The diaphragm is covered next the thorax by a membrane derived from the pleura, and towards the belly it is invested by a production of the peritoneum, which is connected with the liver. Its arteries and nerves have already been noticed in their respective ta bles.
Having now described the contents of the chest, we must consider the nature and disposition of the mem brane that lines the interior of this cavity, and, invelopes the organs of respiration and circulation within its This membrane is called /2/eura, and con sists chiefly of two close bags, one on each side of the chest, united at the sternum. Hence it is generally considered as double, and the two bags which it forms are called the two pleur,e. The whole may, however, be described as one membrane, forming various doub lings. In its texture it is cellular on that surface which is next the ribs, and the contents of the chest, but smooth and polished towards the interior of that cavity. It is firm, dense, and elastic, and more or less trans parent. Its central surface is moistened by a serous fluid which continually ooses from openings which are considered as the mouth of exhalant arteries.
To form a correct idea of the manner in which the pleura is reflected over the different contents of the chest, for the purpose of giving to them a peripheral coat, and connecting them with each other, it will be useful to trace the disposition of this membrane as we have done with the pituitary membrane of the nose, from a certain fixed point, which we shall suppose to be one side of the sternum. Passing from the side of the sternum, the pleura proceeds over the concave surface of the ribs, which it completely invests, but is separated from the intercostal muscles by cellular substance, and by the vessels and nerves that are distributed through the intercostal spaces. It also covers the convex sur face of the diaphragm, and passing from this surffice, and from that of the ribs, it proceeds to the bodies ol• the dorsal vertehrv, forming at the atlantal part of the chest, a close cavity for lodging the smaller extremity of the lungs. Towards the heads of the ribs it covers the nervous ganglions that belong to the chest, and the brain hes to w hich they give origin. It is separated from the bodies of the vertebrx by a considerable quantity of I•at. Arrived at the vertebral column, the pleura of one side approaches that of the other, which has proceeded in a similar direction and between them is formed that Ca% ity which has been called the posterior mediastinum.
Through this passes the gullet the descending aorta, the thoracic duct, &c. inveloped in cellular substance, which entirely tills this space. Proceeding towards the sternum, it approaches the heart, and the great ves sels that arise front, and terminate in, that organ. Iierc it may be conceived either to form the whole of the pericardium, or to invest that membrane with a peri pheral coat, by which it connects it with the diaphragm. Leaving the pericadium at the roots of the pulmonary vessels, it passes over, we shall suppose, the menial, dor sal, lateral, and sternal sides of each lobe of the lungs, till it reaches the side of the pulmonary vessels and pericardium, opposite to that from which we supposed it to he last reflected. It hence passes to the inner sur face of the sternum, to the point from which we first set out, having formed in this sternal part, the anterior mediastinum.
Hence we see that the mediastinum is properly a double partition, formed of two plaits united at the sternum, and diverging from each other as they pro ceed towards the vertebrx. This divergence is most remarkable at the sacral part of the chest, where the heart is lodged. The sternal or angular part of the mediastinum is connected to the sternum in such a manner as to divide this bone into two unequal portions. Supposing it to commence on the atlantal part of the sternum, it is there' fixed near the articulation of the cartilages of the right ribs with the right side of the sternum. Hence it descends obliquely sacrad, till it approaches the articulation of the left cartilages of the ribs. This disposition of the sternal mediastinum affords a useful hint to surgeons in those cases where it is re quired to perforate the sternum, for the purpose of making an opening into one side of the cavity of the chest. If the opening is to be made into the right side, we find, from the disposition of the mediastinum, that the perforation must be made in the sacral part of the sternum, near the ensiform cartilage, but on the left side a perforation of the sternum will scarcely be of advantage, as, in order to make an opening into the left cavity, it would be necessary to perform the opera tion near the atlantal extremity of the sacrum.