74 THE STRUCTURE AND LIFE OF BIRDS cuap, 
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 
