COMMISSURES OF THE CEREBRAL HEMISPHERES. 



519 



carried into effect (Figs. 155, 156). Of this view we shall presently 

 find that there is strong physiological evidence. 



Fig. 156. 



Diagram of the mutual relations of the principal Encephalic centres, as shown in a vertical section : A, 

 Cerebrum; B, Cerebellum; c, Sensori-motor tract, including the Olfactive ganglion olf, the Optic opt, and 

 the Auditory and, with the Thalami Optici thai, and the Corpora Striata cs ; D, Medulla Oblongata; E, Spinal 

 Cord: a, olfactive nerve; &, optic; c, auditory; d, pneumogastric ; e, hypoglossal ;/, spinal : fibres of the 

 medullary substance of the cerebrum are shown, connecting its ganglionic surface with the sensori-motor 

 tract. 



915. The two Hemispheres are united on the median line by several 

 transverse commissures ; of which the Corpus Callosum is the most im- 

 portant. This consists of a mass of fibres very closely interlaced to- 

 gether ; ' which may be traced into the substance of the hemispheres on 

 each side, particularly at their lower part, where they are connected 

 with the thalami optici and corpora striata. It is difficult, if not impos- 

 sible, to trace its fibres any further ; but there can be little doubt that 

 they radiate, with the fibres proceeding from the bodies just named, to 

 the different parts of the surface of the hemispheres. This commissure 

 is altogether absent in Fish, Reptiles, and Bir.ds ; and it is partially or 

 completely wanting in the Mammalia with least perfect brain, as the 

 Rodents and Marsupials. The other transverse commissures rather 

 belong to the Sensory Ganglia than to the Cerebral hemispheres. Thus 

 the anterior commissure particularly unites the Corpora Striata of the 

 two sides ; but many of its fibres pass through those organs, and radiate 

 towards the convolutions of the hemispheres, especially those of the 

 middle lobe. This commissure is particularly large in those Marsu- 

 pials, in which the corpus callosum is deficient. The posterior com- 

 missure is a band of fibres which connects the Optic Thalami ; crossing 

 over from the posterior extremity of one to that of the other. Besides 

 these, there are other groups of fibres, which seem to have similar com- 

 missural functions, but which are intermingled with vesicular substance. 



