454 ANATOMY OF VERTEBRATES. 



ordinary diameter of the small intestines — about 1 inch : these 

 are continued for between 40 and 50 feet to the vent. Broad and 

 well-marked longitudinal folds of the lining membrane extend 

 along the major part of this course : and the same character obtains 

 in other Delphinidce. In Balcenoptera the longitudinal folds are 

 wavy, run into each other, and are connected by smaller oblique 

 or transverse folds : the submucous areolar tissue is very loose 

 and abundant. In Hyperoodon the complexity is carried out to 

 such a degree as to occasion a sacculated structure of the mucous 

 coat through nearly the whole tract of the intestinal canal. 

 The orifices of the larger pouches are directed vent-ward: their 

 cavity is divided into smaller cells. They begin gradually in 

 the duodenum near its last abrupt bend, and subside near the 

 anus. 



In Balcsnoptera the ileum opens in a valvular way into a 

 comparatively short colon, leaving a caecum of about 7 inches in 

 length, and of a simple conical form : in the specimen 1 7 feet 

 long, of Bal. rostrata, dissected by Hunter, he records the length 

 of the small intestine at 28^ yards, of the large intestine 2} 

 yards ; l and notes that he ( never found air in the intestines of 

 this tribe.' 2 



The complicated stomach and long intestinal canal of such car- 

 nivorous Cetacea as the Grampus have other relations than to the 

 nature of the food : they are necessitated in the present order by 

 the amount of nutriment which must be had from it. In no other 

 carnivorous mammals is so great a quantity of blood and fat to 

 be obtained from the raw alimentary material : in none are such 

 active and extensive molecular changes concerned in the produc- 

 tion and maintenance, under adverse external conditions, of so 

 high a temperature of the body. The digestive system and pro- 

 cesses are therefore perfected in these warm-blooded marine air- 

 breathers to meet the contingencies of their aquatic life. 



§ 334. Alimentary canal of Sirenia. — In these more slothful, 

 tropical, or sub-tropical marine mammals, although the food is of 

 a low vegetable kind, the digestive and assimilative tract differs 

 from that of the carnivorous Cetaceans rather by a minor than a 

 major degree of complexity. The stomach, it is true, shows 

 appended sacculi, special glands, and a subdivision of the general 

 cavity, not only through constriction, but by a difference of 

 structure in the lining membrane. It is of considerable length, and 

 nearly equally divided into a cardiac and pyloric portion. In the 

 Manatee the oesophagus terminates at the middle of the cardiac por- 

 1 ccxxxvi, vol. ii. p. 115. 2 xciv. p. 361. 



