THE DOGFISH 23 



limited internally by a thin firmer layer with smooth surface, 

 the tapetum. (In life the tapetum reflects brilliantly and pro- 

 duces the green color of the eye; in preserved specimens it largely 

 loses this power.) Inside the tapetum is the retina, light in 

 color, which in laboratory material has usually separated from 

 the other layers except where it is connected with the optic 

 nerve. Note the radiations from the point where the optie 

 nerve enters. 



In the outer half of the eye recognize the same parts and note 

 also the large lens held in place by a delicate membrane from 

 the margin of the retina. Remove the lens and see that the pi;- 

 mented layer extends farther forward than the retina, this part 

 being the iris with a central opening, the pupil. Observe that 

 there is a space between iris and lens, the posterior chamber of 

 the eye. See also that the irideal portion is free from the outer 

 wall of the eye, there being an anterior chamber in front of it, and 

 that the solera passes into a transparent layer, the cornea, which 

 forms the front of the eye. (Is really covered with a conjunctiva, 

 but this cannot be demonstrated by ordinary dissection.) 



Draw a diagrammatic section of the eye, showing the lens, 

 iris, layers, etc. 



The Cranial Nerves. These are not readily followed in 

 numerical order, as the dissection of one frequently involves 

 the destruction of another. By following the directions here 

 given all will at last be seen. As made out each should be inserted 

 in its proper position in the sketch of the brain. 



First follow the ophthalmicus superficialis nerve (p. 22) 

 forward to the tip of the snout, noting its size and its numerous 

 branches. Does it pass dorsal or ventral to the eye-muscles? 

 Follow it backwards, cutting away the cartilage, to its point of 

 emergence from the brain. From which of the divisions of the 

 brain does it arise? Do you find any indications of a double 

 origin to it, as if it were composed of two nerves? 



Now follow the olfactory nerve, I, from the olfactory lobe 

 forward to its expansion on the wall of the olfactory organ. 



One of the eye-muscle nerves, the trochleans, has already 

 been seen- Look on the ventral surface of the external rectus for 

 the abducens nerve, VI, and follow it back as far as possible 

 without cutting anything. The oculomotor nerve, III, can be 

 found on the ventral surface of the inferior rectus. Trace its 



