396 THE SENSORIAL FUNCTIONS. 



established between the parts on both sides. For this pur- 

 pose there is provided a set of medullary fibres, passing di- 

 rectly across from one side of the brain to the other; these 

 constitute what are called the Commissures of the brain.* 



The question, however, still recurs: What relation does 

 all this artificial intertexture and accumulation of fibres bear 

 to the mental operations of which we are conscious, such as 

 memory, abstraction, judgment, imagination, volition? Are 

 there localities set apart for our different ideas in the store- 

 house of the cerebral hemispheres, and are they associated 

 by the material channels of communicating fibres? Are the 

 mental phenomena the effects, as was formerly supposed, of 

 a subtle fluid, or animal spirits, circulating with great ve- 

 locity along invisible canals in the nervous substance? or 

 shall we, with Hartley, suppose them to be the results of 

 vibrations and vibraliuncles, agitating in succession the 

 finer threads of which this mystic web has been construct- 

 ed? But a little reflection will suffice to convince us that 

 these, and all other mechanical hypotheses, which the most 

 fanciful imagination can devise, make not the smallest ap- 

 proach to a solution of the difficulty; for they, in fact, do 

 not touch the real subject to be explained, namely, how the 

 affections of a material substance can influence and be influ- 

 enced by an immaterial agent. All that we have been able 

 to accomplish has been to trace the impressions from the 

 organ of sense along the communicating nerve to the senso- 

 rium: beyond this the clew is lost, and we can follow the 

 process no farther. 



The principal commissure of the human brain, called the corpus callosum, 

 is seen at a, Fig. 461. Dr. Macartney, in a paper which he read at the late 

 meeting at Cambridge of the British Association for the Advancement of 

 Science, described the structure of the human brain, as discovered by his pe- 

 culiar mode of dissection, to be much more complicated than is generally 

 supposed. He observed that its fibres are interlaced in the most intricate 

 manner, resembling the plexuses met with among the nerves, and establish- 

 ing the most extensive and general communications between every part of 

 the cerebral mass. 



