184 ANATOMY OF THE HEAD. 



iris contracts or dilates. The colour oi' the pupil is de- 

 pendent upon the quantity of light admitted to the bottom 

 of the eye ; being blue or green in a good light ; and per- 

 fectly black in a bad one. At the margin of the iris are 

 seen some little globular bodies or bags containing a 

 black pigment {Fig 19) ; the largest of which are attached 

 to its upper margin. These bodies are called the corpora 

 nigra, and appear designed to stifle a portion of the rays 

 when the sun is too powerful ; which, in a contracted 

 state of the iris, they are able to do by almost filling up the 

 remaining opening, so as to admit but little light ; though a 

 crevice may serve for sight. The iris, therefore, it is 

 evident, is a very important part, for by its actions vision 

 is in a great degree regulated. It is stimulated to contract 

 by the excess of light ; and consist of two orders of muscu- 

 lar fibre. One is circular, and serves to render the pupil 

 small. The other radiates from the margin of the iris 

 towards the centre, and answers the purpose of enlarging 

 the pupillary opening. Light is the stimulus which causes 

 the circular order to contract ; also relaxes the straight 

 order of fibres to which darkness supplies energy, as light 

 stimulates the circular ones. 



The choroid coat {Fig 19 . 8) is a secreting membrane, 

 spread over the internal surface of the sclerotic; and the ciliary 

 processes are plaits or folds of it attached to the ciliary liga- 

 ment. The pigment {Fig 19 . c) is distributed over the greater 

 portion of the internal surface of the choroid coat ; but is 

 deficient on a part named tapidum lucidum. The tapiduni 

 lucidum is of a green appearance, and has a metallic lustre ; 

 it occupies rather the upper portion of the internal membrane 

 of the choroid {Fig 1 9) . Its use is to collect the stray rays of 

 light, and by reflecting them to the opposite retina, to enable 

 the horse to see when feeding off the ground during the night ; 

 as the use of the pigment is to absorb all rays of light that 

 touch it, and thus render vision single, distinct, and clear. 

 The pigment and the tapidum lucidum, therefore, form one 

 coat. The secreting membrane constitutes another ; but 

 besides these two, there is a third called the vena vorti- 

 cosa {Fig 19 . a), which is placed most outward, being 

 immediately underneath the sclerotic. This last of the 



