288 INNERVATION. [CHAP. x. 



groove between the corpus striatum and optic thalarnus. This is 

 called tcenia semicircular is. It may be described as connected with 

 the corpus mamillare, in much the same way as the anterior pillar 

 of the fornix. Traced from this point, it is found to penetrate the 

 optic thalamus, following the general course of the anterior pillar 

 of the fornix, but slightly diverging from it, and to emerge from 

 the thalamus in the anterior part of the groove between it and the 

 corpus striatum, whence it passes backwards, outwards, and down- 

 wards into the inferior cornu of the lateral ventricle. 



Other structures exist in the brain, which seem likewise to act 

 as commissures to the parts between which they are placed. 

 Thus, between the crura cerebri a layer of fibrous matter, mingled 

 with a few vesicles, is placed, which fills up the angle formed at 

 their divergence ; this layer is remarkable for being perforated by 

 numerous foramina, which give passage to the blood-vessels of the 

 locus niger. It is called the pans Tarini ; it probably connects the 

 gray matter of the crura. 



The innermost fibres of the optic tracts are evidently commissural. 

 These fibres form an arch, which crosses the tuber cinereum. In 

 the mole, they are the only fibres of the optic tracts existing : 

 those which form the optic nerves are not present. These fibres 

 connect the quadrigeminal tubercles and the geniculate bodies of 

 opposite sides. 



The tuber cinereum is a remarkable layer of vesicular matter, 

 with which nerve-tubes freely intermingle, which extends from the 

 mamillary bodies forwards to the posterior reflection of the 

 corpus callosum, and has intimate connexions with the anterior 

 pillars of the fornix, the optic tracts, the septum lucidum, and, at 

 the floor of the third ventricle, with the optic thalami. An infun- 

 dibuliform tube passes from it down towards the pituitary gland, 

 which is situate in the sella Turcica. 



It is curious how few are the fibres which seem to connect the 

 cerebrum and cerebellum. The only ones to which this office 

 can be assigned are those which form the processus cerebelli ad 

 testes. Hence these structures may more fitly be denominated 

 cerebro-cerebellar commissures. They extend between the cere- 

 bellum on the one hand, and the optic tubercles and thalami on 

 the other. 



Of the manner in which the commissures connect the various 

 parts between which they are placed, it is difficult to form an 

 exact opinion. Are the commissural fibres directly continuous 

 with those of the segments which they unite ? or do they inter- 



