CHANGES DURING PREGNANCY 557 



tissue of the endometrium, and by others to an increase in the groups 

 of lymphatic glands in the neighbourhood of the genital apparatus. 

 The spleen is not obviously affected, but a remarkable change was 

 observed in rats in the thymus. It was reduced to half its normal 

 size and histologically presented the appearances of involution. 

 These findings confirm the view expressed above that the functional 

 activity of the lymphoid tissue is of special importance during the 

 period of pregnancy. 



According to Spiegelberg and Gscheidlen, the total amount of 

 blood is increased during pregnancy in the dog from IS per cent, to 

 9 per cent, of the body-.weight. There is no essential difference in 

 the specific gravity (Nasse, Blumreich i). The reaction of the blood 

 has been dealt with above (see p. 546). 



(h.) The Heart and Circulation. — Older authorities stated that a 

 true hypertrophy of the heart occurred during pregnancy, and was 

 caused by the increased length and size of the uterine vessels, the 

 placental circulation, and the compression of the aorta by the gravid 

 uterus. Experiments showed, however, that the uterine vessels did 

 not offer a resistance which required an increase in the work of the 

 heart (Engstrom ^), while the compression of the abdominal aorta and 

 the introduction of large quantities of fluid into the abdominal cavi|;y 

 produced no change which could be detected from the pulse 

 (Heinricius ^). The controversy has been a long one, but it does not 

 properly belong to this article. In the rat Herring * failed to find a, 

 hypertrophy or enlargement as the result of pregnancy. 



It has been suggested that the increased work of the heart is 

 due to an increased peripheral resistance from the presence of a 

 vaso-constricting substance in the blood. Whether such a substance 

 exists is very doubtful. The investigation of extracts or the placenta 

 by Lochhead and Cramer^ in 1907, proved that this organ contained 

 no blood-pressure raising substance. The substances extracted by 

 Dixon and Taylor^ from the placenta and observed to have an 

 adrenalin-hke action, were subsequently shown to arise in the course 

 of putrefaction. 



The blood-pressure is not affected in normal pregnancy, but is 

 always raised in labour as a result of the uterine contractions. After 



1 Blumreicli, " Der Einfluss der Graviditat auf die Blutalkalesoenz," Arch. f. 

 Oynak., vol. lix., 1899. 



2 Engstrom, " L'Influence de la grossesse sur la circulation," Arch, de Gyn., 

 vol. ii., 1886. 



' Heinricius, Experimentelle und klinische Untersuchungen iiber Gircvlations- 

 verhalten der Mutter imd der Fraoht, Helsingfors, 1889. 



* Herring, "The Effect of Pregnancy, etc.," British Med. Jour., 11th December 

 1920. \ 



* Lochhead and Cramer, Unpublished observations. 



* Dixon and Taylor, " On the Physiological Action of the Placenta," Proc. 

 Roy. Soo. Med., London, vol. i., 1908. 



