THE PLACENTALIAN EYE 679 



angle and join to the cornea, thus contributing to the 'pectinate ligament' 

 (or sometimes forming the whole of it) . At the other 'end' of the iris — 

 the pupil margin — cystic protrusions of the pigmented retinal layers 

 form the corpora nigra or 'grape-seed bodies' (Fig, 86, p. 219) which 

 are characteristic of the highest artiodactyls (tylopods, and ruminants 

 except Tragulus) and also of the highest perissodactyls (horses) . Where 

 a dilatator is present, the anterior retinal layer is pigmented only slightly 

 or not at all; otherwise, it is as dark as the posterior layer, as in lower 

 vertebrates in which it has not partly differentiated into muscle. The 

 color of the iris is usually dark brown. Where it is not, the color is 

 generally optical, as in the 'blue' human eye; but lipophores and irido- 

 cytes may be present in the stroma, as in the cats and some prosimians. 



The organization of the ciliary body in all placentals is basically the 

 same as has been described earlier for man. A corona (bearing true, 

 vascular, ciliary processes) and an orbiculus (smooth, or bearing only 

 low meridional ridges) can usually be distinguished. In carnivores how- 

 ever, the posterior ends of the processes are practically at the ora (Fig. 

 197, p. 683) ; and in Orycteropus and ungulates, whose corneae are mark- 

 edly ovoid horizontally, the obligation of the coronal zone to remain 

 circular (to 'fit' the lens) results in an encroachment upon the iris, 

 nasally and temporally, by the anterior ends of the processes — so that 

 these portions of the iris serve as extensions of the base-plate of the 

 ciliary body, and are rendered immobile as regards changes involved in 

 the operation of the pupil. In ungulates and in many carnivores the 

 orbiculus is practically eliminated nasally owing to the existence of 

 marked nasad asymmetry (see p. 300). 



Except in very small eyes, the main part of the ciliary body (apart 

 from the processes, that is) gradually thickens toward its anterior end, 

 as in man. This bulk of uveal tissue is not, however, solid muscle as in 

 the primate eye. Muscle — sometimes considerable of it, as in carnivores 

 — is almost always present, but is in the form of slender fascicles inter- 

 spersed with much connective tissue. Anteriorly, the ciliary muscle tends 

 to have two anchorages: one, by means of the meshwork tissue which 

 terminates at the margin of Descemet's membrane (as in man — see Fig. 

 5, p. 10), and another attachment into the anteriormost portion of the 

 base plate, practically in the root of the iris. Between these two anterior 

 leaves of the muscle lies a nearly empty space, best visualized by imagin- 

 ing the human filtration angle to be eroded or extended backward deep 

 into the ciliary body. This space, 'Fontana's space (s)', is traversed by 



