ON THE BLOOD — STOMACH. 51 



and in several instances communicate directly with the 

 lungs, add, of course, to their facility of flight. Even the 

 bones of the Ostrich, although this bird cannot fly, are 

 hollow ; and he is also furnished with air vesicles similar 

 to other birds, which, notwithstanding he cannot leave the 

 earth, enable him, by the assistance of his powerful and 

 muscular legs, to run with astonishing swiftness. Mr. 

 Green informed us in his Lectures on the comparative ana- 

 tomy of Birds at the College of Surgeons, (April, 1827,) that 

 in young birds a medullary substance was often observable 

 in the bones, but that, as they grew up to maturity, it be- 

 came absorbed, and the bone empty. 



It may be stated, too, that the blood of Birds is generally 

 of a brighter colour, and warmer, than that found in the 

 mammalia, and that it circulates with much more rapidity. 

 While the Horse has about forty pulsations in a minute, 

 man from seventy to eighty, in Birds they vary from one hun- 

 dred to one hundred and ten. From the extreme mobility 

 and activity of Birds, it would seem that they are more 

 highly oxygenated than other animals ; in addition to which 

 it may be mentioned, that Birds consume more food in pro- 

 portion to their size, in a given period, than any other race 

 of animals. 



Perhaps, however, one of the most striking peculiarities 

 in the anatomical structure of Birds is the stomach. In those 

 whose food consists principally of grain and seeds, the 

 stomach is cartilaginous, and covered with very strong mus- 

 cles: in this state it is called a gizzard. This structure 

 is necessary, in order that, by its strong action, the food 

 should be comminuted ; but, besides this, birds with such 

 stomachs pick up and swallow, occasionally, small gravel 

 stones, which assist the process of comminution. In a state 

 of nature, the quantity of gravel taken in is regulated, no 



