THE PLEURAE. 



1861 



FIG. 1585. 



Injected lymphatics of pleura, seen from surface. 

 X 75- (Miller.) 



The veins of the visceral pleurae are tributary to the pulmonary system ; those 

 of the parietal pleurae open into the veins corresponding to the arteries. It is 

 important to note that the intercostal spaces have many veins and that the pleura 

 over the ribs has but few, these chiefly communicating with the veins above and 



below them. Owing to the arrangement by 

 which the intercostal veins are kept open, the 

 venous circulation of the parietal pleurae is 

 under the influence of the suction power both 

 of respiration and of the heart. 



The lymphatics are numerous over the 

 lungs and also in the intercostal spaces. 

 Those of the parietes open into both inter- 

 costal and substernal lymph-nodes. 



Nerves. The nerves of the visceral 

 pleurae are from the pulmonary plexuses, con- 

 taining both pneumogastric and sympathetic 

 fibres ; those of the parietal pleurae are from 

 the intercostal, the phrenic, the sympathetic, 

 and the pneumogastric nerves. 



Development of the Respiratory 

 Tract. The respiratory tract develops as 

 an outgrowth from the primitive digestive 

 tube. Early in the third week, in embryos 

 of little over 3 mm. in length, a longitudinal 



groove appears on the ventral wall of the fore-gut, extending from the primitive 

 pharynx above well towards the stomach below. This groove becomes deeper, 

 constricted, and finally separated from the fore-gut as a distinct tube, the differen- 

 tiation resulting in the production of two canals, the respiratory tube in front and 

 the oesophagus behind. Separation and completion of the former proceeds from 

 the lower end of the groove upward as far as the pharynx, into which both cesopha- 

 gus and air-tube open. The cephalic end of the latter becomes enlarged and forms 

 the larynx, the adjoining portion correspond- 

 ing to the trachea. 



The Lungs. The distal extremity of 

 the primary respiratory tube soon enlarges 

 and becomes bilobed, pouching out on each 

 side into a lateral diverticulum which rep- 

 resents the primitive bronchus and lung. 

 These pulmonary diverticula elongate and 

 subdivide, the right one, which is somewhat 

 the larger, breaking up into three secondary 

 divisions and the left into two, thus early 

 foreshadowing the later asymmetry of the 

 lung-lobes. Since the primary air-tube lies 

 medially in the dorsal attachment of the sep- 

 tum transversum, the pulmonary buds extend 

 laterally and backward into the dorsal parie- 

 tal recesses (later the pleural cavities), carry- 

 ing before them a covering of mesoblast. 



The primary lobes increase in size and 

 complexity as additional outgrowths arise by 

 the division of the enlarged terminal part of 

 each diverticulum. The resulting divisions, 

 or new bronchi, are at first equal, but soon 

 grow at an unequal rate, the one elongating 

 most rapidly becoming so placed as to continue the main air-tube, while the less 

 rapidly elongating division becomes a lateral branch. The repeated bifurcation in 

 this manner results in the production of a chief bronchus, traversing the entire 

 length of the lung, into which open numerous lateral tubes or secondary bronchi. 



FIG. 1586. 



CEsophagu 



Lung-tube 



Part of sagittal section of rabbit embryo, show- 

 ing lung-tube growing downward and forward from 

 primitive laryngo-pharynx. X 40. 



