74 THE STRUCTURE AND LIFE OF BIRDS chap. 



sequently, close against an inrush of blood, but allow 

 an outrush to pass. In a bird of any size they are 

 easy to see. 



Some account of a reptile's heart will be found in 

 the chapter on " The Bird within the Egg." 



The Blood. 



Cut off the supply of blood from a limb, and all its 

 power goes. The muscles lose their sensitiveness to 

 stimulus, and eventually rigor mortis, the stiffness of 

 death, sets in. The life of the whole organism and 

 of each of its parts depends upon the blood. The 

 Jew, who will not eat blood " because it is the life," 

 has dimly seen an important physiological fact. 



Blood consists of corpuscles of two kinds, the red and 

 the colourless, commonly called white, and the liquid 

 plasma in which these float. The corpuscles are very 

 minute. It is said that 10,000,000 of the red ones will 

 lie on a space of one square inch. Among birds the 

 Cassowary has the largest, the Humming Bird the 

 smallest. The colourless ones also are always ex- 

 tremely small, though they vary much in size. In 

 shape the red ones, seen under the microscope, are, 

 in birds and reptiles, oval ; in man, round. And 

 the shape is not the only difference. In birds, reptiles, 

 and fishes there is. a nucleus, a small roundish body in 

 the middle. In mammals, except in the case of em- 

 bryos, no sign of this is, as a rule, to be found. Why 

 this nucleus has disappeared, no one has been able to 

 show. It certainly cannot be maintained that there 

 is any superior vitality which we can associate with its 



