The functions of the thorax may be divided into two great heads, — the chemical and the physical ; for an account of the former see RESPIRATION.
Every point of the thorax can move for the purpose of respiration; and hencehas followed a division of these movements, nominated after the parts which respectively carry on their functions, viz. costal breathing, and ab dominal or diaphragmic breathing. These motions are, in health, symmetrical, constant, regular, sensitive, and precisely the same ; otherwise disease must exist.
The breathing movements are also expres sive of mental emotions. The tragedian imi tates them to give force to the character he represents, whether it be the stealthy breathing of the Roman conspirator sharpen ing his knife, or the deep swelling inspiration of Ajax defying the lightning ; these repre sentations, without such movements, would be but dull pictures of the mind of the authors who left such characters on record. It is much easier to become delicately fa miliar with these movements and their cha racteristic differences, than it is to describe them.
The latitude of movement, performed by the walls and floor of the thorax, admits of three common degrees of division : First, extreme expansion or enlargement. Second, extreme contraction or diminution ; and Third, an intermediate condition,—an or dinary or quiescent state.
These three divisions necessarily displace volumes of air of different magnitude.
Were the respiratory movements but two in number, extreme expansion and extreme contraction, the quantity of air moved, and the character of the movement, would be easy of calculation and expression ; but the interme diate breathing or quiescent movement being so limited, so perfectly under the control of the will, so readily affected by mental emo tions and by the animal functions, renders the calculation of the volume of air ordinarily passing through the lungs a very difficult question.
The quantity of air in the thorax, together with those portions which can be added at will, may be arranged and denominated thus :— First, residual air.
Second, reserve air.
Third, breathing air.
Fourth, complemental air.
Fifth, vital capacity.
1st. Of residual air. — After death the lungs contain air, which is not displaced by the last expiration ; this quantity remains in the thorax as long as the lungs maintain their natural structure; therefore we have no control over this volume of air : to it we assign the term "residual air." 2nd. Reserve air is that portion which re mains in the chest after the gentle ordinary expiration, but which may be displaced at will.
3rd. Breathing air is that volume which is displaced by the constant gentle inspiration and expiration.
4th. The complemental air is that volume which can at will be drawn into the lungs by a violent exertion above the moderate effort of ordinary breathing, constituting the deepest possible inspiration. It is only occasionally demanded.
5th. The vital capacity is these last three divisions combined, being the greatest vo luntary expiration, following the deepest inspi ration.* This division of thoracic movements for commanding these different volumes of air may be more clearly illustrated by diagram 703.
Let that portion marked H represent the dual volume, or air left in the lungs, after a complete voluntary expiration ; the part next anteriorly, left white, — the reserve volume, or latitude of movement appropriated for dis placing that air left in the lungs at the end of an ordinary expiration ; the black stripe next anteriorly represents the mobi lity for commanding the volume of breathing air ; and lastly, beyond this another white' stripe shows the extreme limit of inspiration or thoracic mobility commanding the com plemental volume of air. These last three viz. the complemental, breathing, and reserve volumes conjointly— we style vital capacity. The absolute capacity may be considered as all the divisions combined in one.
Whatever limits the mobility of the thorax must modify the volume of air respired. This applies to any or all of the above movements. Therefore the measure of the volume of air displaced becomes a measure of the thoracic mobility ; and as disease affects the mobility of the chest, the measure of the volume of respired air becomes a measure of disease.