THE SPLEEN IN MARSUPIALS. 



seen the various grades of shortening of the mesocolon), 

 and nature does not retrace her steps. From the point 

 of view of "selection" for the evolution of the human 

 type, the arrangement in the Platypus is superior to that 

 of the Echidna. 



The "selection" factors would he: — 



(a) Lieno-mesocolic fold lying ventral to left 



kidney, connecting spleen and mesocolon, 

 and supporting pancreas. 



(b) Pancreas outside the lesser sac. 



(c) Shortening of the great omentum (gastro- 



splenic omentum) between the left extrem- 

 ity of the spleen and stomach fundus, with 

 secondary peritoneal bands between the two. 



(d) Development — not atrophy — of the distal 



colon. 

 As evolution proceeds, the pancreas, which even in the 

 Platypus is fixed dorsally about the root of the mesentery, 

 approaches the dorsum on the right and left sides (in the 

 Echidna direct adhesions may be noted between pancreas 

 and lobus caudatus), which means some approximation of 

 spleen and mesocolon. Adhesions form between the lieno- 

 mesocolic fold and ventrum of the left kidney as in 

 Triehosurus — the fold ending in the Wombat at the lower 

 pole of the kidney. Thus is developed the lieno-renal 

 band. The development of the so-called human "inter- 

 mediate" angle (which really represents the lower limi- 

 tation of the renal ridge) is now easy to understand. 

 Further evolution results in further atrophy or shorten- 

 ing of the lieno-renal (mesocolic) fold. The pancreas 

 and colon become more closely related to the spleen, 

 with an encroachment on the lower portion of the renal 

 concavity and atrophy of the lower part of the renal 



ridge. 



155 



