ANATOMY, PHYSIOLOGY, OF NERVOUS SYSTEM. 423 



this part. This small caiial is known as the aqueduct of 

 Sylvius, or more usually the iter. This iter lies between 

 the copora quadrigemina and the crura cerebri. Immediately 

 upon reaching the cerebrum the iter enlarges into the third 

 ventricle. This ventricle lies between the optic thalami and 

 extends down into the infundibulum already described. Both 

 the fourth ventricle, the iter, and the third ventricle lie in a 

 median position. At the forward end, the third ventricle 

 narrows into two openings, known as the foramina of 

 Monro, and each of these opens at once into a large lateral 

 ventricle which lies within each cerebral hemisphere. These 

 lateral ventricles are relatively very large, and extend from 

 near the front of the brain to the occipital region, while a 

 horn of this ventricle dips down almost to the bottom of 

 the lateral lobes. 



The two lateral ventricles are separated from each other 

 by a thin partition, which partition in front of the third 

 ventricle is called the septum hicidum. This septum luci- 

 dum is really double, enclosing a small space within itself. 

 This space is called the fifth ventricle of the brain. It is 

 necessary, however, to bear in mind that this fifth ventricle 

 is not a true ventricle at all. It has no connection what- 

 ever with the other ventricles, and is really only an acci- 

 dental opening formed in the septum lucidum as the brain 

 developed. . 



Interior Structures. If with a scalpel sections of the cer- 

 ebrum parallel to the base of the brain should be cut off 

 there would soon be reached the bottom of the fissure divid- 

 ing the brain into two hemispheres. Examination of this 

 bottom shows it to be made up of a sheet of white nerve 

 fibers extending from one hemisphere to the other. This 

 sheet of connecting fibers is the corpus callosttm. If after 

 having been laid open, the corpus callosum is gently cut 

 loose and lifted off the structures below it, the two lateral 

 ventricles come to view, the corpus callosum forming the 

 roof of these lateral ventricles. The two lateral ventricles 



