THE CATFISH. 171 



scope. Observe in the groove on the gill-bearing surface 

 of the arch the two blood vessels, branches of the branchial 

 artery and vein. Observe the distribution of the minute 

 blood vessels in the gill filaments. Examine the fifth gill 

 arch. How does it differ from the other four ? 



The Dorsal Surface. Turn the fish over. The bones 

 of the head constitute the skull. Observe that the thick, 

 dark skin of the dorsal surface rests directly upon the top 

 of the skull. Find a narrow backward process from the 

 skull extending to the dorsal spine. 



Cranium and Cranial Nerves. Remove the skin from 

 the right side of the body and head, and from the whole 

 upper posterior portion of the skull. Then cut away the 

 top of the skull and expose the brain. Remove the bone, 

 with great care not to injure the soft, nervous tissues be- 

 neath it. The bony cavity in which the brain lies is the 

 cranium. When the top of the cranium is cut away, it 

 will be seen that the white brain does not fill the cranial 

 cavity, and that it is surrounded within the cavity by a 

 transparent, semifluid substance. 



Study the brain as to its general features. 



Observe that it tapers posteriorly into the spinal cord, 

 which has been already noticed, extending through the 

 spinal column. 



Observe the nerves that start out from the brain. Aris- 

 ing within the cranium, these are called cranial nerves. 

 Observe that the cranial nerves are paired. Several pairs 

 of them may be easily distinguished. The two extending 

 forward from the brain nearest the median line are the 

 olfactory nerves, or nerves of smell. Cut away its bony 

 covering, and trace one of these to its termination in an 

 olfactory bulb, beneath the communicating nostrils. 



The Optic Nerve and the Eye. Just behind the olfac- 

 tory nerves is a larger pair of optic nerves extending direct 



