THE FROG 



77 



The brain is an elongated mass of nervous tissue filling up 

 the whole cavity of the cranium. It is composed of two distinct 

 kinds of tissue, as will be seen if it is cut in section : a layer of grey 

 matter around the outside and white matter within. Looked at 

 from the dorsal aspect, it will be seen that the anterior half, the fore- 

 brain or prosencephalon, is mainly composed of two elongated 

 ovoidal masses, the cerebral hemispheres or telencephalon. These 

 are separated from one another in the middle line by a deep groove, 

 the sagittal fissure. In front the hemisphere is continued forward 

 into the olfactory lobe or rhinencephalon, and these unite in the 



FIG. 24. A, the brain of the frog : dorsal surface. X 4. B, the brain of 

 the frog : ventral surface, x 4. From Marshall and Gamble. 



C., cerebellum ; C.H., cerebral hemisphere ; .C.P., choroid plexus of third ventricle ; F., fourth 

 ventricle ; IN., tuber cinereum ; M., medualla oblongata ; O., olfactory lobe ; O.C., optic chiasma ; 

 O.L., optic lobe ; P., stalk of pineal body ; P. B., pituitary body ; T., thalamencephalon. 



I., olfactory nerve ; II., optic nerve ; III., third or motor pculi nerve ; IV., fourth nerve ;V., fifth 

 or trigeminal nerve ; VI., sixth nerve ; VII. and VIII., combined root of facial and auditory nerves ; 

 IX. and X., combined root of glossopharyngeal and pneumogastric nerves. 



middle line, so forming the anterior limit of the fissure. The 

 posterior limit is formed by the lamina terminalis, the front wall of 

 the median part of the thalamencephalon, as the next part of the 

 brain is called. It is a comparatively small median portion, situated 

 between the divergent hinder ends of the hemispheres and behind 

 them. The top of it is fairly conspicuous in the freshly killed animal, 

 appealing as a reddish area, the anterior choroid plexus, formed by a 

 large increase in the size and number of blood-vessels of the pia 

 mater in this region. From the posterior end of the thalamence- 

 phalon a thin stalk runs forward over the surface of the brain, to 



