Vol. 8, 1922 PHYSIOLOGY: LUNDSGAARD AND VAN SLYKE 
281 
the arteries, plus that of the fraction (1 — a) of arterial blood from the 
lungs. In such a case 
A = {I - a)lT + aV (2) 
A - IT 
OL = (3) 
V -IT ^ ^ 
Substituting V — D for A in (3) and solving for F, 
y = 1 r + (4) 
/ — a 
Substituting A + D for F in (3) and solving for A, 
A =IT ^ (5) 
\ — a 
We now substitute for A and V m (1) their values as expressed in (4) and 
(5), and obtain 
c = /r + ^l^t^ (6) 
2(1 - a) 
which indicates the effects of the four factors, /, T, a, and D on C. 
An interesting point brought out by (6), and in accord with observation, 
is that other factors normal about 40 per cent of venous blood may be 
mixed with arterial {a — 0.4) before the reduced hemoglobin concentra- 
tion in the latter becomes great enough to cause cyanosis. 
The effects on C of separate variations of Z, T, D, and a are indicated 
by partial differentiation of (6). 
bC = TN (7) 
bC = IbT (8) 
bC = — ()a (9) 
(1 - ar 
= dD (10) 
2(1 -a) 
While the reduced hemoglobin concentration in the capillary blood may 
be accepted as the cause of cyanosis, there are various other factors which 
modify the resulting coloration. Such are local skin vascularity, pigmen- 
tation, thickness of epidermis; and also the fact that the mean capillary 
content of reduced homoglobin (A -\- V) only approximately represents 
the average content. With changing conditions the latter may instead of 
being midway between venous and arterial, approximate either more 
