290 VARIOUS MOVEMENTS CONNECTED WITH RESPIRATION.

strongly, and which has prevented our feeling the insufficiency of the ordinary respiratory movements. Hence this action is only occasionally connected with mental emotion. Yawning is a still deeper inspiration, which is accompanied by a kind of spasmodic contraction of the muscles of the jaw, and also by a very great elevation of the ribs, in which the shoulders and arms partake. The purely involuntary character of this movement is sometimes seen in a remarkable manner in cases of palsy, in which the patient cannot raise his shoulder by an effort of the will, but does so in the act of yawning. Nevertheless the action may be performed by the will, though not completely; and it is one that is particularly excited by an involuntary tendency to imitation, as every one must have experienced who has ever been in company with a set of yawners. Sobbing is the consequence of a series of short convulsive contractions of the diaphragm; and it is usually accompanied by a closure of the glottis, so that no air really enters. In Hiccup, the same convulsive inspiratory movement occurs, the glottis closing suddenly in the midst of it; and the sound is occasioned by the impulse of the column of air in motion against the glottis. În Laughing, a precisely reverse action takes place; the muscles of expiration are in convulsive movement, more or less violent, and send out the breath in a series of jerks, the glottis being open. This sometimes goes on until the diaphragm is more arched, and the chest more completely emptied of air, than it could be by an ordinary movement of expiration. The act of Crying, though occasioned by a contrary emotion, is, so far as the respiration is concerned, very nearly the same. We all know the effect of mixed emotions in producing something "between a laugh and a cry."

342. The purposes of the acts of coughing and sneezing are, in both instances, to expel substances from the air-passages, which are sources of irritation there; and this is accomplished in both by a violent expiratory effort, which sends forth a blast of air from the lungs.-Coughing occurs when the source of irritation is situated at the back of the mouth, in the trachea, or bronchial tubes. The irritation may be produced by acrid vapours, or by liquids or solids that have found their way into these passages, or by secretions which have been poured into them in unusual quantity as the result of

COUGHING AND SNEEZING-AQUEOUS EXHALATION. 291

disease; and the latter will be the more likely to produce the effect, from the irritable state in which the lining membrane of the air-passages already is. The impression made upon this membrane is conveyed by the nerves spread out beneath its surface to the spinal cord; and the motor impulses are sent to the different muscles, which they combine in the act of coughing. This act consists, 1st, in a long inspiration, which fills the lungs; 2d, in the closure of the glottis at the moment when expiration commences; and 3d, in the bursting-open, as it were, of the glottis, by the violence of the expiratory movement, so that a sudden blast of air is forced up the air-passages, carrying before it anything that may offer an obstruction.-Sneezing differs from coughing in this, that the communication between the larynx and the mouth is partly or entirely closed, by the drawing-together of the sides of the veil of the palate over the back of the tongue; so that the blast of air is directed more or less completely through the nose, in such a way as to carry-off any source of irritation that may be present there.

343. Every one is aware that the air he breathes-forth contains a large quantity of vapour: this is not perceptible in a warm atmosphere, because the watery particles remain dissolved in it and do not affect its transparency; but in a cold atmosphere they are no longer held in solution, and consequently present the appearance of fog or steam. The quantity of fluid which thus passes off is by no means trifling,probably not less than from 16 to 20 ounces in the twentyfour hours; a portion of it undoubtedly proceeds from the moist lining of the mouth, throat, &c., but the greater part is thrown-off by the lungs themselves. This fluid, when collected, is found to contain a good deal of decomposing organic matter, especially in cases in which the respiratory process has not been carried on with perfect freedom; such matter being oxydized and thrown-off under other forms, when the blood is duly aerated. Various substances of an odoriferous character, which have been taken into the blood, manifest their presence in this exhalation: thus turpentine, camphor, and alcohol, communicate their odour to the breath; and when the digestive system is out of order, the breath frequently acquires a disagreeable taint, from the reception of putrescent matters into the blood, and their exhalation through

292

ABSORPTION OF VAPOUR-POISONOUS GASES.

this channel. Of the water of the blood, from which this exhalation is given-off, a small part is most probably formed by the direct union of the hydrogen contained in the food (especially when this is one of its predominating components, § 153) with the oxygen absorbed. For it has been found by careful experiment, that the proportion of inspired oxygen which disappears (not being contained in the carbonic acid expired, § 305), is much greater in animals that are fed on a flesh diet, than in those living on farinaceous food. Another portion of such oxygen probably unites with the sulphur and phosphorus of the food and tissues, to form sulphuric and phosphoric acids, which are excreted through the kidneys in combination with alkaline bases (§ 367).

