360 FUNCTIONS OF THE NERVOUS SYSTEM. 



Oblongata, and the chain of Sensory Ganglia at the summit of the latter), 

 and thus brought, through the medium of the latter, into relation with the cen- 

 tral terminations of the afferent nerves, and the origins of the motor. On this 

 view, the Cerebrum would receive all its sensory impressions, by the commis- 

 sural fibres that connect it with the ganglia, which are the real centres of these 

 nerves ; whilst it would call the motor trunks into action, by exciting, through 

 another set of commissural fibres, the vesicular matter of the ganglionic cen- 

 tres from which they pass forth.* This question cannot be determined until 

 it shall have been shown, whether there is, or is not, a direct continuity be- 

 tween any of the fibres of the trunks connected with the Cranio-Spinal Axis, 

 and any of the radiating fibres of the Cerebral hemispheres. But the latter 

 view is certainly favoured by the very remarkable fact, in which the results 

 of all experiments agree, that no irritation or injury of the Cerebral fibres 

 themselves, produces either sensation or motion. Even the Thalami and Cor- 

 pora Striata may be wounded, without the excitement of convulsive actions ; 

 but if the incisions involve the Tubercula Quadrigemina or the Medulla Ob- 

 longata, convulsions uniformly occur. These results are borne out by patho- 

 logical observations in Man ; for it has been frequently remarked, when it 

 has been necessary to separate protruded portions of the Brain from the 

 healthy part, that this has given rise to no sensation, even in cases in which 

 the mind has been perfectly clear at the time. 



474. The Commissural fibres constitute two principal groups, the trans- 

 verse, and the longitudinal; the former connecting the two Hemispheres with 

 each other; the latter uniting the different parts of the same Hemisphere. 

 Of the transverse commissures, the Corpus Callosum is the most important. 

 This consists of a mass of fibres very closely interlaced together; which may 

 be traced into the substance of the hemispheres on each side, particularly at 

 their lower part, where their connections are the closest with the Thalami 

 Optici and Corpora Striata. It is difficult, if not impossible, 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 different parts of the cortical sub- 

 stance of the Hemispheres. This commissure is altogether wanting in Fish, 

 Reptiles, and Birds ; and it is partially or completely wanting in those Mam- 

 mals, whose Cerebrum is formed upon the least complex plan the Rodents 

 and Marsupials. 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 Marsupials, 

 in which the Corpus Callosum is deficient. The posterior commissure is a 

 band of fibres which connects together the Thalami optici; crossing over from 

 the posterior extremity of one to that of the other. Besides these, there are 

 other groups of fibres, which appear to have similar commissural functions, 

 but which are intermingled with vesicular substance. Such are the soft 

 commissure, which also extends between the Thalami; the Pons Tarini, 

 which extends between the Crura Cerebri ; and the Tuber Cinereum, which 

 seems to unite the optic tracts with the thalarni, the corpus callosum, the for- 

 nix, &c., and to be a common point of meeting for several distinct groups of 

 fibres. Of the longitudinal commissures, some lie above, and others below, 

 the Corpus Callosum. Upon the transverse fibres of that body, there is a 

 longitudinal tract on each side of the median line, which serves to connect 



* See Messrs. Toilil ami Bowman's Physiological Anatomy, Chap. XI. for a fuller state- 

 ment of this view, and of the ni-Lruments in its favour. See also the General Summary at 

 the conclusion of the present Chapter. 



