In examining the chest, it is of paramount importance that the student should familiarly know this medical floor of the thorax. The sign laid down by the late Dr. Edwyn Har rison, for marking this boundary, we have found strictly correct in every normal-shaped thorax. Namely, take the xiphoid cartilage as a point to start from, and pass the flat hand horizontally from thence to the side of the chest, the index finger, when horizontal, will distinguish a slight depression or sulcus at the maximum lateral bulge of the thorax, then from this point slide the hand slightly upwards (perpendicularly), and it will pass over a bulge, about enough to fill the palm of the hand, into another sulcus, better defined than the former one ; this groove corresponds with the medical base of the thorax, and a probe passed in here would graze the arch of the diaphragm after passing through the moveable inferior edges of the lungs. It is necessary that the hand be kept perfectly horizontal, or it will fall into an intercostal space, which does not correspond with the groove in question. With practice, the hand falls at once into the upper sulcus, without first searching for the lower one ; indeed, latterly, Dr. Harrison allowed that the lower sulcus might be absent, and yet the upper one present.
This groove is higher upon the right side than upon the left, corresponding to the height of the liver.
By external observation, the medical base of the thorax may he known by the slight rota tory motion made upon the diaphragm when a person is walking. A kind of great ball and socket-joint may be conceived to exist between the abdomen and thorax, and the outer garments even exhibit a correspondence to this base of the thorax.
II. Contents V the thorax. —Not only does the thorax contain organs proper to itself, but it lodges and protects others passing to the abdomen ; thus, the food we eat passes through the thorax to enter the stomach, while the eighth nerve both supplies the lungs and then passes onwards to the stomach. Like other cavities, the thorax is lined with a serous membrane (pleura costa/is), which like wise invests the lungs (pleura pulnionalis) (See PLEURA). The heart and its blood vessels, the trachea, and the lungs, almost completely fill the thorax. The remaining organs, viz., the origin of the sterno-hyoid and -thyroid muscles, remains of the thymus !gland, lymphatic glands, mammary vessels, triangulares sterni muscles, the phrenic, the eighth pair, and the splanchnic nerves, the (esophagus, thoracic duct, vena azygos, and vcine cavie, occupy very little space.
Thus the thorax is completely filled. The older anatomists, however, even as late as Benj.
Hoadley's time, considered that there was a considerable space containing air, between the lungs and the walls of the thorax. Now, we know that any considerable accumulation of air in this situation would be destructive ,to the respiratory function.
III. Shape of the thorax.—The shape of the thorax is subject to many varieties which may be conveniently studied under the fol lowing headings : External thorax. — The thorax in the perfect subject is somewhat conical, broader above than it is below ; but when deprived Of the upper extremities and their appen dages, the contrary obtains, for it is nar rower above than it is below (see figs. 662. and 630. et seq.). Therefore the notion we commonly connect with broad shoulders or broad back, has but a feeble relation with the absolute breadth of the thorax. In fact the diameter of the neck corresponds more correctly with the diameter of the upper part or true apex of the thorax. Freeman the American giant measured 26 inches from tip to tip of his shoulders, while the diameter at the lower part of his neck, in the same direction, was only 6 inches. In women the mannum materially add to the dimensions of the chest towards the apex. We have found the measure of a female round the thorax over the mamma; 45; inches, while the root of the neck was 15 inches. Allowing two inches for muscle, Sze., the true thoracic apex would not exceed 13 inches, whilst 45i inches was the apparent circumference of the thorax. The true apex of the thorax is loaded with large muscles, sometimes a vast quantity of fat, the upper extremities, and in the female with large maninive besides. The true base of the chest is comparatively little covered, particularly below the axilla, where the serrati are seen. This is the weakest and thinnest part of the thorax, but it is well protected by the arms.
(a) The anterior or sternal region of the tho rax.—In a well-developed thorax the sternum appears to lie in a fossa formed by the pecto rales and manini?. This region assumes more or less of the perpendicular, according to the carriage of the person ; a perpendicular drawn to the external plane of the sternum would, however, be directed upwards. Laterally to this, the costal cartilages articulate with the sternum ; and, still more laterally, the ribs and their cartilages unite, forming an oblique ridge from above downwards. (See figs. 680. and 681. the line on each side of the sternum.) Still more externally, and parallel to this, may be noticed the projections formed by the anterior angles of the ribs, which bound the anterior region.