CIRCULATING SYSTEM. 343 
observed between the size of these particles, and that of the 
body of the animal. The particles of human blood are 
larger than those of the ox; while those of the ox are only 
equal to those of the mouse. Age appears to exercise some 
influence on their size. Mr Hewson found in a chicken, 
on the sixth day of incubation, the particles larger than in 
a full grown hen; and also larger in the blood of a very 
young viper than in that of its mother, out of whose belly 
it was taken. He could not, however, detect any difference 
in size between those of a child at its birth, and those of an 
adult person *. 
Every one is acquainted with the red colour of the blood 
of the vertebral animals. The colouring matter is not equal- 
ly diffused, but adheres to the particles, which it surrounds 
with a thin film. Mr Baver found an entire particle to be 
t7rsoth part of an inch in diameter, but when deprived of 
its colouring matter, only g,/5,th part. While the bleod of 
some animals, as all the vertebrosa, is uniformly of a red co- 
lour; m many other animals it exhibits a different shade. 
Among the crustaceous animals, as the lobster and shrimp, 
it is white; while, among some insects, as the grasshopper 
and white caterpillar, it is green +. 
When blood, newly drawn from an animal, 1s allowed to 
rest, it thickens, and spontaneously separates into two parts, 
a pale coloured fluid termed Serwm, and a denser coagu- 
lum, termed Clot, Cruor, or Crassamentum. The serum 
is the fluid basis of the blood ; the clot is formed from the 
floating particles; and hence the difference of colour and 
consistence exhibited’ by the two portions. 
During the act of coagulation, a sensible quantity of heat 
is evolved, amounting to about 6° Fahr. This appears 
* Phil. Trans. vol. lxiti. p. 321. 
{~ Hewson, Phil. Trans, vol. Ixiii. p. 321. 
