122 ANATOMY 



The area of skin divided by (body weight, grams) ^ gives 

 values ranging from 9.56-10.37 with a mean of 9.93. The 

 authors conclude that the area of skin in square centimeters may 

 be expressed approximately by (body weight, grams) ^ X 10. 



Hair. The arrangement of the hairs is reported by E. C. 

 Greene ('24 MS.) as follows: 



The hairs on the albino rat, over the greater part of the body, 

 are arranged in groups of nine, made up of three small groups 

 in a row, each containing three hairs growing very close to- 

 gether. One hair of the middle group is coarse and corresponds 

 to the spine in related forms. The others are fine. The hair 

 pattern shows parallel rows with the groups of one row alter- 

 nating with the groups of the next row. 



In the region of the upper lip and nose the bristles are de- 

 veloped as long vibrissae, growing in five or six parallel rows 

 running from the nose backwards along the upper lip. The 

 number in a row varied from five to ten. Some of the vibrissae 

 are longer than the whole head of the animal. 



In addition to the vibrissae in the region of the nose, there 

 are a few others, usually three just above the eye, another 

 slightly posterior and a little below the outer corner of the eye, 

 and two more near the corner of the mouth. 



The eyelashes are very fine and short. 



The palms and soles are devoid of hair but the backs of the 

 paws are sparsely covered with short thin hair. 



The hairs on the tail are reduced to three rather short bristles 

 emerging from under the edge of each scale. 



On the tip of the scrotal sac the hair is sparse and fine. 



The scales on the tail are arranged in rings and of these there 

 may be as many as 260 in large specimens. Sweat glands are 

 present on the soles and palms — Romer (96). 



9. Thoracic and abdoviinal viscera. For the combined weights 

 of the several viscera see tables 153-156 and chart 49 (Nos. 

 1-21). 



G astro-intestinal system. In the rat both the tonsils and the 

 gall bladder are absent. The volumes of the liver and the 

 pancreas cells, with those of their respective nuclei, have been 



