741 



for the replacement of tlie posterior cardinal veins by the post-caval 

 which occurs in all animals above fishes — tetrapoda, as Credner calls 

 them — and since I am not aware that the idea, though simple enough, 

 has been expressed before, I gladly take the opportunity here provided 

 of publishing my conclusions along with the description of an abnorma- 

 lity associated with the structure to Avhich the conclusions refer. 



As leading up to the subject proper, it will first be necessary to 

 consider why the posterior cardinal veins of fishes assume a mid- dorsal 

 position in the body-cavity. Taking the familiar illustration of a flexible 

 lathe, it is an elementary truth that during flexion the substance situated 

 at the sides of the lathe is more subject to pressure (due either to com- 

 pression or distension according to the direction of flexion) than the 

 substance situated near the median vertical plane (supposing the lathe 

 to be placed edge-up) — the pressure attaining a maximum at the 

 extreme periphery and a minimum midway between the two sides 



AB C 



Fig. 4. ^1. lathe viewed in section. The degree of approximation of the lines indicates 

 the intensity of pressure experienced by the suljstance of the lathe in any given re- 

 gion. The some remark applies to B and C, which diagrammatically represent trans- 

 verse sections of atypical fish, illustrating the evolution of the posterior cardinal veins. 

 In B is indicated a tendency on the part of the blood to flow either dorsally on ven- 

 trally on account of the lateral flexion of the body. In C is shown the mid-dorsal 

 course which the blood actually pursues. B. bloodin vascular space; AC. body-cavity. 



(fig. 44). Now the fish-body, although ovate in transverse section and, in 

 the region of the bod)*- cavity, hollow, yet essentially resembles a lathe 

 from the mechanical standpoint, and is therefore similarly affected by 

 flexion such as that involved in swimming. Flexion of the fish-body, 

 like that of the lathe, involves considerable alternate compression and 

 distension of the substance at the sides but not of that in the median 

 vertical plane; hence it follows that intercellular spaces situated in 

 the lateral body-wall — such as vascular channels — will ultimately be 

 obliterated (since, owing to the periodic obstruction offered, the blood 

 will eventually cease to flow through them) and that those situated dor- 

 sally or ventrally to the body-cavity in the median vertical plane will 



