RESPIRATORY ORGANS. 235 
main pancreatic duct (Wirsung’s duct), the dorsal, the accessory 
or Santorini’s duct. The ducts may remain distinct; they may unite 
before entering the intestine or one of them may unite with the bile 
duct. 
For a long time it was supposed that a pancreas was lacking in 
certain vertebrates (some teleosts, dipnoi, cyclostomes), but recent 
studies have shown its presence in many of these. In the case of some 
Fic. 242.—Diagram of developing pancreas of cat, after Thyng. c, ductus coledo- 
chus; d, duodenum; dp, dorsal pancreas; dd, its duct; i, small intestine; s, stomach; vp, 
ventral pancreas. 
teleosts it occurs as a slender tube in the mesentery; in the dipnoi it is 
outside of the muscles in the intestinal wall, while in the cyclostomes 
it is partly concealed at the insertion of the spiral valve, partly (myxi- 
noids) inthe liver. In these forms, owing to the complete disappearance 
of the duct it becomes a gland of internal secretion. The pancreas 
may be elongate, compact, or sometimes extremely lobulated. Usually 
(fig. 241) it lies in a loop of the duodenum. From certain peculiarities 
of structure the queston has arisen as to whether two distinct structures 
are included in the pancreas. 
: THE RESPIRATORY ORGANS. 
The respiratory organs have for their purpose the exchange of gases 
between the blood and the surrounding medium—water or air— 
carbonic dioxide being given off and oxygen being absorbed by the 
circulating fluid. In order that the exchange be readily effected it is 
necessary that the organs be richly vascular, that the walls between the 
blood and the surrounding medium be extremely thin so as to permit 
rapid osmosis, and that the osmotic surface be as great as possible. 
Further, there must be an adequate mechanism for passing the oxygen- 
containing medium over the respiratory surfaces. 
