186 THE STRUCTURE OF MAN 
maintained, as the venous circulation in the lower limbs now has, 
against the action of gravity. This justifiable assumption has led 
me to investigate the intercostal veins in Man closely, by way of 
ascertaining if they possess valves, and my observations in all 
essentials confirm those of Henle recorded in his Handbuch der 
Anatomie. That is, I found great variation both in the number 
and the development. of the valves, so that the impression of a 
retrogressive condition became irresistible. 
It is well known that in other parts of the body, valves of 
the veins appear in a reduced and evidently degenerating or 
vestigial form, and also that in the embryo there arise many 
more valves than attain complete development. [The valves of 
the portal system are among the number thus suppressed, but 
they may be occasionally retained."] 
THE SPLEEN 
Throughout the Mammalian series three lobes of the spleen 
may be detected, viz. an anterior, a posterior, and a middle, all 
of which vary greatly in size and form, in the various types. In 
Marsupials the posterior lobe stretches far down towards the 
rectum. In the Placental mammals the lobes are increasingly 
reduced, and finally, in the Primates, the posterior lobe has 
almost disappeared; but the anterior and the median are repre- 
sented even in Man, while the posterior lobe is in him reduced to 
a projection of its margo obtusus (Klaatsch). 
The average weight of the spleen in the white races is said 
to be 195, and in the black but 171 grs. 
1 [These valves are typically bicuspid. They are most numerous at birth, in 
the vessels of the large intestine. After birth they disappear rapidly, and when 
present in the adult they appear to be most abundant on the small intestine. Cf. 
Hochstetter, Archiv f. Anat. und Phys. 1887 ; Anat. Abth., p. 137; and Bryant, 
Boston Medical and Surgical Journal, vol. cxix. p. 400. Hyrtl long ago drew 
attention (Sitzungsb, Wien. Akad., Bd. 1xi. p. 27) to the existence in the Rodentia 
of a spiral valve-like fold within the portal vein. ] 
