172 



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 imperceptibly into the 

 cord. The latter is a continuous tract of gray matter en- 

 closed within strands of white fibres. 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, 

 it weighs about an ounce and a half. 



Besides these parts, there are also the olfactory and the 

 optic lobes, which give rise respectively to the nerves of 

 smell and sight. 



The parts of the brain are always in pairs ; but in rela- 

 tive development and po- 

 sition they differ widely in 

 the several classes of Ver- 

 tebrates. In Fishes and 

 Reptiles, they are arranged 

 in a horizontal line ; in 

 Birds and Mammals, the 

 axis of the spinal cord 

 bends to nearly a right an- 



Fig. 139.— Brain of g] e j n p ass i n cr through the 

 the Perch, upper o r o & 



view: a, cerebei- brain, so that the lobes no 



him; &, optic . . 



lobes; c, cere- longer lie in a straight line. 



ry n iobes.°f ml In Man, the fore-brain is Fw. MO. -Brain of the 



„,,,,', m _ _ "FYnov nnnw vipw V A.* 



duiia oblongata. g0 developed that it cov. 

 ers all the other lobes. In looking down 

 upon the brain of a Perch, we see in 

 front a pair of olfactory lobes (which 

 send forth the nerves of smell), behind 



Frog, upper view, X 4 : 

 .7, olfactory nerves; I/oi, 

 olfactory lobes ; ifc, cer- 

 ebral hemispheres; Pn, 

 pineal gland; Fho and 

 Srh, third and fourth 

 ventricles; Lop, optic 

 lobes ; C, cerebellum ; 

 lib, medulla oblongata. 



