DIAPHRAGM (human anatomy).—The dia phragm in man is a muscle of great importance (post cor facile prineeps, Haller), being the chief agent by which respiration is carried on, while it assists in the performance of many other im portant processes. It is placed between the thorax and abdomen, forming a convex floor to the former, and a concave ceiling to the latter. Although a single muscle, and situated in the median line, it is not symmetrical ; the right side of it is more extensive than the left. Symmetry, however, was not necessary in an organ which could exert no Influence on the external form; nor was it to be expected in a muscle which is not wholly voluntary. In this article it is intended to describe, 1st, the form, structure, and organization of the diaphragm ; 2nd, its uses; and, 3rd, its malformations and diseases.
For the convenience of description the dia phragm is usually divided into two portions— the upper, which is called the costal, or true or greater muscle ; and the lower, which is named the vertebral, or smaller, and is also well known as the crura or pillars. This division is sanc tioned by the situation, the shape, and the uses of the two portions.
The upper portion, placed tranversely, (sep tum transversum,) is thin, but of great super ficial extent, being connected by its margins to the entire circumference of the inferior outlet of the thorax. Narrow between the sternum and spine, it spreads out on each side into large wings, and its outline bears some resemblance to the figure of eight laid on the side, thus oo . The centre is tendinous ; the border consists of fleshy fibres. The tendinous part (fig. 1, T) (centruin tendincum, s. nerveum, s. phrenicum, cordiform tendon) is of considerable size, and in shape resembles the trefoil leaf. It presents a large semicircular notch behind towards the spine, and is deeply divided on its anterior margin into three lobes, of which one points for wards and one to each side. Of these lobes the right is usually the largest, the left the smallest; the anterior is the shortest, and sometimes the broadest; the left is the narrowest and often the longest. But these proportions will be found to vary in different individuals. The tendon is composed of fibres which pursue various courses. The greater number radiate from the vertebral notch ; these are crossed by others which run in every direction, and which seem to be continuous with the muscular fibres; and others again appear to be laid on the tendon as accessaries, rather than as con tributing to its texture. These last are most distinctly seen in old men, and on the under surface of the right lobe. The tendinous centre forms nearly the highest part of the arch. It is less curved than the fleshy portion, and more fixed in its position. One large opening pre sents itself here, between the right and middle lobes, through which the vena cava passes to the heart.
From the anterior and lateral margins of this tendon the muscular fibres pass off in arches, to be inserted into all the hase of the thorax by digitations which mix with those of the trans versus abdominis.
Beginning in front, we find two slender fasci culi running downwards and forwards to the ensiform cartilage. These are separated from each other by a line of cellular tissue, marking the median line of the muscle; sometimes one or both of these bundles may be absent, pro bably resulting from an arrest of formation. To the outside of these, on each side, a con siderable triangular interval exists, where the pleura and peritoneum are separated only by cellular substance. Here some small branches of the internal mammary artery pass to the ab domen; and in this situation fluids might easily find their way from the cellular tissue of one cavity to that of the other. The fibres next in order, bounding these spaces externally, are much longer ; they pass outwards and down wards to the seventh rib, and are inserted by a broad digitation into the point of the bone and into about one half of the adjoining portion of its cartilage. The next fibres are still longer, usually the longest of all ; they run outwards, then downwards, forming the second digitation, which is attached in a similar manner to the eighth rib. The following fibres becoming shorter as they approach the spinal notch, go to the ninth and tenth ribs, and are similarly con nected. The succeeding ones, still shorter, proceed to the eleventh and twelfth, and attach themselves to a considerable portion of their length. In the two lowest intercostal spaces the diaphragm and transverses abdominis are united by a common aponeurosis, which is very thin ; and here it is not very unusual to meet with a deficiency in the diaphragm. The thin portion of the muscle, near to the crura, has its short fleshy fibres inserted into the ligamentum a reuaturn csteraum.* (Fig.1, d.) This last appel lation is bestowed on a thin aponeurosis which stretches from the inferior margin of the last rib to the point of the transverse process of the first lumbar vertebra. In reality it is nothing more than the anterior layer of the tendon of the transverses abdominis which lies in front of the quadratus lumborum muscle, and is connected to the lowest rib. By pulling the rib outwards the aponeurosis is projected into a fold which looks like a ligament. It is designated es t ernum to distinguish it from another that is much stronger and more truly ligamentous, which arches over the psoas magnus muscle, is attached to the transverse process of the first lumbar vertebra (just where the former ends), and to the body of the second. The latter is known as the ligamentum areuatum internum t (fig. 1,f;) it is also called the true, and the external the fir/se,--names derived from their structure.