Physiology

The Principle of Specific Chemical Response.

There are a number of relatively simple chemical substances, any of which if injected into the body cause the body to produce some other substance which unites with and in a sense neutralises it. To give three examples :—If a fatty acid be administered to the alimentary canal, the acid will be absorbed by the villi, the cells of which will produce glycerine and the result will be a neutral fat. If on the other hand benzoic acid be administered it will get so far as the kidney and there the kidney cells will produce glycine, turning it into hippuric acid. Lactic acid similarly if thrust into the general circulation will be excreted as ammonium lactate, the ammonia being produced not by the cells of the body generally but by those, or certain of those, in the kidney. We have now arrived at the confines of the great subject of immunity —and perhaps outside the region of the present article.

The Principle of Dual Control.

The body has two ways of doing a great many things. Examples of this principle are so numerous as to be found in almost any system of the body. To take the point at which two of the most important systems of the body—the nervous and the respiratory systems—touch, let us consider the nervous control of respiration.

The essence of respiration is a rhythmic outflow of alternate inspiratory and expiratory impulses from the brain to the muscles concerned. That rhythm is probably, but not certainly, inherent in the brain itself, but it is regulated by influences which play upon the brain. These influences fall into two general categories— those which arrive along the nervous paths from the lungs and elsewhere and those which are borne to the brain in the blood stream.

An increase of total respiration may then take place in either of two ways—(a) many forms of irritation in the lung produce an increased total ventilation which is abolished by cutting the vagi and which may therefore be regarded as being due to special stimuli passing up along these nerves, (b) increase of carbonic acid in the blood even in the absence of nervous stimulation will so act upon the respiratory centre as to increase the total ventila tion. During muscular exercise both the nervous and the chemical factors come into play; the nervous is the first to be initiated and therefore it may be regarded as the most sensitive; but even were it not involved a sufficient regulation could probably be carried out by the carbonic acid in the blood.

The respiratory centre furnishes another example. This time it is a combination of a nervous with a physical regulation. The

facts are principally based on the observations of Professor Richet. If a dog lies down in the sun he will very shortly com mence to pant in rapid, shallow respirations passing air over his tongue and through his respiratory passages. According to Pro fessor Richet this tachypnoea is associated with an actual lower ing of the body temperature and is due to stimuli from the warm skin reaching the respiratory centre via the sensory nerves. But even did not this nervous regulation of the body temperature take place another would supervene of perhaps a cruder character, for as soon as the body temperature began to rise, the warmer blood reaching the respiratory centre would itself produce an increased ventilation. Here again the nervous effect is the first to be in voked and presumably the more delicate.

The digestive system abounds with examples of duality. Of the three principal forms of food which are digested, carbohydrates, proteins and fats, the organs for the secretion of the digestive ferments are in all cases reduplicated. Starch is turned into sugar both by the saliva and by the pancreas, protein is broken down to the peptone stage both by the pepsin of the stomach and the trypsin of the pancreas, while to quote Leathes "two fat splitting enzymes are present in the intestinal contents, one being provided by the pancreatic juice and the other by the intestinal juice. . . . The action of the intestinal juice is however overshadowed by that of the pancreatic juice." Not only are the ferments which effect digestion duplicated but there is a measure of duplication about the mechanisms re sponsible for their secretion. The processes of the alimentary canal are actuated, some by nervous stimuli, some by chemical, but in large measure by both. The secretion of saliva is entirely actuated by nervous stimuli; gastric juice is secreted largely as a result of nervous stimuli, but its flow is maintained by chemi cal bodies. In the secretion of pancreatic juice chemical stimuli play the more prominent part, nervous stimuli not however being entirely absent ; whilst the secretion of the intestine is—so far as is known—due entirely to hormones. If the whole process from end to end be reviewed, some of the statements made about the respiratory centre are applicable. The process of digestion is initiated by nervous and maintained by chemical stimuli. As Dr. Anrep has pointed out, the further the food gets from the surface of the body, the less is it under the influence of the nervous, the more is it under the influence of the hormonal system.