THIRD VENTRICLE. 207 



4 



it envelops the cms of the fornix, and ascends to the septum lucidum ; 

 and in the floor of the cavity it exists in abundance, entering into the 

 corpora albicantia. In the middle of the space it reaches from side to side, 

 and forms the soft commissure (o?). 



The anterior commissure of the cerebrum (e) is a round bundle of white 

 fibres about as large as a crow-quill, which passes through both corpora 

 striata, and connects the opposite hemispheres. To see it in one-half of 

 its extent, the following dissection should be made : 



Dissection. On the side on which the corpus striatum has been cut into, 

 the commissure is to be followed into the interior of that body, by scraping 

 away the intraventricular gray matter with the handle of the scalpel. The 

 commissure may be seen then to perforate below the white fibres of the 

 corpus striatum ; and it should be followed through the extraventricular 

 mass of gray matter of the same body. 



The anterior commissure is free in the middle line for about the eighth 

 of an inch, where it lies before the pillars of the fornix. Laterally it per- 

 forates the corpus striatum, passing in succession through the intraven- 

 tricular gray mass, the white fibres, and the extra gray ventricular gray 

 mass. Lastly, the commissure pierces the white layer bounding externally 

 the corpus striatum, and spreads in the hemisphere over the inferior cornu 

 of the lateral ventricle communicating with the temporo-sphenoidal and 

 central lobes. 



The posterior commissure of the cerebrum (e) is smaller than the ante- 

 rior, and is placed above the passage into the fourth ventricle. Laterally 

 it enters the substance of the optic thalamus ; and pierces this body to end 

 in the hemisphere. 



The connections of the thalamus opticus (inferior ganglion of the cere- 

 brum) (b) will be best seen on the side on which the inferior cornu of the 

 lateral ventricle has been opened. It has the form of a cube, and bounds 

 the lateral and third ventricles. 



The upper surface projects in the floor of the lateral ventricle, and is 

 marked in front by a prominence anterior tubercle, near the tienia semi- 

 circularis. The under surface forms part of the roof of the inferior cornu 

 of the lateral ventricle, and into it the crus cerebri is inserted. 



By its inner side it enters into the third ventricle ; and along the upper 

 part lies the peduncle of the pineal body. On the outer side are the cor 

 pus striatum, the tarnia semicircularis, and the substance of the hemisphere. 



The anterior end looks to the foramen of Monro. And the posterior 

 part, which is free in the inferior cornu of the lateral ventricle, presents 

 inferiorly two small roundish tubercles, which are placed one outside and 

 the other inside the bend (genu) of the optic nerve, and are named from 

 their position to it, internal and external geniculate bodies. 



The structure of the optic thalamus will be subsequently referred to, p. 

 211. 



The origin of the optic nerve can now be seen. At the back of the crus 

 cerebri the optic tract receives fibres from the thalamus which it touches, 

 and then divides into two terminal bands: One of these is connected with 

 the gray matter in the external geniculate body, and is continued onwards 

 to one of the corpora quadrigemina (nates); the other is connected with 

 the internal geniculate body. 



Dissection. The origin of the fornix in the optic thalamus may be fol- 

 lowed out next. As a preparatory step the anterior commissure, the 

 anterior part of the corpus callosum, and the commissure of the optic 



