SPIRAL VALVE IN THE GENUS RAIA. 55 
possessed by the intestine. I am enabled to express these differences numerically, 
through the kindness of my friend Mr. Ambrose R. Willis, B.Sc., who has taken the 
trouble to make for me the necessary calculations. 
Taking the intestine represented in fig. 1, Pl. X., as the nearest approach to type A, 
and for B, C, and D, figs. 4 (Pl. X.), 1 & 3 (Pl. XI.) respectively, we have the follow- 
ing dimensions :— 
Length of 
intestine, 
measured from! Internal area te one Area of one | Total absorp- 
Uy ylorus to of intestine, | M”8CneC S°8e| aide of valve tion-area, 
MABE Pyro a of spiral ? 7 
ere =a. erent =b. =a+26. 
rectum. 
em. sq. cm em. sq. cm. sq. cm. 
A, 7 44:3 58°55 46:17 136-64 
B. 9°5 92°5 83-9 69-16 230-82 
C. 12-5 1453 102-1 183 511°3 
D. 10:3 97-1 82-5 163-68 434-46 
In this form, however, the areas, which it must be understood are only approximately 
correct, are not readily comparable, owing to the different sizes of the four specimens; 
and it is convenient to take a common length both for the intestine itself and for the 
attached edge of the valve. Taking as a standard these dimensions as they occur in A, 
in the above Table,.we.have, for all four types, the internal area of the intestine equal 
to 44:3 sq. cm., and the other areas as follow — 
Total 
absorption- 
surface. 
Area of 
one side of 
valve. 
sq. cm. 
46:17 
48°26 
1050 
116-2 
Saeh 
Thus, in intestines of the same size, a valve of the type D will have about two and a half 
times the area of one of the type A, while the entire absorption-surface (7. ¢. area of 
both sides of valve plus internal area of intestine) will be as nearly as possible twice as 
great in the former as in the latter—a fact as remarkable as if (the average length of 
the human intestine being 25 feet) subjects were occasionally found with only 20 feet, 
and others with as much as 40 feet. 
§ 10. The relation, in: respect of absorption-surface, between the four types is stri- 
yor. XI.—PART ul. No. 6.—August, 1880. K 
