230 



ANIMAL PHYSIOLOGY. 



pupil, which, appears black, because few of the rays of light 

 which enter it are reflected from the camera behind. The 

 iris is attached round about to the margin of the sclerotic, 

 where it meets the cornea; and it has two sets of muscular 

 fibres; a circular set round its pupilary margin, which con- 

 tracts the pupil; and a radiating set towards its circumference, 

 by which the pupil is dilated. The circular fibres are 

 governed by the third cranial nerve, the radiating fibres by 

 the sympathetic. Division of the sympathetic in the neck 

 causes the pupil to contract, while stimulation of the divided 

 trunk causes it to dilate. 



A 



Fig. 114. PIGMENT CORPUSCLES Fig. 115. SECTION OF CHOROID 



OF THE CHOROID COAT. A, OF THE Ox, diagrammatic 



Branched corpuscles from the view, a, Arteries and veins; 



connective tissue of the choroid b, arterioles and venous radicles 



coat : the white spot in each is piercing the tapetuin ; c, mein- 



the nucleus. B, Pigmentary brane of Ruysch. 

 epithelium on the inner surface 



. of the choroid coat. 



In making a dissection of the eye, such as every student 

 may easily make for himself on the eye o'f a sheep or an ox, 

 if the sclerotic and cornea be carefully removed, there is 

 brought into view a second coat, the tunica vasculosa, of which 

 the iris is the anterior part, while the posterior part, cor- 

 responding in extent with the sclerotic, is called the choroid. 

 The choroid coat consists of exceedingly closely-set small 

 arteries and veins, imbedded in connective tissue, the branched 

 corpuscles of which are loaded with pigment; and the capil- 

 laries uniting these vessels are thrown inwards to the deep 

 aspect of them, where they form one of the closest networks 

 in the body, the membrane of Ruysvh. Behind the periphery 



