THE CATFISH. 173 



and the several pairs, which, we have seen, extend ante- 

 riorly, are one or more pairs which extend laterally, and 

 send branches to a pair of thin-walled swellings at the 

 base of the hinder part of the skull. These swellings 

 are the ear capsules. Each of these, if cut open, will be 

 found to contain a membranous sac filled with liquid in 

 which floats an ear bone. Each is an organ of hearing, 

 capable of being affected by vibrations, which are so 

 readily transmissible through water. 



Make a drawing of the brain and cranial nerves in 

 place, as seen from above. 



The Brain. Cut off the cranial nerves half an inch 

 from the brain. Lift out the brain, place it in alcohol, 

 and study to make out its parts. 



1. Looking at the dorsal surface of the brain, observe 

 *a large, squarish, anterior division of softer, more satiny 

 appearance than the rest, divided by a shallow median 

 groove into two lateral halves. This is the cerebrum, and 

 its halves are the cerebral hemispheres. 



2. Looking at the ventral surface of the cerebrum, 

 observe two conical swellings at the base of the olfactory 

 nerves. These are the olfactory lobes, the foremost portion 

 of the brain. 



3. Close behind the cerebrum are the two smooth, 

 roundish optic lobes, which are continued downward into 

 two inferior lobes, that are easily seen from one side or from 

 below. Observe the optic nerves taking origin from this 

 part, and crossing ere they leave the cranium. Which side 

 of the brain receives impressions through the right eye ? 



4. On the median dorsal line, close behind and above 

 the optic lobes, observe a conspicuous, roundish single 

 portion, the cerebellum. This part extends backward and 

 downward in two posterior lobes, which surround a deep 

 median depression, the fourth ventricle. 



