SENSE ORGANS^ — BLOOD 141 



Structural conditions. At 12 days the rods and cones are fairly 

 well defined, but the accessory apparatus is less complete. 

 At the time when the eyes open the plexus of blood vessels to 

 the lens is still present, though much reduced. 



Integument — moulting. The rat has a heavy winter and a light 

 summer coat of hair. The winter coat is moulted in the early 

 spring and the summer coat in the early autumn. The amount 

 of hair at the two seasons, and according to age, has not been 

 determined. During the moult the outline of the "hooded" 

 pattern is sometimes revealed by the slower growth of the hair 

 in the area of the pattern. This is more often seen in young 

 animals. 



Seinicircidar canals. When rats are rotated in a horizontal 

 plane for ten seconds at the rate of 120 revolutions per minute, 

 with the body fixed in a glass tube, the center of gravity of the 

 rat centered on the rotator, and the head free, and then the 

 rotation is suddenly stopped, ocular after-nystagmus appears. 

 The number of seconds during which the after-nystagmus con- 

 tinues is given in table 94, where the data are entered according 

 to the age of the animal (J. A. Detlefsen, MS.) 



All the tests here recorded are initial, the rat having had no 

 previous experience of this sort. Table 94 shows an evident 

 decrease in after-nystagmus time with advancing age. In yet 

 older rats the decrease is still more marked. 



5. Circulation. See References. 



6. Blood and lymph. Folin and Morris ('13) examined the 

 blood from six full-grown rats which was collected over a little 

 powdered potassium oxalate; the uric acid, total non-protein 

 nitrogen and urea were determined by the methods of Fohn and 

 Denis ('13), 



The figures obtained for 100 grams of blood were as follows: 

 Uric acid, 2 mgm.; non-protein nitrogen, 38; urea, 22. This 

 experiment was repeated twice, each time using for the analysis 

 the mixed blood of six normal white rats, and 2.4 mgm, and 2.5 

 mgm. respectively of uric acid per 100 grams of blood were found. 

 These are substantially the same figures as Folin and Denis 

 ('13) found for normal human blood. 



