Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/733

NUTRITION 675 ent reactions, and at the same time a sugar which, from its identity with that which is produced in the process of malting of barley, is called " maltose," and which, though possessing many of the reactions of grape-sugar, is an isomer of cane-sugar; when crystallized it has the com position C 12 H 22 O n + H 2 O. The changes which are brought about by the action of the salivary ferment on the starchy constituents of food are hastened by a temperature near that of the mammalian body, Boiling destroys the diastatic power, as also does the presence of strong acids or alkalies. A very feeble acid reaction does not absolutely stop the characteristic action. Whilst the saliva of man and some few animals possesses the remarkable diastatic ferment just referred to, this is absent from the saliva of the majority of animals, so that we are forced to conclude that the saliva is an alimentary juice which subserves mechanical rather than essential chemical functions. Gastric Juice. This is a thin colourless or straw-coloured liquid of strongly acid reaction, and of a mawkish taste. Its specific gravity varies between I OOl and I OIO, and it contains from J to 1 per cent, of solid matters. The gastric juice contains (1) free hydrochloric acid, (2) certain mineral matters, and (3) a ferment or enzyme called pepsin. It has been much disputed whether the gastric juice contains the hydrochloric acid in a free con dition, but the question may be considered to have been settled in the affirmative by many facts, amongst which one of the most striking is that the gastric juice behaves towards certain organic colouring matters as only liquids do which contain free mineral acids. The pepsin, which has never been separated in a pure condition, is soluble in water, weak spirit, and glycerin, and confers upon all these solvents its characteristic property, viz., that of dissolving, in the presence of a dilute free acid, and at a suitable tempera ture, insoluble proteids, and of converting these into soluble and diffusible modifications termed peptones. Peptones differ from other albuminous or proteid bodies in their greater diffusibility, as well as in their much higher solubility in pure water. They are not coagulated by heat or by nitric acid, and are not thrown down by many mineral salts which precipitate other soluble proteids. Gelatin under the same conditions is converted into an ungelatinizable modification, the so-called gelatin-peptone. Boiling destroys the activity of solutions of pepsin. If their reaction ceases to be acid, and, further, if the acid reaction be not due to a free acid, no action is exerted upon proteids. A temperature approximately the same as that of the mammalian body is that most favourable to gastric digestion. The accumulation of the products of gastric digestion, viz., peptones, hinders the continued activity of the juice. Pepsin, though probably containing nitrogen, is not a proteid body. In addition to pepsin the gastric juice contains an entirely independent ferment which is called the "curdling ferment" or the "rennet ferment," because of its power of rapidly inducing at the temperature of the body the coagulation of the casein of milk, which it converts into cheese. Un like pepsin, this ferment will act in solutions which contain no trace of free acid. The following table (Hermann) shows the average com position of the gastric juice of man, the dog, and the sheep. Man. Dog. Sheep. Water 994-4 973-1 971-2 986-1 Hydrochloric acid 0-2 3-3 2 4 1-2 Organic constituents (pepsin, &c.),.. Mineral matters 3 2 2 2 17-1 17-3 6 5 9 1 4-1 8 6 Bile. Bile is a bitter liquid of golden-red colour in man and carnivorous animals, but green in herbivorous animals. It is commonly viscid owing to admixture with mucus derived from the walls of the gall-bladder and hepatic ducts. Its reaction is neutral or faintly alkaline. It contains about 14 per cent, of solid constituents. (1) It includes the sodium salts of certain so-called bile -acids, which in man are glycocholic and taurocholic acids. The former has the formula C 26 H 43 NO G , and is readily decom posed into glycin, C 2 H 5 NO 2 , and an acid termed cholic acid, C 24 H 40 O 5 ; the latter has the formula C 26 H 4 -NSO 7 , and can be split up into a base called taurin, CgH^NSOg, and into cholic acid. (2) It contains bile-colouring matters, of which the chief is a red one termed bilirubin, C 16 H 18 N 2 O 3 , with which is closely connected the green colouring matter of the bile of herbivores, termed biliviridin, C 16 H 18 N 2 O 4 ; bilirubin is unquestionably derived from the decomposition of the blood-colouring matter or haemoglobin. (3) It has fats, and a body called cholesterin, which belongs to the class of alcohols and has the composition C 0(5 H 44 + H 2 O. (4) It includes saline matters which are rich in sodium salts. Whilst the bile-acids and the bile-colouring matters are formed in the liver itself, the cholesterin of the bile is probably derived entirely from the blood, which obtains it from the nervous organs, in the white matter of which it is abundantly present. The proportion of the constituents of the bile present in this liquid is shown by the following analyses. Composition of Human Bile. (i) (2) Water 860-0 822-7 Sodium glycocholate and taurocholate . . . 102-0 107-9 Mucin and colouring matters 26 6 22-1 Cholesterin 1-6 > Fatty matters and lecithin 3-2 J-47 3 Mineral matters... 6-5 10-8 The amount of bile secreted by man has been estimated at from about 136 grains to about 309 grains per kilo gramme (2-20 K>) of body-weight; but in certain cases, through abnormal circumstances, the total amount of bile secreted in twenty-four hours has been found to be less than this, viz., to vary between about 16 and 21 oz. The action of the bile in digestion, it must be admitted, seems trifling and out of proportion to the size of the gland and the amount of the secretion. It is a weak solvent for fats; it has the power of emulsifying fats, especially, perhaps, when added to the pancreatic juice ; and it may help the passage of fat through animal mem branes. At least it is certain that, when filter-paper is wetted with bile, oils filter through it more readily than when it is wetted with water. The most important use of the bile is, however, as an adjuvant to pancreatic digestion. The pancreatic juice, as will be stated more particularly below, contains a ferment which is either a proteid body or inseparably connected with a proteid body. Strangely, though this ferment possesses the power of dissolving proteids under suitable circumstances, in the presence of pepsin and any free acid it is itself destroyed. It would then be acted upon by the pepsin passing into the duodenum from the stomach if the conditions were favourable. To prevent this untoward accident the alkaline bile is poured over the acid contents of the stomach as they pass the duo denum ; these are neutralized, and a precipitate is thrown down which mechanically entangles the pepsin that may be in the mixture. Although the definable uses of the bile are small, the part which it plays in the economy is of essential importance. Animals whose bile is allowed to escape externally soon grow lean and ill-conditioned. The