Practicum VII. The Tunics of the Eye. 47 



drawing this should be below, indicating that it is toward the ventral side 

 of the eye. 



22. The Timics or Coats of the Eye. If the cut edge is gently 

 manipulated at any point it will separate into an ectal, white portion, the 

 SCLERA ( 12), and an ental, the CHOROID and RETINA. 



a. The thick and fibrous sclera, with the cornea which is really 

 only a transparent portion of the same tunic, gives firmness to the eye, 

 protects the more delicate other tunics, and gives attachment to the mus- 

 cles. In birds and in the sword-fish it is more or less completely ossified. 



23. The Retina. Dip the specimen in alcohol. With the tracer, 

 applied near the ventral cut edge, scrape the ental surface gently. There 

 should be separable a delicate, almost filmy layer, the RETINA, constitut- 

 ing the ental of the three tunics. 



c"> 



24. The Choroid. Between the retina and the sclera is the middle 

 tunic, the CHOROID, black and firmer than the retina. 



25. The relative positions and general characters of the three tunics 

 are easily recognized, but some confusion may arise (and indeed existed 

 in former issues of these directions) from conditions now to be stated. 



a. Part of the retina really consists of two layers, an ental white 

 and soft, and an ectal, jet black in color from the abundant pigment. 



b. Even in those regions where the pigmentary layer exists parts of 

 the latter sometimes separate from the white layer and adhere to the 

 choroid, appearing to constitute a part of it. 



c. The ental surfa-ce of the sclera is pigmented and brownish (lamina 

 fusca). 



26. If care be taken the retina can be detached from the choroid at 

 most points, and drawn or pushed toward the center. . 



a. Note the absence of the YELLOW SPOT which, in man, occupies 

 the center just opposite the pupil. 



b. Ventrad of the center (mesad in man) is the place of entrance of 

 the optic nerve, of which the retina is really the expansion, constituting 

 the BLIND SPOT. 



27. The Tapetum and Fovea. When the retina has been removed, 

 part of the choroid will be seen to present a rich purple color, the 

 TAPETUM. 



a. The tapetum is absent in man, but present in many animals. The corresponding 

 area of the retina lacks the pigmentary layer (g 25) so that in the living or fresh eye, the 

 tapetum shows through it and reflects even a very faint light ; hence the glare that in the 

 darkness is so startling in cats and many other animals. The common impression that this 

 ''eye-shine" is peculiar to dangerous species has sometimes caused needless terror to per- 

 sons or led to the killing of innocent quadrupeds. 



b. In man, on the other hand, at the middle of the retina, just opposite the pupil, is 

 a depression which, from its color, is called the YEXI,OW SPOT, and which is the place of 

 most distinct vision ; in the sheep there may be a slight depression, the FOVEA, but not 

 the yellow color. 



28. The Vitreum. In the cephalic half of the alcoholic eye is a 

 jelly-like mass ; this is the VITREUM or vitreous humor ; it is naturally 

 transparent and nearly liquid, much like the white of egg before boiling. 

 It is inclosed in a delicate membrane or capsule. Holding the specimen 

 tilted a little, with the tracer push the vitreum at one side gently toward 

 the middle. It will separate readily from the retina which lines the 

 caudal portion of this half of the eye, but somwhat less so from a dark 

 circular strip of the choroid presenting numerous radiating furrows and 

 ridges, the CILIARY PROCESSES. 



