Chap. XXIV.] PRINCIPAL PARTS OF THE BRAIN. 479 



Very few explanations have as yet been attempted of the 

 mode of origin of this crossed relation between the Brain 

 and the body. The subject is generally passed over in 

 silence, and though our knowledge of the exact anatomical 

 relations existing in lower animals is not yet ripe enough 

 for a thoroughly satisfactory answer, a few suggestions 

 may here be offered which, if they do nothing else, will 

 perhaps serve to direct more attention to this very inte- 

 resting question, and at the same time indicate some 

 of the directions in which more precise information is 

 needed. 



The essential nature of the problem comes out most 

 distinctly if the reader attempts to picture to himself the 

 existence of a double Nervous System in Vertebrates in all 

 respects similar to what it is, except for the fact that 

 neither its sensory nor its motor channels decussate. 

 With the two halves of the Brain and Spinal Cord, as 

 freely connected by transverse ' commissures ' as they are at 

 present, a direct relationship of this kind would seem to 

 be the most natural arrangement, and it is not, therefore, 

 at all clear why such a plan should not exist and work as 

 well for Vertebrates as it does for Invertebrates. The 

 question to be answered, then, is — What conditions have 

 arisen in Vertebrate Animals tending to initiate, and 

 finally to perfect, such a crossed relation between the 

 Brain and the body as we find existing in Man and the 

 higher Mammalia generally ? 



The following considerations seem to the writer to 

 throw some light upon this subject :— 



1. Movements take place in response to sensory impressions of 

 various kinds, and (for our present purpose) they may be divided 

 into two classes : — {a) those in which related muscles on the two 

 sides of the body are called into simultaneous activity — as with the 

 trunk muscles concerned in the locomotions of Fishes and many 



