PRACTICAL DIRECTIONS 191 



PRACTICAL DIRECTIONS. 



The Organs of Special Sense. 



I. Olfactory organ. Notice again the external and internal nostrils. 

 Then remove the skin covering the snout, dissect off the nasal bones, 

 and open up the olfactory sacs. Note the pigmented olfactory epithefaim 

 lining these, and make out the "olfactory nerves and nasal septum. 

 Compare with a transverse section through the sacs. Sketch. 



II. Eye. 



a. Notice again the eyelids, iris, and pupil. Then remove the skin 

 covering the head so as to expose the nearly globular eyeballs, lying in 

 the orbits. 



In the antero- ventral region of the orbit make out the Harderian 

 gland, and the eye-muscles passing from the walls of the orbit to 

 the eyeball. The four recti and two oblique muscles can be 

 more easily seen on a larger animal, and directions will be given for 

 their examination in Part II (see Fig. 126) ; but if you make a dissec- 

 tfon of them in the frog, you should note at the same time the levator 

 and retractor bulbi, the latter underlying the eyeball, and the former 

 situated internally to the recti muscles. 



b. Remove the eyeball from a freshly-killed specimen, noticing as 

 you do so the optic nerve, which is surrounded by the recti and retractor 

 bulbi muscles : dissect away these muscles and note the cartilaginous 

 sclerotic, the cornea, iris, pupil, and the cut end of the optic nerve. 



c. Divide the eyeball into an inner and an outer hemisphere by a rapid 

 cut with scalpel or scissors taken vertically, midway between the cornea 

 and the optic nerve, through the vitreous chamber. Place them both in 

 a watch-glass or small dissecting dish, under water, and examine with a 

 lens (compare Fig. 57). In the inner .hemisphere note the vitreous 

 humoitr, retina, pigmented choroid, and blind spot or entrance of the 

 optic nerve ; and in the outer hemisphere, the crystalline lens and the 

 margin of the retina, or ora serrata. Sketch. Remove the lens, and 

 notice the iris continuous with the choroid, the pupil, and the aqueous 

 chamber. 



d. Examine sections through the wall of the inner hemisphere of the 

 eyeball, prepared as directed on p. 136, first under the low, and then 

 under the high power of the microscope. Note :- 



i. The cartilaginous sclerotic* 



