RELATION OF ARTERIES IN COMMON (lANOIDS 399 



matter, dependent largely ui)()n tlie interaction of blood pres- 

 sure within the vessel and various fon^es that are brought to bear 

 on its outer surface. Besides these passive factors, is the tend- 

 ency of the vessel itself to spread into new regions. 



This hypothesis receives some supjiort from the results of ex- 

 periments made on Polyodon and Scaphirhynchus in which the 

 coeliaco-mesenteric artery can easily be tied.^ There is, as above 

 described, a normal anastomosis between the anterior and the 

 posterior hepatic arteries thus connecting the branchial and in- 

 testinal arteries. There is also an anastomosis, smaller than the 

 other, between the intestinal artery and a branch of the aorta 

 that passes over the posterior side of the swim-bladder. Now 

 when the coeliaco-mesenteric artery is tied ofT, the pressure must 

 be lowered in the intestinal arteries and raised in all others. It 

 would be expected that this would cause any secondary con- 

 nections existing between the two systems to enlarge, and as a 

 matter of fact a new artery adequate to supi)ly the whole intes- 

 tine is ])rom]:)tly formed through the posterioi* anastomosis. In 

 neither form, however, have I been able to increase appreciably 

 the caliber of the artery that passes through the liver, even by 

 a second operation in which the new posterior mesenteric artery 

 is tied. The difference in the behavior of the two anastomoses 

 is very probably due to the fact that one occurs in the substance 

 of the liver while the other is in the loose tissue of the mesentery. 



A somewhat analogous set of interacting factors might easily 

 determine the distriliution of arteries in a growing eml^ryo, and 

 indeed, the evidence, so far as it goes, suggests that the arrange- 

 ment of vessels is determined anew in each individual. That 

 the same scheme tends to prevail in a given species or in related 

 species is due, on this assumption, to the constant relations of 



' Owing to its lack of scah's I'olyodon i.s especially favorable for this kind of 

 experimentation. The ])rocedure has been to o])en the abdomen by cuttinji 

 along the linea alba and then tuining the fish on its left side, when it becomes an 

 easy matter to pass a ligature around the long free strand in which runs the coe- 

 liaco-mesenteric artery, 'i'he o|)era(ion may be done without loss of blood. The 

 fish usually recovers ([uickly and shows no bad after effects except thai the cut 

 in the linea alba is slow in healing. Several operated specimens were kept in 

 the aquarium foi- three weeks before they were finally killed. 



