.562 THE SENSOItlAL FUNCTIONS. 



It is a remarkable fact, that in vertebrated 

 animals all the organs which are subservient to 

 the sensorial functions are double, those on one 

 side being exactly similar to those on the other. 

 We see this in the eyes, the ears, the limbs, and 

 all the other instruments of voluntary motion ; 

 and in like manner the parts of the nervous 

 system which are connected with these functions 

 are all double, and arranged symmetrically on 

 the two sides of the body. The same law of 

 symmetry extends to the brain : every part of 

 that organ which is found on one side is repeated 

 on the other ; so that, strictly speaking, we have 

 two brains, as well as two optic nerves and two 

 eyes. But in order that the two sets of fibres 

 may co-operate, and constitute a single organ of 

 sensation, corresponding with our consciousness 

 of individuality, it was necessary that a free 

 communication should be established between 

 the parts on both sides. For this purpose there 

 is provided a set of medullary fibres, passing 

 directly across from one side of the brain to the 

 other ; these constitute what are called the Com- 

 misstires of the Brain.* 



* The principal commissure of the human brain, called the 

 corpus callosum, is seen at q, 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 peculiar 

 mode of dissection, to be much more complicated than is 



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