234 ADAPTATIONS TO NOCTURNAL ACTIVITY 



shine of the domestic cat has been known literally for millennia : it was 

 the basis of the reverence shown the cat by the ancient Egyptians, who 

 believed that the cat's eyes magically reflected the sun even at night 

 when it was hidden from mere man. Although the typical carnivore 

 tapetum is the same large triangle as the tapetum fibrosum of an un- 

 gulate (Fig. 93a), and resembles it ophthalmoscopically even to the 

 presence of the 'little stars of Winslow', it is very different histologically 

 and in evolutionary origin : 



Endothelial cells, such as lurk in the meshes of any chorioid, have 

 proliferated just outside of the choriocapillaris to form several layers 

 of thin, broad, tile-like cells (Fig. 92b) . Unlike the arrangement of cells 

 in a true stratified epithelium, there is a tendency in the tapetum cellu- 

 losum for the boundaries of each cell to coincide with those of cells in 

 the layers immediately above and below — in other words the courses of 

 brickwork are not staggered. The connecting capillaries, running to the 

 choriocapillaris from vessels in the outlying, normally vascular, pig- 

 mented layers of the chorioid, can consequently take quite straight 

 paths and thus interfere but little with the action of the tapetum. 



The number of layers of cells may be only four or five, as in the 

 wolverine, or as many as 15 as in the cat (the dog has 10, the lion 8 

 to 10). The numbers are higher in the seals, however, ranging from 

 16-18 to 30-35 (in Phoca barbata). In one seal (Halichoerus gryphus) 

 the tapetal cells are so elongated as to simulate a tapetum fibrosum; but 

 they are still cells, not inert connective-tissue fibers as in a true tapetum 

 fibrosum. The seal tapetum covers a great area of the retina, usually 

 extending at least to the equator of the eyeball in all meridians and often 

 much farther than this on the temporal side, the retinal region which 

 looks ahead of the animal. This record-breaking area of tapetum in the 

 seals will appear significant when we consider its special purpose in 

 these animals (pp. 446-8). 



Though the elements of a tapetum cellulosum (unlike those of the 

 true fibrosum type) are Uving cells, there is not room in them for much 

 protoplasm. They are packed with highly refractive threads or rodlets, 

 in some cases long and with crossings and recrossings to form a felt- 

 work, in other cases very short and set in serried rows so that a 'herring- 

 bone' pattern is created (Fig. 93b). These inclusions are formed of 

 some organic substance, perhaps different in different cases, whose 

 chemical nature is unknown; but they appear to be crystalline and homo- 

 geneous. In the cat they are yellowish, about 10[X by 0.5-1 [1, and 



