74 VERTEBRATE ZOOLOGY 



The Ventral Surface of the Brain. Carefully remove the brain 

 from the skull, float it in water, and study its ventral surface. 

 The large paired hemispheres will be seen and back of them the 

 median diencephalon, from which the small optic nerves and the 

 infundibulum proceed. The latter is a large median projection 

 just back of the optic nerves which bears at its distal end a small 

 body called the pituitary body or hypophysis. This structure rests 

 in a depression in the ventral wall of the skull and is apt to 

 be torn away when the skull is removed. Back of the infundib- 

 ulum are the crura cerebri, two swellings which join the medulla ; 

 they form the ventral portion of the midbrain. Note the mid- 

 ventral groove in the medulla and the spinal cord. 



Exercise 12. Draw the ventral aspect of the brain on a scale of 2 or 3. 



The Cavities of the Brain and Spinal Cord. The brain and spinal 

 cord are hollow structures. In the center of the latter is a nar- 

 row passage called the central canal, and the brain contains a 

 series of large spaces of various sizes. In the medulla is the broad, 

 shallow cavity called the fourth ventricle. Its dorsal roof is not 

 composed of nervous matter but is vascular and is hence dark- 

 colored. The cavities of the hemispheres are called the lateral, or 

 the first and second, ventricles. These are joined with each other 

 and also with the small median cavity of the diencephalon, which 

 is called the third ventricle, by a small opening called the foramen 

 of Monro. Joining the third ventricle with the fourth is a median 

 canal called the aqueductus Sylvii, with which are connected a 

 pair of large cavities situated in the optic lobes, or midbrain, and 

 called the optic ventricles. 



The fourth ventricle has already been seen. The lateral ven- 

 tricles are found by removing the dorsal wall of the hemispheres, 

 and the optic ventricles by removing the dorsal wall of the optic 

 lobes. The median spaces are not so easy to find, but may be seen 

 by splitting the brain exactly in the sagittal plane. 



Exercise 13. Draw a diagram showing the cavities of the brain. 



The vascular system is made up of the following organs : ( 1 ) the 

 heart; (2) the arteries, which carry blood to the gills, the lungs, 



