THE LUIs^GS. 269 



expanding into a dome-like aponeurosis or fascia, Avliicli covers or 

 strengthens the pleural cul-de-sac, and is attached to the whole of the 

 inner edge of the first rib. The right pleura is generally stated to 

 reach higher in the neck than the left ; but, in twenty observations 

 recorded by Hutchinson, tho right lung was higher in ten cases, and 

 the left in eight, whilst in two the height was equal on the two sides. 

 Anteriorly the pleural sacs of opposite sides come nearly or altogether 

 into contact behind the second piece of the sternum, and continue so 

 for some distance ; but opposite the lower end of the sternum the right 

 pleura passes beyond the middle line or remains close to it, while the 

 left recedes to a variable distance. Inferiorly the pleura3 do not pass 

 quite down to the attachments of the diaphragm, but leave a portion of 

 its circumference in contact with the costal parietes. Owing to the 

 height of the diaphragm on the right side (corresponding with the 

 greater convexity of the liver), the right pleural sac is shorter than the 

 left ; it is at the same time wider. In the axillary line, the right 

 pleura extends down to the lower edge of the ninth rib, while the 

 left pleura reaches to the lower edge of the tenth (Luschka). 



Structure. — The pleura possesses the usual characters of serous 

 membranes. The costal part is the thickest, and may be easily raised 

 from the ribs and intercostal spaces. It is strengthened here by 

 a layer of subserous areolar tissue of considerable thickness. On 

 the pericardium and diaphragm the pleura is thinner and more 

 firmly adherent ; but it is thinnest and least easily detached upon 

 the surface of the lungs, A difference is also noticeable in the 

 character of the superficial epithelioid layer, for while on the 2^Ieiira 

 costalis this consists of the ordinary flattened cells, on the pinira jml- 

 monalis the cells, at least in some animals, are less distinctly flattened 

 and more granular and polyhedral (Klein). Lymphatic vessels are 

 abundant in and beneath the pleura as in other serous membranes, and 

 they communicate in many parts, by means of stomata, with the cavity 

 of the membrane. In the pleura costalis they are only found over the 

 intercostal spaces ; not over the ribs (Dybkowsky). 



THE LUNGS. 



Form. — Each lung is irregularly pyramidal or conical, with the base 

 downwards, and one side (the inner) much flattened. The broad, concave 

 base is of a semilunar form, and rests upon the arch of the diaphragm. 

 It is bounded by a thin margin, which is received in the angle between 

 the ribs and the diaphragm, and reaches much lower down behind and 

 at the outer side than in front. The ajwx is blunt, and, as already 

 mentioned, reaches into the root of the neck, above the first rib, where 

 it is separated from the first portion of the subclavian artery by the 

 pleural membrane. The outer surface, Avhich moves upon the thoracic 

 parietes, is smooth, convex, and of great extent, corresponding with the 

 arches of the ribs and costal cartilag-es. The inner surface is slightly con- 

 cave, and in part adapted to the convex pericardium. The posterior border 

 is rounded, and is received into the deep groove formed by the ribs at 

 the side of the vertebral column ; measured from above dowmvards, it is 

 the longest part of the lung. The anterior border is thin and overlaps 

 the pericardium, forming a sliarp edge, which, opposite the middle of the 

 sternum, is separated during inspiration from the coiTesponding niargin 

 of the opposite lung only "by the two thin layers of the mediastinal 



