96 



HUMAN BIOLOGY 



where it passes forward into the head and thus forms the 

 rear section of the brain, the medulla oblongata (Fig. i). 

 In front of this is a shght tongue-hke elevation, the cerebel- 



FlG. I. 



Fig. I. Frog's brain seen from above. 



c, cerebellum; H, hemispheres; m, medulla oblongata; o, optic lobes. 



Fig. 2. Frog's brain opened from above to show ventricles. 



c, central canal of spinal cord; h, hemispheres; m, medulla oblongata; o, 

 optic lobes. 



lum, which is followed midway on the length of the brain by 

 a pair of conspicuous prominences, the optic lobes, on the 

 right and on the left. A httle in front of these lobes the stem 

 of the brain branches into two relatively large elongated 

 bodies which, because of their general structural agreement 

 with parts in the brains of the higher animals, are called the 

 hemispheres. These lobes terminate the brain at its front 

 end. 



If a transverse section of the frog's spinal cord is examined 

 under a microscope a small pore can easily be observed near 

 its center. This pore is the so-called central canal which 

 runs lengthwise in the cord. The cord therefore is a hollow 

 structure and may be compared not inappropriately to a 

 very thick-walled tube. The central canal of the cord can be 

 traced forward into the brain where it expands into a suc- 

 cession of chambers known as the ventricles of that structure. 

 As the cord enlarges at its front end to form the medulla 



