CHORIOIDEA 



537 



adheres to the chorioid when that tunic is removed, although, 

 in reality, it is a portion of the retina. 



In the eyes of many mammals, but not in man, the posterior part of the 

 chorioid, when viewed from the front, presents an extensive brightly-coloured 

 area, which exhibits a metallic lustre. The appearance is due to the 

 presence of an additional layer in the chorioid, termed the tapetum. In 

 the horse, elephant, and ox, the tapetum is composed of fibres (tapetum 

 fibrosum) ; in carnivora, it is formed of cells (tapetum cellulosum). In the 

 ox, it is a brilliant green colour with a golden lustre ; in the dog, it is white 

 with a bluish border ; in the horse, it is blue with a silvery lustre. 



The chief bulk of the chorioid coat is composed of blood 

 vessels. They are arranged in two well-marked layers, viz., 



Sinus venosus sclerae 



Anterior ciliary, 

 artery 



Scler 



Vena vorticosa 



Long posterior 

 ciliary artery' 



Anterior ciliary 

 artery 



Ciliary muscle 



_Long posterior 

 ciliary artery 



Vena vorticosa 



.Long posterior 

 ciliary artery 



FIG. 228. Dissection of the Eyeball showing the Vascular Tunic and 

 the Arrangement of the Ciliary Nerves and Vessels. 



a deep, closely-meshed capillary layer called the lamina chorio- 

 capillaris, and a more superficial venous layer composed of 

 the vasa vorticosa. The short posterior ciliary arteries pass 

 forwards between the two vascular layers. 



The eyeball in which the outer surface of the chorioid is exposed should 

 be immersed in water and the pigment washed out of it by means of a 

 camel-hair brush. The vasa vorticosa will then appear as white curved 

 lines converging towards four or five points, from which the venae vorticosse 

 take origin (Fig. 228). 



Corpus Ciliare. The ciliary body is separable into an 

 antero-external part, the orbiculus ciliaris, and a postero- 

 internal part, the corona ciliaris. 



The orbiculus ciliaris consists of the ciliary muscle, the 



