63. The skin forms the envelope of the body, and is lost in the mucous membranes at the entrance of all the cavities; although some assert that these membranes are only a continuation of it, and thus account for the sympathy which subsists between such structures. Be this as it may, it is evident that the interior organs, especially the stomach, alimentary canal, the lungs, the liver, and the kidneys, sympathize in a very remarkable degree with the surface of the body. So striking and constant is the sympathy between these latter organs and the skin, that they appear capable of reciprocally assisting each other in their operations: where cold contracts the pores of the surface, we find the kidneys excited to a greater degree of activity; and changes in the secretions of the skin are attended with corresponding alterations in the alimentary excretions. Its sympathy with the stomach is also evinced by the phenomena which accompany digestion, It has been ascertained by Lavoisier and Seguin, that the cutaneous transpiration is at its minimum during chymification, and at its maximum after its completion.

No organ of the body, however, is in such close and intimate alliance with the skin as the brain; nor is this surprising when we consider the universal expansion of the sentient extremities of its nerves. That the sympathies of the skin are more direct with the brain than with the heart is shown, as Mr. Travers 1 very acutely observes, by the blush preceding the sensible increase of the heart's action, as well as by the pallor which precedes the state of Syncope. "So also," he adds, "we may observe that the fever of Scarlatina, measles, and smallpox precedes the eruption, and that the inflammation of erysipelas precedes the fever. "In a pathological point of view this principle is extremely important, and is capable of explaining hitherto unintelligible or obscure phenomena.

1 "A Further Inquiry concerning Constitutional Irritation," etc. by B. Travers, F.R.S.

64. According to M. Thenard, the liquid that escapes from the skin is composed of a great deal of water, a small quantity of acetic acid, of muriates of soda and potass, a small proportion of earthy phosphate, an atom of oxide of iron, and a trace of animal matter: the skin exhales also carbonic acid. The average amount of perspired fluid during twenty-four hours may be stated at about thirty ounces.

65. Much controversy has existed with regard to the absorbing powers of the skin: the experiments of M. Seguin would seem to show that, as long as the epidermis remains entire, substances in contact with the surface will not pass into the circulation, but that as soon as it is abraded, absorption takes place. If, however, friction be employed, an effect follows which simple application would not produce; it cannot, for instance, be doubted that mercury and other bodies may be thus made to pass into the system. This question is one of great importance, inasmuch as it would enable us to appreciate the value of nourishing baths of milk and broth; but at present considerable doubt hangs over it. In opposition to the opinion of Seguin, Dr. Dill has lately published a paper in the second volume of the Transactions of the Medico-chirurgical Society of Edinburgh, on the subject of cutaneous absorption. His belief in the existence of such a function, is founded in the fact that in some cases, both of health and. disease, an immoderate quantity of urine is discharged, which he maintains cannot be accounted for on any other principle than that of cuticular inhalation; and in order to confirm such an opinion, Dr. Dill instituted a series of experiments, with a view of discovering whether or not the weight of the body is affected by immersion in the warm-bath. I am bound to say that the results he obtained are far from carrying conviction to my mind.

66. I have thus, with as much brevity as the subject will allow, offered a sketch of the different organs by which aliment is converted into blood. My object has been to bring into one point of view all those facts which are capable of practical application, and to exclude, as far as possible, those which have no other than theoretical relations to the subject before us. Minute anatomy is of little service to the physician; but without a knowledge of the positions and localities of the different organs which constitute the seats of the diseases he may be called upon to cure, he will be inevitably led into error. In consequence of such deficiency, a practitioner at once refers a fulness of the right epigastrium to the liver, forgetting that a distended state of the duodenum may account for the symptom; in like manner, he will attribute to the kidney pains which ought to be referred to the posterior edge of the liver. A hundred parallel examples .might be adduced, but those above stated will answer every purpose of illustration.