quickened vitality of all the other leading organs is unavailing for a perfectly healthy tone while the skin is depressed. But it must be also true, that we are in a peculiar degree sensitive to changes in the condition of the skin, owing no doubt to its great supply of nerves.

15. The consequences of Heat are, in nearly every particular, the opposite of those now stated. Acute or intense heats agree with intense colds in being simply destructive and painful. Within the point of injury to the tissues, heat is a pleasurable sensation. The pleasure of heat, like the pain of cold, is voluminous or massive. There are cases, however, distinguished by intensity rather than by quantity; indeed, this distinction of quantity and intensity, used as a part of the description of feelings, has its perfect type in the case of temperature, there being a physical reality corresponding to the mental facts. Sometimes we have great intensity and small quantity, as in the scorching rays of a fire, or a cup of hot tea at other times we have large quantity with low intensity, as in a hot bath, a warm room, a warm bed. The hot bath is the extreme instance. By no other contrivance can such a mass of heat be brought to bear upon the human system consequently this presents the sensation of warmth in its most luxuriant form. It is the intoxication of animal beat. We are unavoidably led to assume that this warmth must act powerfully on the sensitive nerves; for it is hardly to be supposed, that the organic processes are so greatly furthered by the sustained temperature as to exalt the pleasurable consciousness in this remarkable degree. Indeed, we may derange the system by excessive heat, without producing the painful feeling arising from cold.

In the case of morbid activity of the nervous system, warmth is a soothing influence, either by its physical effects, or by the nature of the sensation, or from both combined.

The feelings of Respiration, and those of Heat and of Cold, illustrate in a marked manner the fundamental doctrine of Relativity, or of change as a condition of consciousness. There is no feeling of respiration, unless by increase or

MATERIALS OF FOOD.

123

diminution of the action of the lungs; and if we lived in an even temperature, heat and cold would be alike unknown. The induction of the principle of Relativity as regards these states is complete.

Sensations of the Alimentary Canal.

There

16. Digestion offers all the conditions of a sense. is an external object-the Food; a distinct organ of sensethe Alimentary Canal and its appendages; and a set of Feelings arising from the contact, also distinct and specific. To treat these feelings uuder Taste, is to confound together two senses totally different in their character, although happening to have one common object or stimulant.

The objects of this sense are the materials taken into the body as food and drink. These materials are extremely various, but there is no corresponding variety in their action on the stomach. They can be reduced to a few general heads, according to their composition, it being found possible to assign a few leading substances that comprehend all the different sorts of material serviceable in nourishing the body. The following is an abstract of this classification :

1st. Water and the watery liquids, including substances conveyed in solution, or suspension, in water.

2nd Saccharine substances derived from the vegetable kingdom. These comprehend sugars, starch, gums, vinegar.

3rd. Oily substances. These include the various fats and oils as well as alcohol. Like the former group, they are composed of carbon and the elements of water, but in them the carbon is in a much higher proportion.

4th. Albuminous substances, containing nitrogen: fibrine, gelatine, albumen, caseine (matter of cheese), vegetable gluten. All the materials which make up this group are derived generally from the animal kingdom, with the exception of the last, which is contained in great abundance in wheat; similar if not identical principles exist in other vegetables. Wheat, indeed, consists of two substances-one referable to

the saccharine group, the other to the albuminous, the former consisting of starch, the latter of gluten.'

Milk is found to contain matter of all the four classes : water, sugar, oily matters (butter), caseine.

The three first classes are incapable of nourishing the principal animal tissues, such as nerve, muscle, &c. They are fitted rather for supplying fat, bile, and matters used in the production of the carbonic acid that escapes from the lungs. Being supposed to be mainly destined for the supply of animal heat, by being combined with oxygen, or slowly burned, they were formerly termed calorifacient; but this is now reckoned a too narrow view. Experiments recently made have proved that their combustion is the chief source of muscular power; being an example of chemical combination transmuted into mechanical force, of which a parallel is found in the steam-engine. The same combustion may also be the source of the nerve force; the parallel case being the voltaic circuit, where the electricity is evolved from chemical combination in the cells.

