66 THE NECTURUS 



medulla itself. This swelling is formed by the crura 

 cerebri where the large dorsal paths of the medulla must 

 curve to the sides to get around the fossa rhomboidalis. 

 The ventral surfaces of both the medulla and the spinal 

 cord are marked by a median groove. 



Sagittal Section of the Brain 



Place the train on a piece of cork, wax or other soft 

 material and make a sagittal section with a razor blade or 

 a sharp scalpel. Make this cut by gently pulling the blade 

 along the median line. If the brain is fairly well hardened, 

 this cut is not difficult to make, even in formaldehyde 

 specimens. Place the sections in alcohol or water and study 

 the cut surface. 



The first and second ventricles are in the cerebral lobes. 

 They are long cavities, extending well through this section 

 of the brain. The foramina of Monro or interventricular 

 foramina, are triangular openings that lead from the third 

 ventricle to these paired ventricles of the anterior lobes 

 of the brain. A membranous mass the telea chorioidea 

 will be observed extending into the first and second ven- 

 tricles. This is concerned in the metabolism of the brain. 

 The third ventricle is in the diencephalon or 'twixt brain. 

 The walls are thin on the dorsal side and contain paired 

 cavities; that extend into the optic lobes. These are the 

 mesocoeles. The cavity of the optic lobes is not extensive 

 in this animal. 



The iter is the passage through the mesencephalon. It 

 is rather wide in this form and not the small tube that it 

 is in the higher vertebrates. It continues the passage 

 through the brain to the fourth ventricle. 



The fourth ventricle is a large cavity in the posterior 

 part of the brain. From the dorsal side it is triangular 

 with a long extension running posteriorly along the me- 

 dulla. It is covered with non-nervous material and is 



