muscles: the force by which it is accomplished, and the course of the blood are alike simple.

Changes in the Blood.-The most obvious change which the blood of the pulmonary artery undergoes in its passage through the lungs is 1st, that of colour, the dark crimson of venous blood being exchanged for the bright scarlet of arterial blood; 2nd, and in connection with the preceding change, it gains oxygen; 3rd, it loses carbonic acid; 4th, it becomes slightly cooler; 5th, it coagulates sooner and more firmly, apparently containing more fibrin. The oxygen absorbed into the blood from the atmospheric air in the lungs is combined chemically with the hæmoglobin of the red blood-corpuscles. In this condition it is carried in the arterial blood to the various parts of the body, and brought into near relation or contact with the tissues. In these tissues, and in the blood which circulates in them, a certain portion of the oxygen, which the arterial blood contains, disappears, and a proportionate quantity of carbonic acid and water is formed. The venous blood, containing the new-formed carbonic acid returns to the lungs, where a portion of the carbonic acid is exhaled, and a fresh supply of oxygen is taken in.

Mechanism of Various Respiratory Actions.

It will be well here, perhaps, to explain some respiratory acts, which appear at first sight somewhat complicated, but cease to be so when the mechanism by which they are performed is clearly understood. The accompanying diagram (fig. 160) shows that the cavity of the chest is separated from that of the abdomen by the diaphragm, which, when acting, will lessen its curve, and thus descending, will push downwards and forwards the abdominal viscera; while the abdominal muscles have the opposite effect, and in acting will push the viscera upwards and backwards, and with them the diaphragm, supposing its ascent to be not from any cause interfered with. From the same diagram it will be seen that the lungs communicate with the exterior of the body through the glottis, and further on through the mouth and nostrils-through either of them separately, or through both at the same time, according to the position of the soft palate. The stomach communicates with the exterior of the body through the œsophagus, pharynx, and mouth; while below the rectum opens at the anus, and the bladder through the urethra. All these

openings, through which the hollow viscera communicate with the exterior of the body, are guarded by muscles, called sphincters, which can act independently of each other. The position of the latter is indicated in the diagram.

Sighing.--In sighing there is a rather prolonged inspiration; the air almost noiselessly passing in through the glottis, and by

the elastic recoil of the lungs and chest-walls, and probably also of the abdominal walls, being rather suddenly expelled again.

Now, in the first, or inspiratory part of this act, the descent of the diaphragm presses the abdominal viscera downwards, and of course this pressure tends to evacuate the contents of such as communicate with the exterior of the body. Inasmuch, however, as their various openings are guarded by sphincter muscles, in a state of constant tonic contraction, there is no escape of their

contents, and air simply enters the lungs. In the second, or expiratory part of the act of sighing, there is also pressure made on the abdominal viscera in the opposite direction, by the elastic or muscular recoil of the abdominal walls; but the pressure is relieved by the escape of air through the open glottis, and the relaxed diaphragm is pushed up again into its original position. The sphincters of the stomach, rectum, and bladder, act in the same manner as before.

Hiccough resembles sighing in that it is an inspiratory act; but the inspiration is sudden instead of gradual, in consequence of the diaphragm acting suddenly and spasmodically; and the air, therefore suddenly rushing through the unprepared rima glottidis, causes vibration of the vocal cords, and the peculiar sound.

Coughing. In the act of coughing, there is most often first of all a deep inspiration, followed by an expiration; but the latter, instead of being easy and uninterrupted, as in normal breathing, is obstructed, in consequence of the glottis being momentarily closed by the approximation of the vocal cords. The abdominal muscles, then strongly acting, push up the viscera against the diaphragm, and thus make pressure on the air in the lungs until its tension is sufficient to noisily burst open the vocal cords which oppose its outward passage. In this way considerable force is exercised, and mucus or any other matter that may need expulsion from the air-passages is quickly and sharply expelled by the outstreaming current of air.

It will be evident on reference to the diagram (fig. 160), that pressure exercised by the abdominal muscles in the act of coughing, acts as forcibly on the abdominal viscera as on the lungs, inasmuch as the viscera form the medium by which the upward pressure on the diaphragm is made, and there is of necessity quite as great a tendency to the expulsion of their contents as of the air in the lungs. The instinctive, and if necessary, voluntarily increased contraction of the sphincters, however, prevents any escape at the openings guarded by them, and the pressure is effective at one part only, at the rima glottidis.

