Some other foramina transmit vessels and nerves, but they are very small and irregular. The sympathetic nerve usually passes with the psoas muscle under the internal ligamentum arcuatuni. The right splanchnic slips out of the thorax between the fibres of the right crux, at a point internal, superior, and anterior to the sympathetic. The left splanchnic comes in the same way, or more frequently with the aorta. The lesser splanchnic passes at the outer side of the former, separated from it by a few fibres. Behind the external ligamentum areuatum the last dorsal nerve may be seen. Filaments of the phrenic nerve pierce the muscle in several places, principally its tendinous part, and some pass through the opening for the vena eava. And branches of the internal mammary artery creep through those cellular spaces which are left between the xiphoid cartilage and first .costal attachment.
The upper muscle of the diaphragm is lined for the most part of its under surface by the peritoneum, and on its upper by the pleura and pericardium ; being thus placed between serous membranes. In some points the peri toneum is reflected off to form ligaments for the liver, and there this last organ comes in contact with the muscle. The same thing oc curs .to a small extent in the case of the kid .neys. The upper surface too is for a little way all along its margin destitute of serous covering, and in contact with the ribs, intercostal mus cles, quadratus lumborum, psoas, and triangu laris sterni. Over the serous membranes on the thoracic surface we find on each side the base of the lungs, and in the centre the heart resting on the middle lobe of the tendon and on some muscular fibres to its right. The ab dominal surface is related to the liver, stomach, spleen, and kidneys.
The inferior muscle of the diaphragm has one surface turned back to the spinal column, and in contact with it and with a little of the aorta; the other surface looks forwards, and is covered by the suprarenal capsules, the semi lunar ganglia, and various nerves, the aorta and its principal branches, the ascending cava, the coinmencement of the abdominal vena porta and its tributaries, the pancreas, stomach, duo denum, and occasionally other parts. Little or no peritoneum can touch this portion.
Arteries.—A muscle of so much importance in the animal economy as the diaphragm, and so perpetually in action, requires a large supply of blood. This it receives through numerous channels and from distinct sources ; and as all its vessels inoseulate freely in its substance, no failure in the supply can well occur. The phrenic and internal mammary are distributed to its middle ; the same vessels, with the in tercostal, the 'timbal, and some small aortic twigs, feed the circumference.
reins.—The veins of the diaphragm accom pany the arteries as in other parts of the body ; each artery having one or two versa: comites.
The principal veins, however, correspond to the phrenie artery, and pour their blood into two trunks, a right and a left, which empty them selves into the eava. They are usually seen on the under surface of the tendon, sometimes on the upper; or there may be two above and two below. Occasionally they lie between the two surfaces, so that their entrance into the eava is not seen ; and in some cases they join the hepatic veins.
Lymphatics.—The diaphragm is furnished with lymphatic vessels as other muscles, but there is nothing peculiar in them. They are not easily demonstrated, as they do not form any very distinct trunks, but join with the lymphatics of the neighbouring organs.
Nerves.—The diaphragm receives a great number of nerves. The lumbar send twigs to the crura, the lower dorsal to the broad muscle, and there is a phrenie plexus sent off from the solar, which accompanies the phrenic arteries, and distributes its numerous and delicate fila ments with extreme minuteness to the under surface of the muscle. From the plexus which the eighth pair forms on the stomach, we trace also some fine filaments. But the chief and most important nerves are the phrenic. The phrenic nerve arises from the cervical plexus ; its principal origin is from the fourth cervical nerve, to which there is usually joined a small twig from the third. It runs down along the anterior sealenus, and gets into the thorax be tween the subelavian artery and vein. In the neck it generally receives filaments from each cervical nerve. As it enters the thorax, it com municates with the inferior cervical ganglion, and gets a filament from the deseendens noni and the pneumogastric. The nerve thus formed is conducted by the mediastinum and pericar dium, in front of the root of the lung, to the diaphragm ; the left being a little longer than the right, and thrown somewhat further back by the position of the heart.
It enters the diaphragm at the anterior edge of the tendon in six or seven branches, the largest of which pass backwards. Some go through the muscle, ramify on its under sur face, and anastomose with the solar plexus ; and one may usually be traced through the opening for the cava on to that plexus. The influence which these nerves exert on the organ will presently be adverted to.