The albuminous bodies are undoubtedly the tissueforming material, having a composition fitted for the purpose. But they are not confined to this function; in their final transformations and decay, they may be at last oxidized and become the source of heat, muscular force, and nerve force, like the others.

Certain substances of the saline, earthy, or mineral class, are requisite; most of them being found in the usual articles of food Salts of soda, potash, and lime, as well as iron and phosphorus, are essential ingredients.

The Stimulants are classified into spices, or condiments; vegetable alkaloids, as tea, coffee, cocoa; extractives, as creatin and creatinin, occurring in the juice of meat; and the alcoholic beverages. For the most part, these substances are not directly nutritive; they act as stimulants to the nervous system, and also retard the waste of tissue. The organic vegetable acids,―vinegar, the acids of fruit, and lactic acid, are in extensive use as an ingredient of food.

ORGANS OF DIGESTION.

125

If we

The differences that exist among the infinity of articles used as food are not at bottom so great as they seem. take the different species of grain,-wheat, barley, rye, oats, rice, maize, millet, we shall find they are all composed of the same ultimate materials, gluten and starch, though not in the same proportions. In like manner, the potato is a starchy vegetable, with a very small share of gluten, hence the defective character of it as an article of nourishment. Another difference among vegetables relates to their texture, as fitting them for being acted on during mastication and digestion,a circumstance, however, that cooking can modify. Thus the potato is a much looser texture than grain. A third print of distinction among alimentary substances, is the extraneous essences that may enter into them, and affect the sense of taste, and the general relish, as in the difference between mutton and beef, chicken and venison, brandy and

rum.

17. I extract from Quain's Anatomy the following general view of the Organs of Digestion.

'The digestive apparatus includes that portion of the organs of assimilation, within which the food is received and partially converted into chyle, and from which, after the chyle has been absorbed, the residue or excrement is expelled. It consists of a main or primary part named the alimentary canal, and of certain accessory organs.

'The alimentary canal is a long membranous tube, commencing at the mouth and terminating at the anus, composed of certain tunics or coats, and lined by a continuous mucous membrane from one end to the other. Its average length is about thirty feet, being about five or six times the length of the body. The upper part of it is placed beneath the base of the skull, the succeeding portion is situated within the thorax, and the remainder is contained within the cavity of the abdomen. In these several situations, its form, dimensions, and connexions, its structure and functions, are so modified that certain natural divisions of it, bearing different names, have been recognized by anatomists.

'It may be considered as composed of two parts: one situated

above the diaphragm, and the other below that muscular partition, and therefore within the abdomen. The first division consists of the organs of mastication, insalivation, and deglutition; and comprises the mouth, the pharynx, and the oesophagus, or gullet. The second division consists of the organs of digestion, properly so called, and of those of defecation; viz., the stomach, the small intestine, and the great intestine.

'The accessory parts are chiefly glandular organs, which pour their secretion into it at different points. They consist of the salivary glands (named the parotid, submaxillary, and sublingual), the liver, and the pancreas. Besides these large glandular organs, a multitude of small glands, compound, follicular, or tabular, are collected together at certain points, or scattered over large portions of the inner surface of the alimentary canal: these are described along with the mucous membrane of each part. The remaining accessory organs are the teeth, the jaws, the tongue, and the spleen.'—Vol. III. p. 85.

18. The physiology of digestion must be very briefly stated here. The first stage is mastication, which serves the double purpose of breaking down the food and mixing it with saliva; the function of the saliva is now known to be to convert the starch into grape sugar, by a process of the nature of fermentation. The effort of mastication is purely voluntary; but when the food gets upon the back part of the tongue, it is passed into the bag of the pharynx and propelled down the gullet into the stomach by involuntary muscular contractions. In the stomach, it is exposed to the action of the gastric juice. This peculiar action is not as yet fully understood, but so far as the researches of physiologists have yet gone, the most reasonable conclusion is, that 'in man and the carnivora the fluid secreted by the stomach during digestion simply dissolves animal and vegetable substances of the azotized kind, so as to render them capable of absorption, without materially altering their chemical constitution, leaving starchy, oily, saccharine, and the allied substances but little or not at all acted on.' The matter that leaves the stomach to pass into the intestines, is known by the name of chyme. This is very soon mixed up with