344. Certain gases act as violent poisons, even when respired in very small proportion. Thus, a Bird is speedily killed by breathing air which contains no more than 1-1500th part of sulphuretted hydrogen; and a Dog will not live long in an atmosphere containing 1-800th part of this gas. The effects of carburetted hydrogen are similar; but a larger proportion is required to destroy life. Both these gases are given-off by decomposing animal and vegetable matter; the neighbourhood of which is consequently very injurious to health. Several cases of arsenical poisoning have occurred, from the accidental inhalation of a small quantity of arseniuretted hydrogen, the amount of arsenic contained in which must have been so minute as to be scarcely appreciable.

CHAPTER VII.

OF EXCRETION AND SECRETION.

General Purposes of the Excreting Processes.

345. WE have seen that the Blood, in the course of its circulation, not only deposits the materials that are converted into the several fabrics of which the body is composed, but also takes-up into itself the products of the decomposition which is continually going-on in its various parts; and it is to replace this, that the constant Nutrition of the tissues is required. In order that the blood may retain its fitness for

OF EXCRETION AND SECRETION.

293

the purposes to which it is destined, it is requisite that these products should be drawn-off from the current of the circulation, as constantly as they are received into it; and this is accomplished by the various processes of Excretion, which are continually taking place in different parts of the body. The uninterrupted performance of these is even more essential to the maintenance of life, than is an uninterrupted supply of nutritive materials; for an animal may continue to exist for some time without the latter, but it speedily dies if either of the more important excretions be checked. We have a striking instance of this in the case of the Respiration, which may be regarded as a true function of Excretion, having for its object to set free Carbonic acid from the blood in a gaseous form, thereby contributing to the introduction of Oxygen into the blood, for the various important actions to which that element is subservient, especially the maintenance of Animal Heat. (Chap. IX.) The effects of the suspension of the respiratory process, even for a few minutes, in a warmblooded animal, have been shown (§ 338) to be certainly and speedily fatal; and they are as certainly fatal in the end in cold-blooded animals, though a longer time is required to produce them.

346. The products of excretion are the same, as to their essential characters at least, through the whole Animal kingdom; and for this it is not difficult to find a reason. It will be remembered that the ultimate elements of the Animal tissues are four in number: oxygen, hydrogen, carbon, and nitrogen; and that the materials which make up the chief part of the fabric of different classes of animals—albumen, gelatin, fatty matter, &c.-contain these elements united in constant proportions, from whatever source we obtain them. Hence we should expect to find the products of their decomposition also the same; and this is, for the most part, the case. Of these four ingredients, oxygen can never be said (in the healthy state at least) to be superfluous in the body; for a large and constant supply of it is required, to unite with the others and carry them off in their altered conditions. Thus, unless oxygen were continually introduced into the system, for the sake of uniting with the carbon that is to be thrown off by Respiration, that excretion must be checked; and it is required, in like manner, for uniting with hydrogen

294

NATURE AND OBJECTS OF EXCRETORY ACTIONS.

to form water, and with compounds of nitrogen to form urea. Hence there is no need of an organ to carry off the superfluous oxygen; but an organ to introduce it is rather required; and this purpose, as we have seen, is answered by the Respiratory apparatus. But we find organs of excretion specially destined to carry off the carbon, hydrogen, and nitrogen, which are set free, under various forms, by the decomposition of the tissues. Thus the Respiratory organs, as we have seen, throw off carbon in the form of carbonic acid, and hydrogen which has been in like manner united with oxygen so as to form water. The Liver has for its office partly to separate these same elements from the blood in a different form, throwing them off in the condition of a peculiar fatty matter, which consists almost entirely of carbon and hydrogen. But it has another function of no less importance in animals whose respiration is active; for by its agency the hydro-carbonaceous matter circulating in the blood is brought into a state in which it readily combines with oxygen to form carbonic acid and water; and thus the liver may be said to prepare the pabulum for the combustive process. Lastly, the Kidneys have for their chief object to throw off the azotized compounds which result from the decomposition of the tissues; these contain a very large proportion of azote or nitrogen, which is united with the other elements into the crystalline compounds, urea, and uric or lithic acid, the latter of which is usually thrown off in combination with soda or ammonia. And the kidneys further serve as the channel through which soluble matters of various kinds, which have found their way into the current of the circulation, and are foreign to the composition of the blood, are eliminated from it.

347. It is obvious that, when an animal has retained its usual weight for any length of time without change, the total weight of its excretions must be equivalent to the total weight of the solids and fluids it has taken-in. If these last have been no more in amount than was absolutely necessary for the maintenance of the body during that period, all the azotized portion of the food was first appropriated to the formation of the azotized tissues; whilst the non-azotized portion was used-up in maintaining the respiration (§ 157). Consequently, no part of the food would pass at once into the biliary and urinary excretions; and these would have no other function than to