46 Practicum VII. The Humors of the Eye. 



16. The Optic Nerve. With the fingers and forceps tear apart the 

 muscular masses surrounding the optic lu-rve and remove them with the 

 scissors. Note the fibrous constitution of the nerve and the firmness of 

 the sheath. 



a. Although the adult optic nerve is solid, excepting a small artery 

 which traverses it, it was developed as a tubular outgrowth from the brain, 

 as described in the lectures. 



b. The place of attachment of the nerve is eccentric, i. e. } neare 

 one side of the ball ; in the sheep it is ven trad of the center ; in man, 

 mesad. 



17. The Aqueous Humor. Compress the scleral part gently but 

 firmly so as to render the cornea tense. With a very sharp scalpel-point, 

 (borrowed from tlie instructor if necessary) slit the cornea for the middle 

 third of its long axis, not letting the point enter more than two mm , and 

 relaxing the pressure just as soon as any liquid escapes ; this liquid is the 

 AQUEOUS HUMOR ; as its name implies it is naturally watery and clear, but 

 now it probably contains black particles dislodged by manipulation. 



18. The Iris and Pupil. With the forceps carefully raise either cut 

 edge of the cornea, making sure that only it is grasped, and with the 

 scissors clip away the cornea, piecemeal, to within not less than i mm. of 

 the brown boundary line. This will expose the IRIS, a dark lamina, co- 

 extensive with the cornea, and presenting a central orifice, the PUPIL. 



a. In the sheep the pupil, like the cornea, is elliptical, and the long axis horizontal ; 

 in the cat the long axis is vertical ; in man the pupil is circular. The human iris varies in 

 color, whence the name, signifying a rainbow. 



b. T^he contraction of the pupil in response to an increase of light 

 is familiar to all ; it may be observed by closing the eyes in a dark room, 

 keeping them closed while entering a light one and approaching a mirror; 

 when opened the dilated pupil rapidly diminishes in size. 



c. When a cat is about to spring, even in play, the pupils commonly 

 dilate. 



d. Make a diagram of the cephalic aspect of the ball including the 

 appearances now presented. 



19. The ^Anterior" Chamber. By inspection and careful use of 

 the probe or tracer determine that the periphery of the iris is attached at 

 or near the corneal margin ; the space between the iris and the cornea 

 was filled, naturally, with the aqueous humor already mentioned, and is 

 called the "anterior chamber." The manipulation ( 18) may have 

 crowded the iris cephalad so as to nearly obliterate the interval. 



a. Through the pupil will be seen part of the CRYSTALLINE LENS, 

 naturally transparent but rendered opaque by the alcohol ; it will be ex- 

 amined presently. 



20. Transecting the Eye. The other eye is to be transected like an 

 orange, as follows : Hold firmly but with the least possible pressure ; at 

 any point on what might be called the equator, apply the sharpest attain- 

 able blade and use with sawing movement till the sclera is divided and a 

 drop of liquid escapes. Then insert a scissors-blade not more than 5 

 mm.; cut for this distance, withdraw the blade and reinsert in the same 

 way, until the first incision is reached. As the two halves separate keep 

 the cephalic side down and lift off the caudal half like a hemispheric 

 lid or cup ; place the cephalic half carefully in weak alcohol. 



S 21. Examine the cephalic (ental) aspect of the caudal half. Near, 

 but not at, the center is a spot of more or less distinct radiation corres- 

 ponding with the place of attachment of the optic nerve ( 16); in the 



