232 AtflMAL PHYSIOLOGY. 



transparent substance, a soft, whitish, pulpy membrane, the 

 retina, or internal tunic of the eyeball. The retina is a 



nervous structure, containing 

 the distribution of the optic 

 nerve, a layer of nerve-corpus- 

 cles, and the nerve-terminations 

 on which the rays of light act; 

 it is adherent to the other tunics 

 at the optic pore, the place where 

 the optic nerve pierces; and it 

 conies to an apparent margin 

 in front, not far from the ciliary 

 processes. This margin, in the 

 human eye, is scalloped, and 

 called the ora serrata. In the 

 human eye also, when a per- 

 Fig. 116. SECTION OP HUMAN fectly fresh specimen is ex- 

 Em a. Sclerotic; b, cornea; am i lie d the retina will be seen 

 c, conumctiva ; a, iris : c, , -, -, -. 



crystalline lens closely in- **> be nearl 7 transparent, and 

 vested with its capsule, and, of a delicate pmk tint; and 

 above and below, sections of there will be noticed, directly 

 the canal of Petit, bounded opposite the centre of the pupil, 

 in front by suspensory liga- .-i . . j. 1 * 



ment, and behind by hyalSid *M 1S ^ say, a tenth of an 

 membrane. The radiating inch outside the optic pore, a 

 white lines round the lens structure which does not exist 

 are the most prominent parts i n domestic animals, namely, the 

 of the ciliary processes, the 77 . / .. rf 



only parts uncovered with 2/f/T f* / Sfymmnff, an 

 dark pigment ; /, ciliary elliptical mark, of a yellow 

 muscle in section ; g, retina, colour, with a depression in 

 with ora serrata in front ; h, the middle, called fovea cm- 

 optic nerve. . -i , 



The retina, notwithstanding its being so thin, is one of 

 the most complex structures in the body, and reveals this 

 complexity when, after suitable preparation, it' is examined 

 under the microscope in sections made vertically through, it. 

 Close to the surface which rests on the transparent media 

 of the eye, is a layer of fine nerve fibres, the expansion 

 of the optic nerve, and beneath this a layer of multipolar 

 nerve-corpuscles; and in these strata are the ramifications of 

 the retinal artery, which breaks UD into branches at the 



