THE FROG 



79 



downwards and backwards, it is termed the infundibulum and forms, 

 when viewed from the outside, the projection on the ventral surface 

 of the brain known as the tuber cinereum. The front wall of the 

 third ventricle is formed by the lamina terminalis, in which runs 

 the anterior cerebral commissure, and the lateral ventricles of the 

 hemispheres here open by two apertures, known as the foramina of 

 Munro. The fourth ventricle is situated in the medulla, and is a 

 large cavity whose roof is formed by the posterior choroid plexus. 

 It is joined to the preceding cavity by a passage running through the 

 mid-brain. In the frog it is fairly large and continuous with two 

 spaces in the optic lobes, the optic ventricles, but in the higher ani- 

 mals it is only a small hole, consequently it is termed the aqueduct 



AC. 



AM 



FM. 



PC. 



C.H. 



01- 



C.C. 



H. 



FIG. 25. Median longitudinal section of brain, Rana, adapted from Gaupp. 



A.C., anterior choroid plexus ; A. Co., anterior commissure ; A.M., anterior medullary velum ; 

 C., cerebellum; C.C., crura cerebri ; C.H., median surface of cerebral hemisphere; B.C., epi- 

 physis cerebii (pineal body) ; F.M., foramen of Munro ; H., hypophysis cerebri (pituitary body) ; 

 I., infundibulum ; L.T., lamina terminalis ; M., medulla oblongata ; O.C., optic chiasma ; 

 O.L., optic lobe ; OL, olfactory lobe ; O.N., optic nerve; O.V., optic ventricle ; P.C., posterior 

 choroid plexus ; P.Co., posterior commissure ; P.R., pre-optic recess ; S.Co., superior commis- 

 sure ; III., third ventricle ; IV., fourth ventricle. 



of Sylvius or the Iter (a contraction of Iter a tertio ad quartum 

 ventriculum) . 



The spinal cord is a continuation of the brain and, like it, is 

 hollow, the small cavity running through it being called the central 

 canal or canalis centralis. The cord itself is a dorso-ventrally 

 flattened, thick-walled tube situated in the neural canal of the 

 vertebral column. It is not of uniform diameter throughout its 

 course, but swells out in the region of the second vertebra to form 

 the brachial enlargement, and again in the region of the sixth or 

 seventh vertebra to form the lumbar enlargement. After this it 

 quite sharply narrows off to form a fine thread, the filum terminate, 

 which is continued on into the urostyle. It is composed of the same 

 two substances as the brain, i.e. white and grey matter, but their 



