xi 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, using a lens, the pigmented olfactory epithelium lining 

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

 Compare with a transverse section through the sacs. Sketch. 



II. Eye. 



a. Notice agam the eyelids, iris, and pupil. Then remove 

 the skin covering the head so as to expose the nearly 

 globular eyeballs, lying in the orbits. Examine with a lens. 



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 dissection 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 nene, 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 hemi- 

 sphere 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 humour, 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 or a 

 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 hemi- 

 sphere of the eyeball, prepared as directed on p. 136, first 



