I. Physiological Relations

The red corpuscle of the blood is the oxygenating or respiratory element of the body. The physical part of respiration is carried on by means of the chest and respiratory passages, a fresh supply of oxygen being continually presented to the red corpuscles, and carbonic acid, water, and heat given off from the plasma.

The red corpuscle and the chest are brought into functional relation with each other by means of a special nervous mechanism, called the respiratory centre, a portion of nervous matter in the medulla oblongata which is peculiarly irritable in the presence of oxygen, and sends motor impulses through the cord to the respiratory muscles.

The less the amount of oxygen admitted to the respiratory centre, the more powerfully is it stimulated, and the chest moved; the greater the amount of oxygen admitted to the centre, the less powerful its discharges, and the more weak or superficial is the breathing. Now the amount of oxygen in the arteries of the medulla is the same as in the systemic arteries generally; and we thus find that the state of oxygenation of the arterial blood governs the respiratory movements through the medium of the respiratory centre. The fundamental canon in the physiology of respiration is that the condition of the red corpuscle is the prime mover of all respiratory acts. Carbonic acid has no direct effect on the respiratory centre.

The term "centre" implies that certain influences meet in this point, originating in a circle of which it is the middle point; and this is the case. Falling into the respiratory centre are impressions conveyed by afferent-usually sensory-nerves, from every part of the body, modifying its activity, and reflexly influencing the respiratory movements. The vagus is peculiarly capable of stimulating the centre; thus irritation of the larynx immediately causes the reflex respiratory act called cough. The vagus is therefore said to be the special afferent nerve of respiration; the whole surface of the respiratory passages, and probably the lungs, being abundantly supplied with rootlets of the vagus, which are incessantly collecting impressions for transmission to the centre. Every change in the distension of the lungs, and in the quantity and quality of the pulmonary blood, thus instantly tells on the respiratory movements. It must also be carefully noted in this connection that diminished oxygenation of the blood, whilst increasing the respiratory activity, stimulates the other two great centres in the medulla, increasing the arterial resistance through the vaso-motor centre, and slowing the heart through the cardiac centre.

The afferent impressions from the lungs and respiratory passages, besides falling into the respiratory centre, also reach, if sufficiently powerful, the convolutions, where they are felt as various sensations, referred more or less accurately to the respiratory organs. In health these sensations of common sensibility are feeble; and we do not appreciate them until they are converted into sensations of pain, oppression, distress, or irritation, in disorder or disease.

Amongst the nerves of the respiratory muscles one group demands special notice, viz. those distributed to the bronchi. These are motor filaments of the vagus, which originate in the respiratory centre and supply the muscles regulating the calibre of the air-tubes. They bring the bronchi under the control of the medulla, and thus of the afferent impressions, especially of those very impressions which originate in the respiratory passages, the seat of their own distribution.

II. Pharmacodynamics

The extensive relations of the respiratory organs to the external air, to the blood and circulation, and to the nervous system, afford us abundant means of influencing their mode of action. These means we will now review in their natural physiological order:

1. The Air

The Air. The air which comes in contact with the organs of respiration may be altered in five different respects, each of which will have a physiological effect upon the functions of the lungs, viz. as regards (a) its absolute amount, (b) its chemical composition, {c) its temperature, (d) its moisture, and (e) its pressure.

(a) The supply of air, like that of the food, may be entirely arrested for a time, another gas with different physiological properties, such as Nitrous Oxide, being allowed to take its place. Or the amount respired may be simply reduced, by administering rarefied air; or increased, by admitting oxygen or compressed air into the lungs. The same effects may be produced by ordering little or much muscular exercise respectively.

(b) The chemical composition of the atmosphere, physiologically speaking, relates only to the amount and quality of the oxygen. The proportion of oxygen to nitrogen in the air may he modified by arrangements for special inhalation, but practically this is seldom attempted, mountain and ocean climates affording us a much more satisfactory supply of pure air.

(c) The temperature of the air respired may be modified either by selecting particular climates-tropical, sub-tropical, temperate, or cold; by artificial regulation of the atmosphere of the room-ventilation, heating, etc.-or by arrangements for warming or cooling the ingoing current of air only, by means of so-called "respirators," and by recommending nasal breathing only, or oral breathing only, as the case may be.

(d) The amount of moisture in the air respired can be altered at pleasure, whether by residence in a dry climate or in a moist climate, or by varying the amount of watery vapour in the air of the room, or in the individual inspiratory draughts, by means of steam kettles, hot-water inhalations, etc.

(e) Lastly, the pressure of the air is completely under our command; and this again either by means of climate (elevated mountain residence), or by local artificial arrangements such as the air-bath and pneumatic apparatus. The compressed air-bath, at a pressure of 1/5 to 1/2 of an atmosphere above the normal, increases the amount of oxygen admitted into the blood, as well as the vital capacity and the size of the lungs, whilst it renders respiration less frequent and more easy. A rarefied atmosphere is never given as a bath; on elevated mountains it increases the depth and frequency of respiration and the vascularity of the lungs, so that there is a tendency to haemorrhage from the alveoli. The pneumatic apparatus, a small gasometer, admits air under artificial pressure to the respiratory passages only, the patient breathing into, or out of, a valved tube connected therewith. Inspiration of air compressed by about 1/50 atmosphere increases the amount of air entering the chest, and eventually the vital capacity, the size of the chest, and the respiratory force, whilst it diminishes the vascularity of the lungs and raises the arterial pressure. The other methods of aerotherapeutics do not require mention here.