls of the third ventricle. 

 In front and to the sides of the optic thalami are seen the corpora striata 

 (C. 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 gray matter placed on the crus 

 cerebri as this, diverging from its fellow, begin to spread out into the cere- 

 bral hemisphere, the corpus 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 thala- 

 mus 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 rela- 

 tions of these two important bodies we must study sections taken through a 

 cerebral hemisphere in various planes (Figs. 139-146). 



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

 consisting largely of gray matter, and a ventral portion or pes consisting 

 exclusively of longitudinally disposed fibres. The tegmentum ends partly 

 in structures lying ventral to the thalamns, 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 forward and 

 outward, 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 thala- 

 mus (Fig. 139), are seen leaving their previous position ventral to the sub- 

 stantia nigra (8n~) and passing (Op} 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 gray matter which, roughly triangular in section, is divided by 



