OF TI1E THORAX IN GENERAL.— The ster num, the ribs and their cartilages, together with the dorsal vertebra, are so united to gether as to compose the frame-work of the thorax. (fig. 662.) Their arrangement is such as collectively to admit of great mobility, and at the same time to protect completely the or gans of respiration and the heart. How well adapted this mechanism is for protecting those vital organs is well shown by the impunity with which the prize-fighter receives for many hours the trained blows of his antagonist.
Although the range of motion between each vertebra and the ribs attached to it, is very limited, yet the whole frame-work of the chest enjoys such mobility, that, by a deep inspiration, its capacity is sometimes more than doubled. This bony frame-work is by no means destined solely to cover the respira tory organs, for it extends considerably down wards, composing part of the abdominal walls within which lie the liver, spleen, kidneys, stomach, duodenum, and part of the colon ; hence the distinction between thoracic and diaphragmatic ribs. In fact it will be found that about one half only of the costal surface is destined to compose the thorax. The shade running transversely across the ribs in fig. 632. marks the bottom of the thoracic cavity.
In each individual the thoracic cavity cor responds exactly with the volume of the heart and lungs. But there is no relation between the volume of the lungs and the vigour of the constitution —nor between the size of the ca vity of the thorax and the amount of air which can be respired, as will be hereafter shown. There is likewise no relation between the vo lume of the thorax and that of the abdominal cavity. No doubt the vigour of aeration in the lungs is at all times exactly commensurate with the vigour of the alimentary canal, so that the one harmonises with the other; but this vigour bears no relation to space or size. A small thorax may in some individuals admit of the inspiration of a greater volume of air than a larger thorax in others. In fact it may be commonly noticed that where there is a large abdomen there is generally a small thorax, and that the volume of air which can be expelled at one effort from the lungs of those who have a large abdomen, is less than from those with a small abdomen. The alimentary canal receives at once a given quantity of food, and there it remains for an indefinite time, the thorax large or small, it matters not which, aerates the blood from this food by movements, quick or slow, long or short ; therefore the respiratory movements only, need be relative to the abdominal cavity, in the same way as the volume of the blast from a pair of bellows is more dependent on their mobility than on their absolute size. The above remarks are applicable to the thorax of either male or female.
I. Boundaries of the thoracic cavity.— The thoracic cavity, situated between the shoul ders and below the neck, extends but a short way downwards, in the male about seven inches, and in the female about eight inches, below the clavicle, so that a horizontal line drawn about an inch below the axilla, corres ponds (roughly) with its floor. The floor of the chest, therefore, is much higher up in the trunk of the body than is commonly supposed. The thorax is bounded anteriorly by the sternum and costal cartilages ; laterally by the bodies of the ribs and the intercostal muscles ; posteriorly by the vertebrm and angles of the ribs, and inferiorly by a thin tendinous and fleshy floor— the diaphragm. The su
Mor aperture of the chest is about sixteen inches in circumference, this is the smaller end, and thence called the apex of the tho rax. It is bounded laterally by the two 1st ribs, anteriorly by the upper edge of the sternum and inter-articular ligament, and posteriorly by the last cervical and first dorsal vertebra;. The inferior aperture is about thirty or thirty-one inches in circumference, and forms the base of the chest. Anatomists describe this part as bounded in front by the cartilaginous extremity of the sternum or xiphoid cartilage, and the cartilaginous extremities of the last true and false ribs, and more laterally by the 11th and 12th ribs, pos teriorly by the last dorsal and first lumbar vertebrae. But as they assume to themselves the privilege of giving a bone a surgical neck as well as an anatomical one, so may we take a similar liberty in describing the thorax for medical purposes. In the examination of the chest during life, too exclusive attention to anatomical boundary has probably led to the error, of regarding the chest as much deeper than it really is, and thence to examining for disease of the lung where really little or no lung exists. A sharp instrument, piercing the chest laterally, at the cartilaginous extremities of the last true ribs, would most probably pene trate no lung, for the liver, spleen, stomach, &c. are contained within these points. The bottom of the chest is so moveable and so much arched (See art. DIAPHRAGM, fig. 3.), that in the different stages of inspiration, the lung,*assumes different positions This may be demonstrated by percussing over the 5th rib at its junction with its cartilage, first after a deep expiration and then after a deep inspira tion ; in the latter the sound is " clear," in the former it is strikingly " dull." Therefore, instead of taking the insertion of the dia phragm as the bottom of the thorax, it will be found more convenient for examining the chest to take the top of the arch of this muscle as the lower thoracic boundary, or the shadeil line crossing the ribs in fig. 4., for the medical base. This may be described as corres ponding in front with the xiphoid cartilage ; laterally to different osseous portions of the 7th, 8th, 9th, 10th, 11th, and sometimes the 12th rib (fig. 682.), and posteriorly to the 8th and 9th dorsal vertebra;. This will place the bottom of the thorax in a very different posi tion from what is generally supposed ; for, if we express the distance from the 1st rib, to the lowest point of the 10th rib as 13.5, that from the 1st rib to the arch of the diaphragm or medical boundary will be only 6.25, less than one half the depth of the thorax as ana tomically described. The medical base of the thorax forms a nearly horizontal plane, which extends between the sternum and the bo dies of the 9th or 10th dorsal vertebra:, its posterior being somewhat higher than its an terior. But on each side of the bodies of the vertebrx there is a deep groove formed by the angles of the ribs. In that part of this groove which extends below the above-mentioned in clined plane, a wedge-shaped process of lung is lodged, which varies in size in different sub jects, and consequently will be found to ter minate at different points in the dorsal re gion, as already noticed, sometimes hanging down like a broad, thick flap, and at other times forming only an insignificant process.