Le eS S—“‘ PO 
THE STORY OF THE RED BLOOD CORPUSCLES. 115 
Under some abnormal conditions—for example, after a 
severe hemorrhage, when there is a great demand for more 
blood—nucleated corpuscles may be found in the general 
circulation. Possibly the more watery blood passing through 
the marrow in such circumstances loosens the corpuscles in 
the vascular channels from their attachment before they 
have reached maturity. 
The bone-marrow, then, is the birth-place of the red 
corpuscles, which are derived from specialised cells which 
give rise to nucleated coloured cells. These ultimately lose 
their nuclei and pass into the general circulation. 
The Red Corpuscle in Circulation.—When the red 
corpuscle passes into the circulation, it becomes, so to speak, 
lost in the crowd. For a time its life is spent in a pilgrim- 
age through the blood vessels in the capacity of a carrier 
of oxygen to the tissues. Though still living, it possesses 
none of the active propensities of its white brethren, but is 
passively carried along with the circulating blood. 
Here I should just like to remark that the circulation is 
not the mechanical thing which we are apt to represent it. 
The heart is a pump, the blood vessels are tubes, and the 
circulation of the blood consists in the pumping of the blood 
through these tubes so that it comes back again to its point 
of departure. Such is the idea one is apt to form in one’s 
mind, but such an idea is very far indeed from the truth. 
The circulation of the blood is very far from being a mere 
mechanical process. Rather we should picture it to our- 
selves as altering every instant of the day. The vessels are 
under the control of nerves, and by them the exact supply 
of blood to each organ of the body is regulated to a nicety. 
All over the body the blood vessels are continually dilating or 
contracting, so as to increase or diminish the blood supply 
of each part. We cannot think a thought, or breathe a word, 
or move a limb, without at the same instant altering the 
supply of blood to the parts concerned. 
The precise duration of life of a red corpuscle varies con- 
siderably, and is not easy to determine accurately. Sutffice 
it to say that it is not indefinite. Some corpuscles are 
destroyed within the body, others may be lost by hemor- 
rhage. 
