DIGESTION 241 



Glycogen. — This substance, although especially abundant in the liver, 

 is rather widely distributed in the body, being a constituent of the white 

 cells of the blood, the muscles, and various embryonic or growing and 

 developing tissues. It is a white, tasteless substance resembling ordinary 

 starch, except that on the addition of iodine it assumes a port-wine red 

 instead of a blue colour. It appears that the liver can produce glycogen 

 either from the saccharine or from the nitrogenous group of alimentary 

 substances, but it is not increased when fats are given in excess. The 

 livers of rabbits fed on carrots, and of dogs fed on lean meat, both contain 

 abundance of glycogen. It is a store of easily oxidizable material, which, 

 after being converted into sugar, can be absorbed by the blood at the liver, 

 and being carried to the muscles is there burnt off, giving rise to the 

 liberation of muscular force and to animal heat. It disappears both in 

 muscles and in the liver after prolonged and violent exertion. 



The pancreas. — This gland, commonly but incorrectly spoken of as 

 the "sweetbread", is deeply seated in the abdomen, lying above the 

 stomach, between it and the spine, and occupies the space enclosed by the 

 loop of the duodenum or first division of the small intestine. It is of 

 pinkish-gray colour, and weighs about H lb. It is a highly important 

 gland, its secretion exercising a powerful digestive action on starches, oils, 

 and proteids. 



Food. — The natural food of the wild horse is the fresh moist succulent 

 grasses and cereals of the temperate zones. These include many species of 

 the Graminaceae, and plants like the mallow, rumex, and ranunculus from 

 other orders. In the domesticated animals, however, whilst grass forms 

 part of the food for part of the year, many working horses in large towns 

 live on the dried stems, flowers, and fruit of the grasses, with some addition 

 from the seeds of the leguminous plants. 



The food of an average-sized horse performing a moderate amount of 

 work may be taken in England at 12 lbs. of hay, 10 lbs. of oats, 2 lbs. of 

 crushed maize, and 8 lbs. of chaff, with five or six gallons of water, accord- 

 ing to the requirements of the animal. 1 In considering the changes wrought 

 in the food during the process of digestion, it is to be borne in mind that 

 the main constituents of the various plants eaten by the horse, whether 

 in the open field or in the form of hay, corn, and beans, are reducible to a 

 few groups which are named proteids, farinaceous and saccharine com- 

 pounds, oils, vegetable acids, water, and salts. 



The proteids constitute the most important of the food constituents 

 because they contain nitrogen, sulphur, and phosphorus. They are repre- 



1 In Frame, in 188">. the omnibus companies supplied each horse with 10 lbs. of oats, 8 lbs. of bruised 

 maize, and about 2 lbs. of feverole, a sort of small bean, the product of Vidafaba. 



