IX ALIMENTARY CANAL 303 



a temporary store-house for carbohydrate or starchy material. After a 

 meal the carbohydrate absorbed from the food is transported to the 

 liver in the blood-stream, in the form of dextrose or grape-sugar. As 

 the blood passes through the capillary network of the liver the carbo- 

 hydrate is extracted from it by the activity of the liver-cells and stored 

 up in their cytoplasm in the form of glycogen — a substance of the same 

 chemical composition as starch but apparently possessing some slight ' 

 difference in physical constitution. The carbohydrate, which reaches 

 the liver in a large instalment after each meal, is doled out again to 

 the circulating blood as it is required for the metabolism of the 

 tissues. 



The other bulky gland associated with the alimentary canal is the 

 pancreas (Fig. 125, p), an elongated organ of a characteristic whitish 

 colour 1 lying in the bend between stomach and intestine and opening 

 into the latter near its front end by the pancreatic duct. In Acanthias 

 the pancreas is remarkably variable, being in some cases large and well 

 developed, while in others it is so slightly developed as to be hidden 

 from view in the thickness of the intestinal wall. 



The pancreas is the most important of the digestive glands in existing 

 vertebrates. Its secretion is strongly alkaline and contains a number of 

 powerful digestive ferments, each with its specific role in the digestion 

 of some particular type of food material. Thus one type of ferment 

 (tryptic) is concerned with the digestion of protein, another with that of 

 fat, another with that of starch, and so on. By the action of these 

 ferments the various food substances in the cavity of the intestine are 

 reduced to soluble form and made ready for absorption by the intestinal 

 lining. 



A glandular thick-walled caecum — the rectal gland (Fig. 125, r.c) — 

 opens into the rectum on its dprsal side. 



Along with the glandular appendages of the alimentary canal already 

 mentioned there must be grouped the remarkable organ known as the 

 thyroid gland, although in the adult vertebrate it has no longer any 

 obvious connexion with the alimentary canal. This makes its appearance 

 in the embryo as a median pocket-like downgrowth of the floor of the 

 pharynx, between the level of the first and second gill-clefts. The 

 connexion with the pharynx becomes narrowed and finally obliterated, 

 and the closed sac so formed becomes divided up into a large number of 

 small spherical vesicles, embedded in a framework of connective tissue. 

 The lining of these vesicles produces a clear colloid secretion which 

 accumulates in the cavity of the vesicles, the original outlet to the 

 1 As seen in the " sweetbread " of the kitchen. 



