382 CONVERGING FIBRES. 



stitute both the cerebrum and cerebellum, by their increase and 

 developement, another set of fibres are found to exist, which have 

 for their office the association of the symmetrical halves, and distant 

 parts of the same hemispheres. 



These are called from their direction converging fibres, and from 

 their office commissures. The commissures of the cerebrum and 

 cerebellum are the 



Corpus callosum, 

 Fornix, 



Septum lucidum, 

 Anterior commissure, 

 Middle commissure, 

 Posterior commissure, 

 Peduncles of the pineal gland, 

 Processus e cerebello ad testes, 

 Valve of Vieussens, 

 Pons Varolii. 



The Corpus callosum is the commissure of the hemispheres. It is 

 therefore of moderate thickness in the middle, where its fibres pass 

 directly from one hemisphere to the other ; thicker in front, where 

 the anterior lobes are connected ; and thickest behind, where the 

 fibres from the posterior lobes are assembled. 



The Fornix is an antero-posterior commissure, and serves to con- 

 nect a number of parts. Below it is associated with the thalami 

 optici ; on each side, by means of the corpora fimbriata, with the 

 middle lobes of the brain ; and, above, with the corpus callosum, 

 and consequently with the hemispheres. 



The Septum lucidum is a perpendicular commissure between the 

 fornix and corpus callosum. 



The Anterior commissure traverses the corpus striatum, and con- 

 nects the anterior and middle lobes of opposite hemispheres. 



The Middle commissure is a layer of gray substance, uniting the 

 thalami optici. 



The Posterior commissure is a white flattened cord, connecting the 

 thalami optici. . 



The Peduncles of the pineal gland must also be regarded as com- 

 missures, assisted in their function by the gray substance of the 

 gland. 



The Processus e cerebello ad testes are the means of communica- 

 tion between the white substance of the cerebellum and cerebrum ; 

 and the linguetta laminosa and valve of Vieussens perform the same 

 office to the gray substance. 



The Pons Varolii is the commissure to the two lobes of the cere- 

 bellum. It consists of transverse fibres, which are split into two 

 layers by the passage of the fasciculi of the corpora pyramidalia 

 and olivaria. These two layers, the superior and inferior, are col- 

 lected together on each side, in the formation of the crura cerebelli. 



