COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 309 



disposed of in the more anterior portions of the brain. Near the anterior end of 

 the somatic sensory column, just posterior to the acustico-lateral area, will be 

 seen, by pressing the latter toward the middle, the roots of a number of nerves. 

 These are the roots of the fifth, seventh, and eighth cranial nerves, to be studied 

 in more detail later. At the posterior end of the somatic sensory column, just 

 anterior to the point where the walls of the medulla close together, will be noted 

 the stout root of the tenth cranial nerve. In the lateral wall of the medulla, just 

 ventral to the somatic sensory column, is another longitudinal area marked by a 

 row of rounded elevations ; this area is the visceral sensory column. As its name 

 implies, it is associated with sensations from the viscera. In fishes the gills are 

 important visceral structures so that a considerable portion of this column is 

 connected with the gills ; in fact, each of the little elevations is said to be a center 

 for one visceral arch. Ventral to the visceral sensory column is the very slender 

 visceral motor column, from which impulses go to the visceral muscles. We have 

 already learned that the visceral muscles are the muscles of the gill region. In 

 the floor of the fourth ventricle are the two conspicuous somatic motor columns, 

 separated by a median groove. The somatic motor columns are the places of 

 origin of impulses to the somatic or parietal muscles. As previously stated, 

 these in the head consist chiefly of the six eye muscles. 



The fourth ventricle narrows posteriorly and is finally roofed over by the 

 fusion of the walls of the medulla. Shortly beyond this point the medulla is 

 continuous with the spinal cord. The posterior end of the medulla marks the 

 posterior end of the brain but is not sharply defined. 



Draw the dorsal aspect of the brain. Place it in the middle of the page so 

 that the cranial nerves can be added later. 



4. The cranial nerves. — There are ten cranial nerves in fishes. They are to 

 be dissected with great care and their distribution noted and drawn. This 

 distribution is in general similar in all vertebrates except that certain nerve 

 trunks present in fishes disappear in the land vertebrates. One of the most 

 striking examples of homology is found in this distribution of the cranial nerves 

 which still in man continue to supply the same parts as in the fish. 



Add to your drawing of the dorsal side of the brain an outline of the head of 

 the animal, putting in outline the olfactory sacs, eyes, ears, and gill slits. As you 

 dissect the cranial nerves according to the directions to be given immediately, 

 add each to this drawing, showing as accurately as possible the location and 

 course of each of these nerves and the parts of the head which they supply. It 

 is necessary to enter a nerve on one side only, and by using the two sides of the 

 drawing for different nerves it will be possible to enter all of them. 



a) The first or olfactory nerve: This nerve has already been noted. It arises 

 from the olfactory cells in the lamellae of the olfactory sac and passes by very 

 short branches into the olfactory bulb. These branches are practically invisible. 



