HEAD AND NECK OF THE HORSE 



121 



carried by fibres derived from the oculomotor through the short ciliary 

 nerves. Radially arranged fibres {m. diUdaior pu^nllo/) diverge from 

 the circular sphincter to the periphery of the iris, and are governed 

 by fibres from the sympathetic nervous system through the short ciliary 

 nerves. 



The iris has two margins. One (margo pupillaris) circumscribes the 

 pupil, and carries darkly pigQiented masses (granula iridis), varying in 

 number and size, along both margins of the pupil. Those of the upper 

 margin are much the larger. The ciliary border (margo ciliaris) is 

 connected with the ciliary body and, by the ligamentum pectinatnm 

 iridis, with the posterior lamina of the cornea and the corneo-scleral 

 junction. 



Iris. , 



Crystalline lens 



^Granules of iris. 



Choroid. 



Fig. 51. — Iris and adjoining part of the choroid viewed from the front. 



The iris is supplied with blood by the long and anterior ciliar}^ 

 arteries. The two long ciliary vessels (page 114) pierce the sclera close 

 to the entrance of the optic nerve, and run forwards between the sclera 

 and the choroid. Near the periphery of the iris each divides into two 

 branches, which, by anastomoses of their subdivisions with each other 

 and with twigs from the anterior ciliary arteries, form the circidus 

 arteriosus major. Branches from this circle supply the ciliary body, 

 while others pass towards the pupillary border of the iris, and there 

 form a circidus arteriosus minor. 



Dissection. — The retina may he demonstrated in several ways. By 

 carefully removing the vitreous body from tlie eyeballs that have Ijeen 

 sectioned, a good view of the retina from within may be obtained. It 

 may also be studied in the specimen in which the sclera and cornea have 

 been removed for the exhibition of the vascular tunic. In this specimen, 

 the choroid, ciliary body and iris must be cautiously removed bit by bit. 



The pigmentary (outermost) layer of the retina'^ is so closely 

 adherent to the deep face of the choroid that it is removed along with 



1 Bete [L.], a net ; retina [low L.], something to hold fast, to keep ; (in the 

 retina images are captured). 



