678 MAMMALS 



and to a lesser extent in some other nocturnal prosimians (e.g., Nycti- 

 cebus), the eye is 'tubular' (Fig. 84, p. 213). In large-eyed mammals, 

 it is common for the lens and cornea to be shifted nasally as in birds, 

 and for the ciliary body to be consequently narrower nasally than tem- 

 porally, chiefly at the expense of the nasal orbiculus, which may be quite 

 abolished (see cougar. Fig. 71). 



The sclera never contains any traces of cartilage. It is usually thickest 

 in the fundus and thinnest at the equator; but the cornea may be much 

 thicker than any part of the sclera, or much thinner — the local differ- 

 ences in the thickness of the fibrous tunic are so various that they cannot 

 be covered in a few words. A Bowman's membrane is seldom discrimin- 

 able; but Descemet's layers are always present, and the elastic membrane 

 may be enormously thick in large eyes. An exceptional cornea is that of 

 the armadillo (Dasypus novemcinctus), in which the substantia propria 

 contains many capillaries, even at the apex. These are perhaps required 

 by the fact that the corneal epithelium, being strongly keratinized, can 

 derive no sustenance from the tear fluid. 



Except where a tapetum lucidum has been produced in it, the chorioid 

 is usually built as it is in man, but is seldom so thick. It is exceptionally 

 thin in the squirrel family; but the most unusual chorioid is that of the 

 large bats (Megachiroptera). In these forms there are 20,000-30,000 

 conical, vascular papillae which are protrusions of the chorioid, inter- 

 digitated with the retina and deforming the latter's visual-cell layer (see 

 Fig. 102a, p. 255). Kolmer found this situation in all sixteen of the 

 species he studied, but not in any of an equal number of microchiropteran 

 species. Five structural types of papillae can be recognized; and more 

 than one type may occur in one species, in different retinal regions. 



The iris in large eyes (carnivores and seals, ungulates, whales, pri- 

 mates) has essentially the same constituents as in man. All of these 

 mammals have a dilatator, histologically and embryologically resembling 

 that of man, but with a topographical arrangement which depends upon 

 the shape of the contracted pupil (see Fig. 85, p. 218). In well-adapted 

 aquatic placentals (otters, seals, whales), and also in the pigs, the 

 sphincter occupies the entire width of the iris; and the dilatator may 

 send fibers into the ciliary body for firmer anchorage. In the smaller, 

 nocturnally-adapted eyes of all the lower orders of placentals, a dilatator 

 is ordinarily lacking; but the sphincter is always in evidence and some- 

 times very large, though always compactly massed near the pupil margin. 

 Toward the root of the iris, stromal strands may cross the filtration 



