130 THE BRATN OF REPTILES 



several parts and the spinal cord in the same horizontal 

 plane, and by the small size of the Brain as compared with 

 the latter structure. Still, the brain is more nearly equal 

 in weight to the cord than it is in Fishes, and it also 

 bears, in the majority of Reptiles, a greater proportion to 

 the total body- weight. 



But in BIRDS we find the Brain attaining a notably 

 greater size in proportion to the bulk of the Spinal Cord 

 than it has among Reptiles, and also presenting other 

 signs of increased development. 



According to Leuret, the average proportional weight of 

 the brain to the body in the four undermentioned classes, 

 as deduced from numerous observations on different 

 representatives of each, may be stated to be as follows : 



In FISHES as 1 to 5,668 In BIRDS as 1 to 212 



In EEPTILES as 1 to 1,321 In MAMMALIA... as 1 to 186 



These figures must, of course, be regarded merely as 

 approximate averages. 



No peculiarity worthy of note exists in the Spinal Cord 

 of Birds, except that in the situation of its posterior 

 enlargement, corresponding with the attachment of the 

 great nerves of the legs, the posterior columns of the 

 cord diverge from one another, and shortly again approxi- 

 mate so as to form a space, known as the ' rhomboidal 

 sinus.' This, however, is an anatomical peculiarity to 

 which no physiological significance is attached. 



The Medulla Oblongata, from the back of which the 

 cerebellum is developed, is, in Birds, decidedly broader 

 than the spinal cord. As in lower vertebrates, the diver- 

 gence of the upper or posterior columns of the cord 

 leaves at the corresponding surface of the medulla the 

 space known as the ' fourth ventricle,' which becomes much 



