478 THE FUNCTIONAL RELATIONS OF THE 



1.— The Cross Eelation existing between the Cere- 

 bral Hemispheres and the Lateral Halves of the 

 Body. 



The bodies of the great majority of Invertebrates, as 

 well as of Vertebrate Animals, are bilaterally symme- 

 trical — at least as regards all external organs and 

 all parts of their Nervous System. So that if a median 

 vertical plane were to divide one of these animals in a 

 longitudinal direction, each half of the body would be 

 found to be similar to the other in all respects externally, 

 and each, also, would contain the half of a Nervous System 

 similar to that of its fellow. 



So far as we know at present, however, the relation 

 which the double Nervous System of the Invertebrate 

 bears to its double body is different altogether from 

 the relation subsisting between the same parts in the 

 Vertebrate. In the former the half of the Brain con- 

 tained in either half of the body is in immediate connec- 

 tion with the sensory organs and surfaces, as well as 

 with the motor nerves and muscles, of the same side of the 

 body. In Vertebrates, on the other hand, it is not so. 

 In lower members of the series to some extent, and in 

 the higher forms (including Quadrumana and Man) 

 to a more perfect extent, a cross relation exists between 

 the Brain and the body, so that each half of the Brain 

 is connected with the Sensory Organs and surfaces of 

 the opposite half of the body, and also with its Muscles. 

 The former relation is brought about by the ' sensory ' 

 channels decussating at the base of the Brain and along 

 the Spinal Cord ; and the latter is due to the fact that 

 the nerve-channels for ' outgoing ' or motor stimuli pass 

 from each half of the Brain to the opposite side of 

 the body, decussating with one another in the Medulla. 



