760 A MANUAL OF VETERINARY PHYSIOLOGY 



cannot be acted upon by the digestive juices. This breaking down 

 is accomplished by laceration during the process of mastication, but 

 also by a subsequent digestion of the covering, by which means it is 

 removed and the food substance exposed. The digestion of cellulose 

 is a physiological puzzle, for the reason that no vertebrate is known 

 to secrete a cellulose-dissolving enzyme. In certain invertebrates 

 a true cellulose enzyme (cytase) is met with, and is known to be 

 secreted in the intestinal canal. Inasmuch as the herbivora are 

 capable of dealing with cellulose, the question of its solution is of 

 the greatest interest. Bunge has shown that sheep can digest 

 from 30 to 40 per cent, of the cellulose of sawdust and paper when 

 mixed with hay. There is every reason to think that all the herbiv- 

 ora deal quite as thoroughly with the cellulose naturally found in their 

 food. The question has been before us in dealing with Digestion 

 (p. 193), in which it was shown that in the case of oats the grain 

 provided its own cellulose-dissolving ferment. It is hardly likely 

 that the sawdust or paper in Bunge's experiment provided their 

 own enzyme, so that it is probable the hay furnished it. Cellulose, 

 however, may be digested by the action of putrefactive organisms, 

 either outside or inside the body. In both cases it is attended by 

 the formation of acetic and butyric acids, and the evolution of 

 marsh gas, carbon dioxide, and other substances. We have previously 

 studied the facilities which exist within the body for the necessarily 

 slow maceration of cellulose, which is the essential prelude to its solu- 

 tion (see pp. 195, 216, 220). 



Disaccharides (C 12 H 22 O n ). — Saccharose, or cane-sugar, is not found 

 as part of the animal body, but exists largely in plants, and forms 

 a well-known supply of carbohydrate to the system. Cane-sugar 

 does not give some of the characteristic sugar reactions ; among others, 

 it has no reducing action upon salts of copper, but by boiling with 

 dilute mineral acids it is converted into equal parts of dextrose and 

 laevulose, and the same change may be effected by enzymes in the 

 stomach and small intestines. This conversion of cane-sugar is 

 recognised by the changed action of the solution on polarised light, 

 the rotation of the plane of polarisation being now left-handed 

 instead of right-handed, as it was previously to the conversion ; 

 that is to say, it is inverted, hence the name invert sugar. If cane- 

 sugar be injected into the circulation, it passes out of the system 

 unaltered. Before this sugar can be assimilated, it must be con- 

 verted into dextrose (see p. 248). 



Maltose is formed by the action of malt extract (diastase) on starch 

 paste, also by the action of saliva and pancreatic juice upon starch 

 paste and glycogen. In its reactions it corresponds closely to 

 dextrose, but it has a one-third less reducing action upon Fehling's 

 solution, and, unlike it, does not reduce Barfoed's reagent.* Its 

 specific activity in rotating the plane of polarised light is considerably 

 greater than that of dextrose, being about + 140 , as against 4- 5 2° 

 for dextrose. Maltose yields an osazone when heated with phenyl- 

 hydrazine hydrochloride. When heated the crystals (phenyl-malto- 

 sazone) melt at 206 C, and this, together with the shape of the 

 crystals and their specific solubility in 75 parts of boiling water, 

 renders the identification of maltose easy. Maltose, like cane- 

 sugar, is non-assimilable, for if injected into the circulation it is 

 excreted unchanged. Before absorption it has to be converted into 

 dextrose, and this is effected by a ferment, maltase (p. 256). 



* A solution of cupric acetate to which acetic acid is added. 



