RESPIRATION 297 



explained above, it seems evident that anything but a very ex- 

 treme increase in viscosity will at once be compensated for by 

 more free opening of arterioles and capillaries. The resistance 

 to flow of blood in the living body is regulated physiologically, 

 and cannot for a moment be compared to the mechanical resist- 

 ance in a system of lifeless tubes. 



The rapid variations in blood volume from diminution or in- 

 crease in the vasoconstrictor (pressor) influence of the vaso- 

 motor center is perhaps shown most strikingly by the effects on 

 the blood of section of the spinal cord below the vasomotor center 

 in the medulla. Cohnstein and Zuntz found that very quickly 

 after section and consequent fall of blood pressure the proportion 

 of red corpuscles fell to about half, while the proportion rose 

 rapidly again on stimulation of the cord just below the section, 

 with consequent rise of blood pressure. 38 The blood appears to 

 take up. or lose plasma rapidly when the capacity of the blood 

 vessels is diminished or increased. 



It was discovered by Lorrain Smith with the help of the carbon 

 monoxide method that in chlorosis and in secondary "anaemias" 

 the blood volume is increased without any diminution, or with 

 only a very slight one, in the total haemoglobin in the blood. The 

 anaemia is thus in reality a hydraemia or dilution of the haemo- 

 globin. 39 Boycott and I found the same condition in the ''anaemia" 

 of ankylostomiasis. 40 Miss FitzGerald found later that in chlorosis 

 the alveolar CO 2 pressure is not diminished but normal, so that 

 in this form of anaemia there appears to be no anoxaemia during 

 rest. 41 These facts suggest that the apparent anaemia is due to 

 some cause leading to abnormal dilation and consequent increased 

 capacity of the blood vessels, with the natural sequence of hydrae- 

 mia, but so that the oxygen pressure in the tissues is not dimin- 

 ished. Possibly, therefore, the anaemia is produced through the 

 vasomotor nervous system, or through substances, or the deficiency 

 of substances, which act primarily on the blood vessels. The facts 

 that salts of iron have a striking curative action in chlorosis, and 

 that iron is a constituent of haemoglobin, have led to the idea that 

 the anaemia is caused by the absence of sufficient iron for a normal 

 formation of haemoglobin; but in the cure of chlorosis by iron 

 Lorrain Smith could find no appreciable increase in the total 



38 Cohnstein and Zuntz, P finger's Archiv., 88, p. 310, 1888. 



39 Lorrain Smith, Trans. Pathol. Soc. of London, LI, p. 311, 1900. 



40 Boycott and Haldane, Journ. of Hygiene, III, p. 112, 1903. 



41 FitzGerald, Journ. of Pathol. and Bacterial., XIV, p. 328, 1910. 



