434 FLOW OF THE UEMANIAN FLUIDS 



pighian system, a rather violent supposition, it would seem that the 

 excretion, as such, if poured into the intestine at all, should be poured in 

 posteriorly. But in didelphs, -fuscus, panicus, such a supposition would 

 pour it in near the anterior end. Or, if it be supposed that the demanian 

 system is simply an emunctorium accessory to the intestine and emptying 

 outward and backward, then why the attachment to the fore part of the 

 intestine as in fuscus! 



If the demanian system is excretory, then it is necessary to assume that 

 the necessities of adult females in the way of excretion are different from those 

 of the male or the young female. No reason has been advanced for such an 

 assumption. 



Deduction by Elimination. In the demanian system of 0. pristiurns 

 three ducts come together at a single point, indicated by X in 

 figure 4, ducts in each of which a fluid may conceivably flow 

 in either direction; i.e., there are six different paths along 

 which fluid may be conceived to flow. The assumption is, of 

 course, that, when the organs are functioning normally, fluid 

 passes constantly in one direction only in each of the three 

 tubes, 1, 2, 3, Fig. 4. 



Indicating the six possible paths by arrows lettered a, b } c, 

 d, e, f (Fig. 4), mathematically, there are twenty possible 

 combinations in groups of three as follows: 



abc, abe, acd, acf, adf, Me, cde, cef, bef, bcf. 



abd, abf, ace, ade, bed, bdf, cdf, aef, bee and def; O f the sixpos- 

 this is according to the formula for combinations, s ^'^ c currents. 



n(n-l) (n-2) (n-r + 1) GX5X4 



- = 



Fig. 5. Diagram showing the direction of flow of the fluid in the demanian system. 

 The intestine and the posterior end of the uterus are shown near bde . The backward 

 flow of the fluid in the enteric and uterine efferents is indicated at b and d, and the 

 backward flow of their combined products at e. The outward flow of the fully elabo- 

 rated secretion after it has passed the moniliform glands is shown by the two oblique 

 black arrows. 



It is evident that a combination containing a and 6 represents a physical 

 impossibility, i.e. represents opposite currents simultaneously in the same 

 duct, duct number 1 ; and so with combinations containing c and d, and e 



