Part 144

The =thoracic cavity= (Cavum thoracis) is the second in point of size of the body cavities. In form it is somewhat like a truncated cone, much compressed laterally in its anterior part, and with the base cut off very obliquely.

The =dorsal wall= or =roof= is formed by the thoracic vertebræ and the ligaments and muscles connected with them.

The =lateral walls= are formed by the ribs and the intercostal muscles.

The =ventral wall= or =floor= is formed by the sternum, the cartilages of the sternal ribs, and the muscles in connection therewith. It is about one-half as long as the dorsal wall.

The =posterior wall=, formed by the diaphragm, is very oblique and is strongly convex.

The =anterior aperture= or =inlet= (Apertura thoracis cranialis) is relatively small, and of narrow, oval form. It is bounded dorsally by the first thoracic vertebra and laterally by the first pair of ribs. It is occupied by the longus colli muscles, the trachea, œsophagus, vessels, nerves, and the prepectoral lymph glands.

The cavity is lined by the =endothoracic fascia= and by the =pleuræ=.

A longitudinal =septum=, termed the =mediastinum= (Septum mediastinale), extends from the dorsal wall to the ventral and posterior walls, and subdivides the cavity into two lateral chambers which contain the lungs. Each of these chambers is lined by a serous membrane called the pleura, and is called a =pleural cavity= (Cavum pleuræ). The mediastinum is, for the most part, not median in position; this is largely due to the fact that the largest organ contained in it, the heart, is placed more on the left side; consequently the right pleural cavity and lung are larger than the left. Practically all the organs in the thorax are in the mediastinal space with the exception of the lungs, the posterior vena cava, and the right phrenic nerve. The part in which the heart and the pericardium are situated, together with that above it, is usually called the middle mediastinal space; the parts before and behind this are termed the anterior and posterior mediastinal spaces respectively.

THE PLEURÆ

The =pleuræ= are two serous sacs which line the pleural cavities and are reflected at the roots of the lungs to invest those organs. We, therefore, distinguish =parietal= and =visceral= parts of the pleuræ.

FIG. 358.—CROSS-SECTION OF THE THORAX OF NEW-BORN FOAL.

Tr., Trachea; S, left brachial artery; B, brachiocephalic artery; l.g., lymph gland. Pleura indicated by red line. ]

The =parietal pleura= (Pleura parietalis) lines the cavity in which each lung is situated (Cavum pleuræ). On the lateral thoracic wall it is adherent to the ribs and intercostal muscles and is termed the =costal pleura= (Pleura costalis). Behind it is closely attached to the diaphragm, forming the =diaphragmatic pleura= (Pleura diaphragmatica). The part which is in apposition with the opposite sac or covers the mediastinal organs is termed the =mediastinal pleura= (Pleura mediastinalis); where this layer is adherent to the pericardium, it is distinguished as =pericardiac pleura= (Pleura pericardiaca).

The pleura is reflected at the root of the lung, which it covers, constituting the =visceral= or =pulmonary pleura= (Pleura pulmonalis). Behind the root of the lung a considerable triangular area is not covered by the pleura, the two lungs being attached to each other by connective tissue in this situation.

The =ligament of the lung= (Lig. pulmonale) is a fold formed by the reflection of the pleura from the mediastinum and the diaphragm to the lung, behind the triangular area of adhesion just mentioned. It is seen when the base of the lung is drawn outward. It contains elastic tissue, especially in its posterior part.

FIG. 359.—CROSS-SECTION OF THORAX OF NEW-BORN FOAL.

A, Aorta; Br., bronchi; P.A., pulmonary arteries; l, lymph gland; R.A., right atrium; L.A.; left atrium; R.V., right ventricle; L.V., left ventricle of heart. ]

The right pleura forms a special =sagittal fold= (Plica venæ cavæ) about a handbreadth to the right of the median plane, which incloses the posterior vena cava in its upper edge and gives off a small accessory fold for the right phrenic nerve. The fold arises from the thoracic floor and from the diaphragm below the foramen venæ cavæ, and intervenes between the mediastinal lobe and the body of the right lung. It is delicate and lace-like.

The posterior mediastinum is very delicate below the œsophagus, and usually appears fenestrated; when these apertures are present, the two pleural cavities communicate with each other.

The pleural sacs contain a serous fluid, the =liquor pleuræ=; in health there is only a sufficient amount to moisten the surface, but it accumulates rapidly after death.

FIG. 360.—CROSS-SECTION OF THORAX OF NEW-BORN FOAL. Ribs are numbered. ]

THE LUNGS

The =lungs= (Pulmones) occupy much the greater part of the thoracic cavity. They are accurately adapted to the walls of the cavity and the other organs contained therein. The two lungs are not alike in form or size, the right one being considerably larger than the left; the difference is chiefly in width, in conformity with the projection of the heart to the left.

The lung is soft, spongy, and highly elastic. It crepitates when pressed between the finger and thumb, and floats in water. When the thoracic cavity of the unpreserved subject is opened, the lung collapses immediately to about one-third of its original size, and loses its proper form; this is due to its highly elastic character and the fact that the tension of the lung tissue caused by the air pressure in its cavities has been relieved by the external air pressure.

The =color= varies according to the amount of blood contained in the lung. During life the lung has a pink color, but in subjects which have been bled for dissection it is light gray or faintly tinged with red. In unbled subjects it is dark red; the depth of color varies, and is often locally accentuated by gravitation of blood to the most dependent parts (hypostasis).

The =fœtal lung=, since it contains no air and has a relatively small blood-supply, differs from that of an animal which has breathed in the following respects: (1) It is much smaller; (2) it is firmer and does not crepitate; (3) it sinks in water; (4) it is pale gray in color.

In =form= the lungs are like casts of the pleural cavities in which they are situated. When well hardened in situ, their surfaces present impressions and elevations corresponding exactly to the structures with which they are in contact. Each lung presents two surfaces, two borders, a base, and an apex.

FIG. 361.—LEFT LUNG OF HORSE, COSTAL SURFACE. Specimen hardened in situ. ]