at the commencement of the baking, when the temperature of the oven is little over that of the body, and forms dextrin and sugar from the starch, and then the alcoholic fermentation, due to the action of the yeast, begins. The bubbles of carbonic acid, burrowing passages through the bread, make it light and spongy. This enables the digestive juices subsequently to soak into it readily and affect all parts of it. In the later stages of baking, the gas and alcohol are expelled from the bread, the yeast is killed, and a crust forms from the drying of the outer portions of the dough. White bread contains, in 100 parts, 7 of proteid, 55 of carbohydrates, 1 of fat, 2 of salts, and the rest water. COOKING OF FOOD The cooking of foods is a development of civilisation, and much relating to this subject is a matter of education and taste rather than of physiological necessity. Cooking, however, serves many useful ends : 1. It destroys all parasites and danger of infection. This relates not only to bacterial growths, but also to larger parasites, such as tapeworms and trichina. 2. In the case of vegetable foods it breaks up the starch grains, bursting the cellulose and allowing the digestive juices to come into contact with the granulose. 3. In the case of animal foods it converts the insoluble collagen of the universally distributed connective tissues into the soluble gelatin. By thus loosening the binding material, the more important elements of the food, such as muscular fibres, are rendered accessible to the gastric and other juices. Meat before it is cooked is generally kept a certain length of time to allow rigor mortis to pass off. Of the two chief methods of cooking, roasting and boiling, the ormer is the more economical, as by its means the meat is first surrounded with a coat of coagulated proteid on its exterior, which keeps in the juices to a great extent, letting little else escape but the dripping (fat). Whereas in boiling, unless both bouillon and bouilli are used, there is considerable waste. Cooking, especially boiling, renders the proteids more insoluble than they are in the raw state; but this is counterbalanced by the other advantages that cooking possesses. In making beef tea and similar extracts of meat it is necessary that the meat should be placed in cold water, and this is gradually and carefully warmed. In cooking a joint it is usual to put the meat into boiling water at once, so that the outer part is coagulated, and the loss of material minimised. LESSON V SALIVA 1. To a little saliva in a test-tube add acetic acid. Mucin is precipitated in stringy flakes. 2. Filter some fresh saliva to separate cells and mucus, and apply the xanthoproteic or Millon's test to the filtrate; the presence of proteid is shown. 3. Put some 0.5-per-cent starch solution into two test-tubes. Add some filtered saliva to one of them, and put both in the water-bath at 40° C. After five minutes remove them and test both fluids with iodine and Trommer's test. The saliva will be found to have converted the starch into dextrin and sugar (maltose). 4. The presence of potassium sulphocyanide (KCNS) in saliva may be shown by the red colour given by a drop of ferric chloride. This colour is discharged by mercuric chloride. 5. The reaction of saliva is alkaline to litmus paper. The saliva is the first digestive juice to come in contact with the food; it is secreted by three pairs of salivary glands, the parotid, submaxillary and sublingual. The secretions from these differ somewhat in composition, but they are mixed in the mouth, the secretion of the minute mucous glands of the mouth and a certain number of epithelial cells and débris being added to it. The so-called salivary corpuscles' are derived from the glands themselves. The secretion of saliva is a reflex action; the taste or smell of food excites the nerve endings of the afferent nerves (glossopharyngeal and olfactory); the efferent or secretory nerves are contained in the chorda tympani (a branch of the seventh cranial nerve) which supplies the submaxillary and sublingual, and in a branch of the glossopharyngeal which supplies the parotid. The sympathetic branches which supply the blood vessels with constrictor nerves contain in some animals secretory fibres also. The parotid gland is called a serous or albuminous gland; before secretion the cells of the acini are swollen out with granules; after secretion has occurred the cells shrink, owing to the granules having been shed out to contribute to the secretion (see fig. 12). The submaxillary and sublingual glands are called mucous glands: heir secretion contains mucin. Mucin is absent from parotid saliva. B C FIG. 12.-Alveoli of serous gland: A, loaded before secretion; B, after a short period of active secretion; C, after a prolonged period. (Langley.) FIG. 13.