430 CONVERGING FIBRES. 



the deep transverse fibres of the pons Varolii and become apparent in 

 the floor of the fourth ventricle, under the name of fasciculi teretes or 

 posterior pyramids. From this point they are prolonged upwards 

 beneath the corpora quadrigemina into the crura cerebri, of which they 

 form the upper and outer segment, and are thence continued through 

 the thalami optici and corpora striata into the hemispheres. The 

 locus niger of the crus cerebri is a septum of grey matter interposed 

 between these fasciculi and those of the corpora pyramidalia. 



CONVERGING FIBRES. In addition to the diverging fibres which 

 are thus shown to constitute both the cerebrum and cerebellum, by 

 their increase and development, 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 fares, 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, 



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 fibres which curve back- 

 wards into the posterior lobes from the posterior border of the corpus 

 callosum have been termed forceps, those which pass directly out- 

 wards into the middle lobes from the same point, tapetum, and those 

 which curve forwards and inwards from the anterior border to the 

 anterior lobes, forceps anterior. 



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

 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 connects 

 the anterior and middle lobes of opposite hemispheres. 



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

 thalami optici. 



The Posterior commissure is a white rounded cord, connecting the 

 thalami optici. 



