108 DR W. B. DRUMMOND. 
of its own, only a part of which is spent in the circulating 
blood. Each corpuscle has a place and period of birth; 
each corpuscle has a period of growth and development, a 
period during which it displays all the essential char- 
acteristics of living things—respiration, absorption, assimila- 
tion, excretion—a period, lastly, of decay and death. 
The whole story of the life-history of a blood corpuscle 
involves the study of many other structures besides the 
blood itself. It is some chapters of this story which I desire 
to bring before your notice to-night. 
The Structure of the Blood.—Blood consists of a fluid, the- 
plasma or liquor sanguinis, in which float corpuscles of 
several kinds. The corpuscles are chiefly of two sorts, 
coloured and colourless; and in addition to these there are 
very minute particles, much smaller than the corpuscles, 
known as the blood platelets. To these latter we need not 
again refer, 
The white or colourless corpuscles are found in the- 
blood of all animals. In the invertebrates all the corpuscles 
are colourless, and the same is true of the blood of the 
lowest vertebrate, Amphiowus lanceolatus. In all other 
vertebrates both white and red corpuscles are present,. 
and the red are the more numerous. 
The history of these white corpuscles has during recent 
years excited the keenest interest, and it is with regret 
that we pass by the romance of their emigration from the 
blood vessels, of their wanderings in the tissues and spaces 
of the body, of the battles they fight with the invading 
hosts of bacteria, of their devotion even to death in the 
cause of the body they serve. Of these things we may 
not speak now. A brief description, however, of the 
chief varieties of white corpuscles will assist us in under- 
standing the structure of human blood. 
All white corpuscles are minute nucleated masses of 
protoplasm. Several kinds of these can be recognised 
without difficulty. 
1. The Lymphocytes—These form from 20 to 25 per 
cent. of all the white corpuscles in normal human blood. 
They are small cells, spherical or nearly spherical in form,. 
and their protoplasm is very scanty, forming, as a rule,. 
