758 A MANUAL OF VETERINARY PHYSIOLOGY 



by means of the food, and the poisonous action of choline is probably 

 prevented by the substances being broken up by the bacteria of 

 the intestines into carbonic acid, marsh gas, and ammonia. It has 

 been supposed that the presence of lecithin in the blood cells may 

 prevent the escape of haemoglobin. Halliburton suggests that 

 corpuscular destruction may result from the products of the fer- 

 mentative decomposition of lecithin. Cholesterin contains neither 

 nitrogen nor phosphorus, and in this respect resembles fat. It is an 

 alcohol, and the only one which occurs in the body in a free state. 

 It is widely distributed in the tissues and cells, in the white sub- 

 stance of the nervous system, and in the liver, where it forms the 

 main constituent of gall-stones. In the brain it may occur in a free 

 state, and is recognised by the silvery fish-scale-like deposits in the 

 pia mater of the cerebellum and the choroid plexus of the horse, where 

 it not infrequently gives rise to growths in the lateral ventricles. In 

 the bile it is kept in solution by the bile acids. Observations on the 

 hemolytic action of cobra venom suggest that cholesterin may assist 

 in retarding cell dissolution. In the wool-fat of sheep and in sebum a 

 form of cholesterin, known as iso-cholesterin, exists, which replaces 

 the glycerine constituent of fat. Iso-cholesterin is found in lanoline 

 (see pp. 311, 312), but is not identical with cholesterin, as it does not 

 yield Salkowski's test (see below), and turns the plane of polarised 

 light to the left instead of to the right. Cholesterin is found in 

 some plants, and it has been shown that animal cholesterin belongs 

 chemically to the terpene series, which hitherto has been considered 

 as exclusively connected with vegetable substances. It has been 

 common to regard cholesterin as a waste product in the body, but 

 it is likely, from what has been said above, that this view may 

 require to be modified. That some of it is excreted is undoubted, 

 for it is found in the fasces, but by then it has probably performed 

 its functions. Cholesterin is readily soluble in cold acetone, ether, 

 chloroform, and bile salts. It is insoluble in water and cold alcohol. 

 It can easily be obtained in characteristic crystals. The two chief 

 tests for its presence are performed by dissolving the substance in 

 chloroform, and acting on it with sulphuric acid. If its solution in 

 chloroform be shaken with an equal volume of strong sulphuric acid, 

 the solution turns red, then purple, and passes through blue and 

 green to yellow (Salkowski's test) ; or if sulphuric acid be added 

 drop by drop to the solution of cholesterin in chloroform, a red 

 colour turning to bluish-green results (Liebermann's test). Galacto- 

 sides contain phosphorus, but no nitrogen ; they can be extracted 

 from the brain, but little is known regarding them or their function. 

 They yield on decomposition a reducing sugar known as galactose. 



Carbohydrates. — This important class is of the greatest interest 

 to the physiologist, inasmuch as the bulk of material consumed 

 as food, especially in the herbivora, consists of carbohydrate matter. 

 It is an extensive group of bodies consisting of such substances as 

 starch and its derivatives, the various forms of sugar, and cellulose. 

 Though so much carbohydrate material enters the body, but little 

 can be found in the tissues. An animal starch (glycogen) is found 

 in the liver and other organs, minute amounts of sugar are found in 

 the blood, and a sugar exists in milk ; but very much less carbo- 

 hydrate is recoverable from the body than enters it as food, for the 

 reason that the bulk of it becomes converted into fat, or is rapidly 

 oxidised to carbonic acid and water as a source of heat and energy 

 to the body. 



