382 



COMPARATIVE ZOOLOGY 



The medulla oblongata is the connecting link between 

 the cerebrum and cerebellum and the spinal cord. In 

 structure, it resembles the spinal cord the white 

 matter being external and the gray internal. The 

 former lies beneath or behind the brain, passing through 

 the foramen magnum of the skull, and merging im- 

 perceptibly into the cord. The latter is a continuous 

 tract of gray matter inclosed within strands of white 

 fibers. It usually ends in the lumbar region of the 

 vertebral column, but in fishes it reaches to the end of 

 the tail. In fishes, amphibians, and reptiles, the cord 

 outweighs the brain ; in birds and mammals, the brain 

 is heavier than the cord. In man, the cord weighs 

 about an ounce and a half. 



Besides these parts, 

 there are also the olfac- 

 tory and the optic lobes, 

 which give rise respec- 

 tively to the nerves of 

 smell and sight. 



The parts of the brain 

 are always in pairs ; but 

 in relative development 

 ^^ position they differ 

 widely in the several 

 classes of vertebrates. 



FIG. 336. Brain of 

 the Perch, upper 

 view : a, cerebel- 



b, optic 



c, cere- 



lum ; 

 lobes 



Skr 



brum; i, olfactory In fishes and TCptileS, 

 lobes' p- medulla ^.r. j 



the Y are arranged in a olfactory lobes . Hc 

 horizontal line; in birds 



FIG. 337. Brain of the 

 Frog, upper view; X 4: 

 r , olfactory nerves : Lol, 



c D erebral hemispheres; 



Pn, pineal gland; Fho 



and mammals, the axis of the spinal and shr, third and 



. . , fourth ventricles ; Lop, 



cord bends to nearly a right angle in op tic lobes -, c cerebei- 

 passing through the brain, so that the J u n m gat f '* medulla ob ' 

 lobes no longer lie in a straight line. 

 In man, the fore brain is so developed that it covers all 

 the other lobes. In looking down upon the brain of a 



