116 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



con 



stitutes about 50 per cent, of wood, the other half being composed of 

 cellulose, upon which the lignin is deposited. Iodine stains cellulose of a 

 yellowish color unless hydriodic acid, potassium iodide, zinc iodide, sul- 

 phuric acid, phosphoric acid, or zinc chloride are added with the iodine. 

 With any of these reagents, combined with the iodine, the cell-membrane 

 or cellulose is stained blue. It is not known, however, in what way these 

 agents assist the reaction. 



3. Dextrin (C 6 H 10 O 6 ).— Dextrin, or British gum, is the name given 

 to a group of substances which may be regarded as intermediary products 

 in the conversion of starch into sugar. It may also be obtained by boil- 

 ing starch with dilute acid, although in this operation the sugars are also 

 obtained. There is some doubt as to whether it exists ready formed as 

 a constituent of vegetable cells. In commerce it is manufactured by 

 heating dry starch up to 400°. Through the action of the dry heat the 

 starch becomes yellowish in color and soluble in water. Dextrin is in- 

 soluble in alcohol and ether ; it should not reduce the sugar test unless, 

 as is apt to be the case, it is associated with sugar. It rotates the ray 

 of polarized light strongly to the right, from which it derives its name 

 ( dexter = right), and is readily converted by the action of dilute acids, 

 or the diastatic ferments, into sugar. According to Bernard, dextrin is 

 found in the blood of both the herbivora and carnivora, though in greater 

 amount in the former. When found in the animal body it originates 

 partly from the glycogen of the liver and partly from the food. The test 

 for dextrin is the formation of a mahogany-red color when iodine is added 

 to its solutions. When heated this color disappears and does not return on 

 cooling, — a point of importance as serving to distinguish dextrin from 

 glycogen, another member of this group. Dextrin is precipitated out 

 of its watery solutions, which are always turbid, by alcohol, lime-water 

 and ammonia, and acetate of lead. With iodine in solution in potassium 

 iodide, dextrin gives a violet coloration. 



4. Glycogen. — Glycogen , or animal starch, or, more properly speaking, 

 animal dextrin, will be discussed at length under the subject of Special 

 Physiology. 



5. Inulin (C 6 H 10 O s ). — In its composition and characteristics inulin 

 is closely allied to starch. It is found in the roots of the Lobeliacese, 

 Gampanulacese, and Gordeniacese ; it owes its name to the fact that it was 

 first discovered in the root of the Inula helenium. Dried dahlia-bulbs 

 contain 42 per cent, of inulin. In the autumn inulin is found in 

 greatest amount ; in the spring it becomes converted into kevulose. 

 Inulin is only found dissolved in plant-juices, and never as a solid 

 deposit ; and since inulin by itself is insoluble in water, it must then owe 

 its solubility to the presence of some other substance. 



Inulin may be obtained by boiling dahlia-bulbs in water, enough 



