Diaphragm. The Diaphragm is a thin musculo-fibrous septum, placed obliquely at the junction of the upper with the lower two-thirds of the trunk, and separating the thorax from the abdomen, forming the floor of the former cavity and the roof of the latter. It is elliptical, its longest diameter being from side to side, somewhat fan-shaped, the broad elliptical portion being horizontal, the narrow part, which represents the handle, being vertical, and joined at right angles with the former. It is from this circumstance that some anatomists describe it as consisting of two portions, the upper or great muscle of the diaphragm, and the lower or lesser muscle. This muscle arises from the whole of the internal circumference of the thorax, being attached, in front, by fleshy fibres to the ensiform cartilage; on either side, to the inner surface of the cartilages and bony portions of the six or seven inferior ribs, interdigitating with the Transversalis; and behind, to the ligamentum arcuatum externum and in-ternum. The fibres from these sources vary in length; those arising from the ensiform appendix are very short and occasionally aponeurotic; but those from the ligamenta arcuata, and more especially those from the ribs at the side of tho chest, are the longest, describe well marked curves as they ascend, forming an arch on each side with the concavity downwards, this concavity being deeper on the right than on the left side. These fibres converge, to be inserted into the circumference of the central tendon. Between the sides of the muscular slip from the ensiform appendix and the curtilage of the adjoining rib, the fibres of the diaphragm are deficient, the interval being filled by areolar tissue, covered on the thoracic side by the pleurae, on the abdominal by the peritoneum. This is, consequently, a weak point, and a portion of the contents of the abdomen may protrude into the chest, forming phrenic or diaphragmatic hernia, or a collection of pus in the mediastinum may descend through it so as to point at the epigastrium.

The Ligamentv.nl Arcuatum Internum is a tendinous arch, thrown across the upper part of the Psoas magnus muscle, on each side of the spine. It arises from the outer side of the body of the first, and occasionally from the second lumbar vertebra, being continuous with the outer side of the tendon of the corresponding crus, and, arching across the Psoas muscle, is attached to the front of the transverse process of the second lumbar vertebra.

The Ligamentum Arcuatum Externum is the thickened upper margin of the anterior lamella of the transversalis fascia; it arches across the upper part of the Quadratus lumborum, being attached by one extremity to the front of the transverse process of the second lumbar vertebra, and by the other to the apex and lower margin of the last rib.

To the spine the Diaphragm is connected by two crura, which are situated on the bodie of the lumbar vertebrae, one on each side of the aorta. The crura at their origin are tendinous in structure; the right crus, larger and longer than the left, arising from the anterior surface of the bodies and intervertebral substances of the second, third, and fourth lumbar vertebra; the left from the second and third; and both blending with the anterior common ligament of the spine. A tendinous arch is thrown across the front of the vertebral column, from the tendon of one crus to that of the other, beneath which passes the aorta, vena azygos major, and thoracic duct. The tendons terminate in two large fleshy bellies, which, with the tendinous portions above alluded to, are called the crura, or pillars of the diaphragm. The outer fasciculi of the two crura are directed upwards and outwards to the central tendon, but the inner fasciculi decussate in front of the aorta, and then diverge, so as to surround the oesophagus before ending in the tendinous centre. The most anterior and larger of these fasciculi is formed by the right crus.

The Central or Cordiform Tendon of the Diaphragm is ti thin tendinous aponeu-rosis, situated at the centre of the vault of this muscle, immediately beneath the pericardium, with which its circumference is blended in adults. It is shaped somewhat like a trefoil leaf, consisting of three divisions, or leaflets, separated from one another by slight indentations. The right leaflet is the largest; the middle one, directed towards the ensiform cartilage, the next in size; and the left the smallest. In structure, it is composed of several planes of fibres, which intersect one another at various angles, and unite into straight or curved bundles, an arrangement which affords additional strength to the tendon.

The Openings connected with the Diaphragm are three large and several smaller apertures. The former are the aortic, ocsophageal, and the opening for the vena cava.

The Aortic Opening is the lowest and the most posterior of the three large apertures connected with this muscle. It is situated in the middle line, immediately in front of the bodies of the vertebras. It is an osseo-aponeurotic aperture, formed by a tendinous arch thrown across the front of the bodies of the vertebrae, from the crus on one side to that on the other, and transmits the aorta, vena azygos major, thoracic duct, and occasionally the left sympathetic nerve.

The (Esophageal Opening, elliptical in form, muscular in structure, and formed by the two crura, is placed higher, and, at the same time, anterior, and a little to the left of the preceding. It transmits the oesophagus and pneumogastric nerves. The anterior margin of this aperture is occasionally tendinous, being formed by the margin of the central tendon.

The Opening for the Vena Cava is situated the highest; it is quadrilateral in form, tendinous in structure, and placed at the junction of the right and middle leaflets of the central tendon, its margins being bounded by four bundles of tendinous fibres, which meet at right angles.

