56 



MR. T. J. PARKER ON THE INTESTINAL 



kino-ly shown in the annexed diagram, in which the areas given above are expressed as 

 squares. The square ah c d represents the internal area of the intestine itself, or, in 

 other words, the absorption-surface of a valveless gut ; « J' c' (V- is the entire absorption- 

 surface of A, aV(? cP of B, a P c" d^ of C, and a h' c* d* of I). By taking away the 

 square abed we have the gnomons d c' b, d (? h, d & h, dc*b representing the entire 



d? 



« .-•■' 



c' ,•••■■ 

 c ..-•■' 



i'6= 



i' i* 



areas of the spiral valves of A, B, C, and I) respectively, and, of course, the trapezia 

 d c c^ d\ dc Cj d', d c c^ d', dc c* d* representing the areas of one side of those valves. 



§ 11. But these numbers by no means adequately express the full advantage accruing 

 to the possessor of a perfect form of spiral valve ; for, in the complicated forms, not only 

 is the absorption-surface greater than in the less complicated, but the resistance offered 

 to the passage of food is immensely increased, and, consequently, the time to which it 

 is exposed to the action of the digestive fluids. For instance, in fig. 1, PI. X., there is 

 a clear central passage from one end of the intestine to the other nearly as wide, pos- 

 teriorly, as the corresponding space in a rabbit's caecum ; in fig. 4 the whole of the 

 food is compelled to take a spiral course, the columelliform arrangement of the free 

 edge of the valve quite preventing any direct passage ; in fig. 1, PI. XI., although there 

 is a central passage, yet it is so constricted at one part of its course, namely at the 

 apex of the conical cavity formed by the third turn, that a very small aperture is left, 

 so small, indeed, that a dried specimen of it often becomes entirely closed ; moreover a 

 good deal of food, taking a spiral course, will become wedged in between the closely 

 adhering turns of the valve, and very considerably delayed ; finally, in fig. 3, PI. XI., a 



