i86o 



HUMAN ANATOMY. 



Fig. 1584. 





teriorly. The former teaching, according to which the left pleura describes at the 

 front a curve somewhat similar to that of the left lung, is quite wrong. However, the 

 point at which it leaves the sternum, the extent to which it is in contact with the 

 right pleura, and the distance the latter advances under the sternum are all very- 

 uncertain. The most important point is the extent to which the pleura covers the 

 pericardium. According to Sick's' observations on twenty-three bodies of adults, 

 the reflection of the left pleura at the fifth cartilage was in seventeen either behind 

 the sternum or just at its border ; thus it left the sternum at a higher point only six 

 times. At the sixth cartilage the pleura was ten times behind the sternum and less 

 than I cm. from it in six. At the seventh cartilage it was five times at the border of 

 the sternum or behind it and five times not over i cm. external to it. It left the 

 sternum close to the seventh cartilage five times. Tanja,^ however, found the left 

 pleura leaving the sternum at the fourth cartilage in four of fourteen bodies ranging 



from eight years upward. The left pleura may ex- 

 ceptionally cross the median line, and, it is said, may 

 not extend forward as far as the sternum ; but such 

 a condition must be very exceptional. There is con- 

 siderable variation as to the depth of the descent 

 posteriorly. Tanja never found the lower fold at 

 the back in the adult higher than the middle of the 

 last thoracic vertebra. It may descend to the first 

 lumbar and even to the second. 



Structure. — The pleura, like other serous mem- 

 branes, consists of a stroma-layer composed of bun- 

 dles of fibrous tissue intermingled with numerous 

 elastic fibres. The general disposition of the con- 

 nective-tissue bundles is parallel to the free surface, 

 although the bundles cross one another in various 

 directions. The free surface of the pleura is covered 

 with a single layer of nucleated endothelial cells 

 (from .020-. 045 mm. in diameter), which rest upon 

 a delicate elastic limiting membrane differentiated 

 from the stroma-layer. The existence of definite 

 openings, or stomata, between the endothelial plates, 

 leading into the numerous lymphatics of the pleura, 

 is doubtful. 



The subserous layer is very thin over the lung 

 where it is continuous with the elastic interlobular 

 tissue. In the mediastinum it has a firm fibrous 

 backing so as to make a strong and dense membrane. 

 The cervical pleura is extremely thick and resistant, being strengthened by fibrous 

 or muscular bands from the system of the scaleni muscles spreading into it from 

 behind, as well as by expansions from the areolar tissue about the trachea, oesopha- 

 gus, and subclavian vessels. The costal pleura has a subserous layer, known as the 

 fascia endothoracica, through which it is attached to the thoracic walls less closely 

 than elsewhere. This fascia is thickest near the top. The ribs show clearly through 

 the pleura of the opened thorax, appearing light in contrast to the congested inter- 

 costal spaces. The subserous layer is hardly existent beneath the diaphragmatic 

 pleura, but at the sides of the thorax there is a considerable space below the reflection 

 of the pleura from the diaphragm, occupied by areolar tissue connecting the dia- 

 phragm and walls. 



Blood-Vessels. — The arteries of the visceral pleurae have been shown by 

 Miller to come from the system of the pulmonary arteries instead of from that of 

 the bronchial, as previously believed. They form a fine net-work over the lung. 

 Those of the parietal pleurae come from the aortic and superior intercostals, the in- 

 ternal mammaries, the mediastinal, the oesophageal, the bronchial, and the phrenic 

 arteries. 



1 Archiv f. Anat. u. Phys., Anat. Abth., 1885. 

 ^ Morphol. Jahrbuch, 1891. 



IiiteraUeolar 

 wall 



Endothelium 

 or free surface 



Connective-tissue 

 stroma of pleura 



Section through free edge of lung, show- 

 ing visceral pleura. X 150. 



