number of highly refracting, transparent, oily globules floating in a transparent liquid; these globules are contained in a very thin investing membrane, which is not visible till the milk is treated with dilute acetic acid. Agitation of the milk with ether does not dissolve the fat, but if a small quantity of potash be added to milk, and it be then shaken up with ether, the fat is completely dissolved, the pellicle which envelopes the globules having been removed by the action of the potash.

The object of churning, in the preparation of butter from cream, is to break up mechanically the little sacculi which enclose the oily matter, and thus to facilitate the agglomeration of the fatty particles into masses, and to favour the separation of the liquid constituents which enter into the formation of the buttermilk. The more completely the albuminous and cascous matters are washed away, and the more compactly the butter is pressed, the longer may the butter be preserved without becoming rancid, since the cascous matter acts as a ferment, which speedily effects a partial decomposition of the fat. The addition of salt to the butter preserves the fats unaltered for a considerable period, by retarding the decomposition of the azotised matter.

In addition to oily matter, milk contains two other characteristic ingredients, viz., milk-sugar and casein, both of which are held in solution. Albumin is absent from this solution in its usual condition, but it is abundant in the colostrum, or milk which is first produced after the birth of the young animal. The quantity of albumin in the colostrum of the cow is so considerable, that it coagulates when heated. The colostrum is denser than normal milk; it has a yellowish colour, and approaches serum in quality. It is worthy of observation that the sugar contained in milk is of a kind which does not undergo alcoholic fermentation, and consequently is not liable to produce an evolution of carbonic anhydride during digestion, so that distension of the tender stomach and intestines of the young animal from this cause is guarded against.

The salts contained in milk consist of the chlorides of sodium and potassium, of phosphates of the alkalies, and of a certain proportion of soda and potash, which are combined with the casein and confer solubility upon it; in addition to which, phosphates of calcium and of magnesium are also found in considerable quantity. These phosphates are essential to the development of the bones of the young animal. A minute quantity of ferric oxide, amounting, according to Haidlen, to o'47 per cent. of the entire quantity of ash, is also found in milk. No sulphates, lactates, or salts of ammonium, are present in fresh milk. The quantity of salts

found in cow's milk usually amounts to about o'7 per cent. In woman's milk it is about o2 per cent. The composition of milk varies, not only in the different species of animals, but also in the same animal at different periods of lactation, the proportion of butter being especially liable to fluctuation, and being greatly under the influence of diet: it also appears that the milk last yielded during the act of milking is the richest. The following table comprises the results of the analysis of 100 parts of milk in various species of animals, and will give an idea of the general nature of this liquid :

The spontaneous acidification of milk when kept, and the consequent curdling of the milk, as well as the action of acids generally, in coagulating milk, and the effect of various neutral salts of the earths upon this liquid, have already been mentioned (1119, 1657).

B. Liquids concerned in Digestion.

(1688) 1. THE SALIVA. This secretion is poured out by the parotid and other glands, the ducts of which empty themselves into the cavity of the mouth. Its chief use appears to be to lubricate and moisten the food, and to facilitate the act of deglutition. The saliva varies in composition considerably in different animals. In man it is an opalescent, somewhat viscid liquid, which froths remarkably on agitation. The quantity of fixed solids which it contains in solution was found by Lehmann to lie between 0388, and o'841 per cent., but it has been observed as high as 16 per cent. In the dog the solids amount to about 103, and in the horse to 1 per cent. In addition to epithelial particles, and to the mucus derived from the mouth itself, the saliva contains a peculiar organic principle, termed ptyalin (from TTUEλov, spittle), which resembles sodic albuminate, and which is very prone to putrefaction. Ptyalin constitutes about one-third of the soluble solids of the saliva. It is characterized by its power of converting starch, even in the granular form, into dextrin and into sugar.

The saliva in health has a feebly alkaline reaction; its alkalinity is increased during mastication, and diminishes after the process of digestion is completed. In inflammatory affections of the prime via the saliva is, however, generally acid, and the same fact has been observed in various other forms of inflammatory disease. The inorganic constituents of the saliva consist of salts of potassium and sodium, with a considerable quantity of the salts of calcium, which last is deposited from the saliva of many animals (the horse, for example) in the form of crystallized carbonate, when the secretion is exposed to the air and allowed to absorb carbonic acid. It likewise occasionally contains lactates of the alkali metals. Saliva also always contains a small quantity of calcic phosphate. This calcic phosphate mingled with a variable proportion of calcic carbonate and of salivary mucus, constitutes those calculous concretions which sometimes form upon the teeth, and are known as tartar. Minute quantities of potassic sulphocyanide are also present in the saliva, as may be proved by the red colour which it yields on the addition of a dilute solution of ferric chloride. When sulphur has been taken medicinally, Wright found the quantity of the sulphocyanide to be considerably increased. The sulphates are almost entirely absent from the ash of saliva.