-Mucous cells from a fresh submaxillary gland of dog: a, loaded with mucinogen granules before secretion; b, after secretion: the granules are fewer, especially at the attached border of the cell; a' and b' represent cells in a loaded and discharged condition respectively which have been irrigated with water or dilute acid. The mucous granules are swollen into a transparent mass of mucin traversed by a network of protoplasmic cell-substance. (Foster, after Langley.) FIG. 14.-Section of part of the human submaxillary gland. (Heidenhain.) To the right is a group The granules in the cells are larger than those of the parotid gland. 1 In a section of a mucous gland prepared in the ordinary way the mucinogen granules are swollen out, and give a highly refracting appearance to the mucous acini (see fig. 14). COMPOSITION OF SALIVA On microscopic examination of mixed saliva a few epithelial scales from the mouth and salivary corpuscles from the salivary glands are seen. The liquid is transparent, slightly opalescent, of slimy consistency, and may contain lumps of nearly pure mucin. On standing it becomes more cloudy owing to the precipitation of calcium carbonate, the carbonic acid, which held it in solution as bicarbonate, escaping. Of the three forms of saliva which contribute to the mixture found in the mouth the sublingual is richest in solids (2.75 per cent.). The submaxillary saliva comes next (2.1 to 2.5 per cent.). When artificially obtained by stimulation of nerves in the dog the saliva obtained by stimulation of the sympathetic is richer in solids than that obtained by stimulation of the chorda tympani. The parotid saliva is poorest in total solids (0-3 to 0.5 per cent.), and contains no mucin. Mixed saliva contains in man an average of about 0.5 per cent. of solids; it is alkaline in reaction, due to the salts in it; and has a specific gravity of 1,002 to 1,006. The solid constituents dissolved in saliva may be classified a. Mucin this may be precipitated by acetic acid. c. Proteid of the nature of a globulin. d. Potassium sulphocyanide. e. Sodium chloride: the most abundant salt. f. Other salts: sodium carbonate, calcium phosphate and carbonate; magnesium phosphate; potassium chloride. THE ACTION OF SALIVA The action of saliva is twofold, physical and chemical. The physical use of saliva consists in moistening the mucous membrane of the mouth, assisting the solution of soluble substances in the food, and in virtue of its mucin lubricating the bolus of food to facilitate swallowing. The chemical action of saliva is due to its active principle, ptyalin. This substance belongs to the class of unorganised ferments, and to that special class of unorganised ferments which are called amylolytic (starch splitting) or diastatic (resembling diastase, the similar ferment in germinating barley and other grains). A general description of ferments will be found at the end of this lesson. The starch is first split into dextrin and maltose; the dextrin is subsequently converted into maltose also: this occurs more quickly with erythro-dextrin, which gives a red colour with iodine, than in the other variety of dextrin called achroo-dextrin, which gives no colour with iodine. Brown and Morris give the following equation: = 4nC12H22O11+(C6H10O5)n+(C6H10O5)n [maltose] [achroo-dextrin] [erythro-dextrin] Ptyalin acts in a similar way, but more slowly on glycogen; it has no action on cellulose; hence it is inoperative on uncooked starch grains, in which the cellulose layers are intact. Ptyalin acts best at about the temperature of the body (35-40°). It acts best in neutral medium; a small amount of alkali makes but little difference; a very small amount of acid stops its activity. The conversion of starch into sugar by saliva in the stomach stops after 15-30 minutes, before any free acid appears; the acid which is just poured out combining with the proteids in the food, and acid proteid retards salivary action. Free hydrochloric acid immediately destroys ptyalin, so that it does not resume work when the semi-digested food once more becomes alkaline in the duodenum. FERMENTS The word fermentation was first applied to the change of sugar into alcohol and carbonic acid by means of yeast. The evolution of carbonic acid causes frothing and bubbling; hence the term 'fermentation.' The agent yeast which produces this is called the ferment. Microscopic investigation shows that yeast is composed of minute rapidly growing unicellular organisms (torulæ) belonging to the fungus group of FIG. 15.-Cells of the yeast plants. The souring of milk, the transformation of urea into ammonium carbonate in decomposing urine, and the formation of vinegar (acetic acid) from alcohol are produced by very similar organisms. The complex series of changes known as putrefaction which are accompanied by the formation of malodorous |