THE KEKVOUS SYSTEM 265 



bulb, * whose central cavity is the fourth ventricle. Part of 

 the dorsal wall of this ventricle forms the cerebellum, which 

 in the frog is only slightly developed, but which in higher 

 vertebrates (birds and mammals) becomes a large and con- 

 spicuous organ. Anteriorly the fourth ventricle is connected 

 with the third by a tube, the aqueduct of Sylvius. The thick 

 walls of this aqueduct contain various masses of gray matter 

 whose names need not detain us ; the walls of the third ven- 

 tricle are similarly composed of large masses of gray matter 



FIG. 106. Diagrammatic median longitudinal section of a mammalian brain 



After Edinger 



For convenience the cerebrum, with its lateral ventricle, is represented as a single 



organ in the median plane instead of two hemispheres on either side of this 



plane and each with its own lateral ventricle. The division into forebrain, 



'tweenbrain, midbrain, and hindbrain is marked by the broken lines 



scattered among the fibers of the white matter. Still farther 

 forward two openings from the third ventricle, one on the 

 right and one on the left side, lead into the large lateral 

 ventricles, the nervous tissue of whose walls is the cerebrum, 

 or the cerebral hemispheres. It is convenient to divide the brain 

 into the forebrain, surrounding the lateral ventricles ; the 

 'tweenbrain, surrounding -the third ventricle ; the midbrain, 

 surrounding the aqueduct of Sylvius ; and the hindbrain, 

 surrounding the fourth ventricle. 



1 The older term for the bulb is the medulla oblongata, to distinguish 

 it from the medulla spinalis, or spinal cord. 