Sneezing. The same remarks that apply to coughing, are almost exactly applicable to the act of sneezing; but in this instance the blast of air, on escaping from the lungs, is directed, by an instinctive contraction of the pillars of the fauces and descent of the soft palate, chiefly through the nose, and any offending matter is thence expelled.

Speaking.—In speaking, there is a voluntary expulsion of air through the glottis by means of the expiratory muscles. The vocal cords are put, by the muscles of the larynx, in a proper position and state of tension for vibrating as the air passes over them, and thus sound is produced. The sound is moulded into articulate speech by the tongue, teeth, lips, &c.--the vocal cords producing the sound only, and having nothing to do with articulation.

Singing. Singing resembles speaking in the manner of its production; the laryngeal muscles, by variously altering the position and degree of tension of the vocal cords, producing the different notes. Words used in the act of singing are of course framed, as in speaking, by the tongue, teeth, lips, &c.

Sniffing. Sniffing is produced by a rapidly repeated but incomplete action of the diaphragm and other inspiratory muscles. The mouth is closed, and the whole stream of air is made to enter the air-passages through the nostrils. The alæ nasi are, commonly, at the same time, instinctively dilated.

Sobbing. Sobbing consists of a series of convulsive inspirations, at the moment of which the glottis is usually more or less closed.

Laughing.-Laughing is made up of a series of short and rapid expirations.

Yawning.-Yawning is an act of inspiration, but is unlike most of the preceding actions, as it is always more or less involuntary. It is attended by a stretching of various muscles about the palate and lower jaw, which is probably analogous to the stretching of the muscles of the limbs in which a weary man finds relief, as a voluntary act, when they have been some time out of action. The involuntary and reflex character of yawning probably depends on the fact that the muscles concerned are themselves at all times more or less used involuntarily, and require, therefore, something beyond the exercise of the will to set them in action. For the same reason, yawning, like sneezing, cannot be well performed voluntarily.

Sucking. Sucking is not properly a respiratory act, but it may be most conveniently considered in this place. It is caused chiefly by the depressor muscles of the os hyoides. These, by drawing downwards and backwards the tongue and floor of the mouth, produce a partial vacuum in the latter: and the weight of the atmosphere then acting on all sides tends to produce equili

brium on the inside and outside of the mouth as best it may. The communication between the mouth and pharynx is completely shut off by the contraction of the pillars of the soft palate and descent of the latter so as to touch the back of the tongue; and the equilibrium, therefore, can be restored only by the entrance of something through the mouth. The action, indeed, of the tongue and floor of the mouth in sucking may be compared to that of the piston in a syringe, and the muscles which pull down the os hyoides and tongue, to the power which draws the handle.

Influence of the Nervous System in Respiration. Like all other functions of the body, the discharge of which is necessary to life, respiration is essentially an involuntary act. Unless this were the case, life would be in constant danger, and would cease on the loss of consciousness for a few moments, as in sleep. It is, however, also necessary that respiration should be to some extent under the control of the will. For were it not so, it would be impossible to perform those voluntary respiratory acts which have been just discussed, such as speaking, singing, and

the like.

The respiratory movements and their rhythm, so far as they are involuntary and independent of consciousness, as they are on all ordinary occasions, are under the governance of a nerve-centre in the medulla oblongata which corresponds in position with the origin of the pneumogastric nerves; that is to say, the muscles concerned in the respiratory movements, are excited by stimuli which issue from this part of the nervous system, and which are conveyed by the various motor nerves supplying the muscles. These nerves are the phrenics and intercostals chiefly. On division of one phrenic, for example, the corresponding half of the diaphragm supplied by it ceases to take part in the respiratory movement, and on division of both nerves, the whole muscle ceases to act. Similarly, division of the intercostal nerves one by one produces cessation of action of the muscles supplied by them. To what extent the medullary centre acts automatically, i.e., how far the stimulus originates in it, or how far it is merely a nerve-centre for reflex action, is not certainly known.

It is clear, however, that the medullary centre is bilateral or double, since the respiratory movements continue after the medulla at this point is bisected in the middle line.