(1689) 2. THE GASTRIC JUICE. This important secretion is the principal agent in effecting the digestion of the albuminoid portions of the food, but it exerts scarcely any action upon the starchy and fatty constituents. The gastric juice, as its name. (from yaorup, the stomach) implies, is poured out from the lining membrane of the stomach. Its composition varies at different times, in consequence of which, when obtained from an empty stomach, its reaction upon litmus paper is either neutral or slightly alkaline; but after the ingestion of food into the stomach, it is always acid. In this, which may be considered its normal state, the gastric juice is a clear colourless liquid, which has a peculiar odour, and a slightly saline and acid taste. It does not become turbid when boiled, and it is remarkable for its antiseptic powers. It may be kept for many days at 100° F. without exhibiting any tendency to become putrid. The nature of the acid contained in the gastric juice has been much disputed, some chemists maintaining with Prout that it is the hydrochloric, others with Blondlot, that it consists of phosphoric acid in the form of superphosphate of calcium, and others, that it is composed of lactic acid.

In truth it appears generally to consist of a mixture of

880

THE GASTRIC JUICE-PEPSIN.

hydrochloric and lactic acids. The principal saline matters present are common salt, with small quantities of calcic and magnesic chlorides, sodic lactate, and traces of calcic and ferric phosphates. The quantity of calcic phosphate is very small, and the sulphates and phosphates of the alkali metals are almost entirely wanting. In addition to these bodies, however, the gastric juice contains a small quantity of a peculiar organic compound, which has been termed pepsin (from πéfis, cooking), to which, in conjunction with the free acid, the remarkable solvent and digestive powers of the gastric juice are owing.

Pepsin is an albuminoid body, soluble in water, but insoluble in alcohol. Its aqueous solutions are precipitated by corrosive sublimate, by salts of lead, and by solutions of tannic acid. When boiled it loses its peculiar power of effecting digestion. An artificial gastric juice, which acts as a solvent upon muscular fibre, boiled eggs, and albuminoid substances generally, may be obtained by digesting the mucous membrane of the stomach with a warm but very dilute solution of hydrochloric acid. Such a solution, if maintained at a temperature of 100° F., will in the course of six or eight hours dissolve pieces of hard-boiled egg and of beef; but the solutions thus obtained do not coagulate on the application of heat. They contain what Prout terms incipient albumin, which does not acquire the ordinary properties of albumin until after it has been subjected to the action of the pancreatic and biliary secretions. In preparing this artificial digestive liquid the mucous membrane of the stomach of the pig is generally employed as the source of the pepsin, the most active portion being the glandular layer, extending chiefly along the greater curvature towards the cardia.

In the appendix to Lehmann's work (vol. iii. p. 503), it is stated on the authority of Gruenewaldt, who had an opportunity of making experiments upon a woman suffering from a gastric fistula, that the quantity of the gastric juice secreted in twentyfour hours amounted to the enormous quantity of 31 lb. ; but the greater portion of this liquid was, of course, re-absorbed by the mucous surface. In this case the gastric juice was carefully analysed by Schmidt. The following are his results. It had a sp. gr. of 1020. When heated, it emitted an odour of butyric and propionic acids, and was found to contain in 1000 parts:—

No free hydrochloric acid was found in this analysis; but when the stomach of this patient was irritated by introducing peas instead of masticated food, free hydrochloric acid was obtained from the gastric juice, which was secreted unmixed with saliva.

Schmidt found the quantity of free hydrochloric acid in the gastric juice of dogs, when unmixed with saliva, to amount to from 0'245 to 0'423 per cent. Small quantities of chloride of ammonium were also present in it.

(1690) 3. THE PANCREATIC FLUID.-The secretion from the pancreas resembles the saliva in some respects. It has an alkaline reaction, and putrefies rapidly. It possesses the power of saccharifying starch in an eminent degree, so that it appears to assist in an important manner in the assimilation of the amylaceous portion of the food, which is not rendered soluble by the action of the gastric juice. Bernard considers that one of its chief functions is to aid in the assimilation of fatty substances, but the investigations since made in Germany by several careful and experienced observers have thrown some doubt upon the accuracy of this view. It is, however, certain that the pancreatic fluid furnishes a very perfect emulsion when agitated with oil, and it has been found that such an emulsion continues to exhibit the same milky appearance if left at rest for fifteen or twenty hours. Under these circumstances the emulsion, which at first has an alkaline reaction, generally becomes acid, and butyric and other volatile acids are found in the liquid. The liquid from the pancreas generally becomes coagulated when heated; it is one of the few secretions which contains albumin in the soluble form, the proportion of albumin having been found by Tiedemann and Gmelin, in some cases, to amount to 4 per cent. of the entire liquid. They also ascertained the presence of albumin in this secretion in the horse, the sheep, and the dog.

(1691) 4. MUCUS.-Those cavities of the body which communicate with the external surface are lined with a peculiar kind of membrane, distinguished as the mucous membrane. The material of which these membranes consist appears closely to resemble albu