The Right Cms transmits the sympathetic and the greater and lesser splanchnic nerves of the right side; the left crus, the greater and lesser splanchnic nerves of the left side, and the vena azygos minor.

The Serous Membranes in relation with the Diaphragm are four in number; three lining its upper or thoracic surface, one its abdominal. The three serous membranes on its upper surface are the pleura on either side, and the serous layer of the pericardium, which covers the upper surface of the tendinous centre. The serous membrane covering its under surface is a portion of the general peritoneal membrane of the abdominal cavity.

Peculiarities. The portion of the muscle described as arising from the last rib is occasionally aponeurotic in structure. The sternal attachment of the muscle is sometimes partially or entirely deficient.

Relations. Its upper or thoracic surface is convex on each side, and corresponds with the pleura and lungs, more flattened at the centre where it supports the heart. The convexity of this surface is greater on the right than on the left side, reaching in the former situation as high as the junction of the fifth rib with the sternum, and in the latter as high as the sixth rib. It reaches much higher in the fostus than in the adult.

Its under or abdominal surface is concave, more so on the right side, where it is in relation with the convex surface of the liver, than on the left, where it corresponds to the spleen and great end of the stomach behind; it is also in relation with the kidneys, supra-renal capsules, transverse portion of the duodenum, pancreas, and the solar plexus.

Nerves. The Diaphragm is supplied by the phrenic nerves.

Actions. The Diaphragm is the most important inspiratory muscle, being the only one brought into action in tranquil respiration. During inspiration, when the fibres of the Diaphragm contract, the muscle descends, forming an inclined plane, which extends from the ensiform cartilage to the tenth rib. During this action, the cavity of the thorax is enlarged considerably from above downwards, and the abdominal viscera are pushed into the lower and fore part of the abdomen, which is much diminished in size. If the abdominal muscles and Diaphragm act together, the viscera are compressed and forced to the lower part of the abdominal cavity, as in most expulsory efforts, which are usually accompanied by a deep inspiration. During expiration, when the Diaphragm is relaxed, the muscle is convex, encroaching considerably on the cavity of the chest, particularly at the sides, its upper border, in a forced expiration, being on a level with the lower border of the fourth rib on the right side, and with the fifth on the left. During the action of the Diaphragm the oesophagus is compressed, the aperture through which it passes being chiefly muscular; the apertures for the vena cava and aorta are also compressed, but only to a very trifling extent, as the openings for the passage of these vessels are completely tendinous. Hiccough and sobbing are the result of spasmodic contraction of this muscle; and laughing and crying are produced by its rapid alternation of contraction and relaxation, combined with laryngeal and facial movements.

Dissection of Pectoral Region and Axilla. The arm being drawn away from the side nearly at right angles with the trunk, and rotated outwards, a vertical incision should be made through the integument in the median line of the chest, from the upper to the lower part of the sternum; a second incision should be carried along the lower border of the Pectoral muscle, from the ensiform cartilage to the outer side of the axilla; a third, from the sternum along the clavicle, as far as its centre; and a fourth, from the middle of the clavicle obliquely downwards, along the interspace between the Pectoral and Deltoid muscles, as low as the fold of the armpit. The flap of integument may then be dissected off in the direction indicated in the figure, but not entirely removed, as it should be replaced on completing the dissection. If a transverse incision is now made from the lower eud of the sternum to the side of the chest, as far as the posterior fold of the armpit, and the integument reflected outwards, the axillary space will be more completely exposed.

FASCIJE OF THE THORAX

The Superficial Fascia of the thoracic region is a loose cellulo-fibrous layer, continuous with the superficial fascia of the neck and upper extremity above, and of the abdomen below; opposite the mamma it subdivides into two layers, one of which passes in front, and the other behind this gland; and from both of these layers numerous septa pass into its substance, supporting its various lobes: from the anterior layer, fibrous processes pass forward to the integument and nipple, enclosing in their areolae masses of fat. These processes were called by Sir A. Cooper, the ligamenta suspensoria, from the support they afford to the gland in this situation. On removing the superficial fascia, the deep fascia of the thoracic region is exposed: it is a thin aponeurotic lamina, covering in the outer surface of the great Pectoral muscle, and sending numerous prolongations between its fasciculi: it is attached, in the middle line, to the front of the sternum, and above to the clavicle: it is very thin over the upper part of the muscle, somewhat thicker in the interval between the Pectoralis major and Latissimus clorsi, where it closes in the axillary space, and divides at the margin of the latter muscle into two layers, one of which passes in front and the other behind it; these proceed as far as the spinous processes of the dorsal vertebras, to which they are attached. At the lower part of the thoracic region this fascia is well developed, and is continuous with the fibrous sheath of the Kecti muscles.

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