426 MR F. G. PAKSONS. 



it is perfectly smooth. In the chordate animals below the fish 

 (Cyclostomata and Acrania) no spleen has been found. 



With regard to human embryology, T have examined many 

 foetuses from three months upward, and my experience is that 

 notches are less frequent tlian in the adult, at any rate I 

 am convinced that they are not more frequent, and neither in 

 the human fretus nor in the lower animals have I seen anything 

 corresponding to the distinct fissures on the parietal surface 

 already described in the adult. To me, as I have already said, 

 it seems that the notches and fissures of the human spleen must 

 have one of two meanings, either they are the remnants of an 

 ancestral arrangement such as lobulation, or they are formed by 

 some mechanical cause connected with the growth of the spleen 

 and surrounding viscera, a cause which may not act as strongly 

 in one individual or in one group of animals as in another. If the 

 notches have any atavistic meaning it is probable that they 

 would be found more markedly in the foetus than in the adult, 

 as is the case with the kidneys, but my experience of fffital 

 spleens is that they are less freely and less deeply notched than 

 they are after birth. Again it will be noticed from the tables 

 that in subjects below two years the notches are often remark- 

 able for their slight depth and are not prolonged into fissures 

 running across the viscus. Comparative anatomy shows how 

 far back one has to go for a completely lobulated spleen, right 

 back to the elasmobranch fish, and even here lobulation is by 

 no means constant. The comparison l:)etween the Carnivora 

 and Ungulata is interesting, because it is well known that in 

 riesh-eating animals the spleen is larger than in vegetable 

 feeders, and it seems that notching is correlated with the in- 

 creased size of the organ. All the evidence therefore goes to 

 show that the notches are mere crumplings of the spleen due to 

 its growth and to the pressure of the surrounding viscera. 



There must, however, be some cause for this crumpling 

 occurring at one point more than another, and this I think is 

 the distribution of the main vessels. In looking at the figures 

 of the spleens of the iguana and seal, one is struck by the way 

 in which most of the notches correspond to vessels (figs. 5 and 7), 

 and in a polecat's spleen which I lately examined I found two 

 vessels running ventralward to the stomach, each of which 



