HUMORS; UVEAL TRACT 13 



in position but only slightly altered in shape, and it is held firmly in 

 place by ligaments which the lower fishes lacked; but the differentiation 

 in consistency of the intra-ocular media has never been abandoned. This 

 is probably fortunate, as otherwise, in animals above the fishes, the evolu- 

 tion of a muscular iris and mobile pupil might have been inhibited. 



The Uveal Tract — The layer of the eyeball wall next inside the fibrous 

 tunic, clinging closely to the sclera but swinging inward away from it at 

 the sclero-corneal junction, is the uveal tract or uvea. The part of the 

 uvea which is attached to the sclera is a thin, deeply pigmented layer 

 consisting mostly of blood vessels, with connective tissue binding them 

 into a membrane. It is called the chorioid coat (Fig. 3; Fig. 4a). The 

 pigmentation of the chorioid prevents internal reflections and keeps light 

 from getting through the wall of the eyeball indiscriminately, and the 

 rich vascularity of the tissue is concerned with the nutrition of the highly 

 metabolic retina. 



Against the inner surface of the uveal tract, throughout its extent, 

 lies the retina (Fig. 3). Where it is in contact with the chorioid, the 

 retina is thick (pars optica) and is sensitive to light. The anterior por- 

 tions of the uveal tract are lined with a thin, insensitive continuation of 

 the retina (pars caeca), which thus really terminates at the rim of the 

 pupillary aperture. 



The sensory part of the retina has the form of a cup whose lip, the 

 'ora terminalis' (Fig. 3; Fig. 5, oO, is an important landmark inside the 

 eyeball. From the ora forward, both retina and uvea are profoundly 

 modified. The chorioid, at that point, thickens and ceases to be so 

 heavily pigmented and vascularized. The thickened region forms, in a 

 sagittal section, a slender triangle with its narrow angle aimed pos- 

 teriorly to merge into the chorioid. This thickened zone of the uvea is 

 called the ciliary body (Figs. 3 and 5), and it is characterized by the 

 presence of many involuntary muscle fibers and, on its inner surface 

 anteriorly, a large number (70-80) of radially arranged fin-like struc- 

 tures, the ciliary processes. Each ciliary process (Fig. 3; Fig. 6c; Fig. 7g) 

 is essentially a fold of non-sensory retina covering both sides of a flat 

 sheet of small blood vessels. Retina and uvea thus intimately cooperate 

 to form the ciliary processes. The anterior part of the ciliary body which 

 bears them is termed the corona ciliaris. At the posterior ends of the 

 processes they diminish in height and fade down to the level of the val- 

 leys between them. This leaves, between the hind ends of the processes 



