904 ^ THE LUNGS. 



from the pneumogastric nerves, joined by others from the sympathetic 

 system. The fine nervous cords enter at the root of the lung, and follow 

 the air-tubes. Their final distribution requires further examination. Accord- 

 lug to Remak, whitish filaments from the par vagum follow the bronchia as 

 far nearly as the surface of the lung, and greyish filaments, proceeding from 

 the sympathetic, and having very minute ganglia upon them in their course, 

 pass both to the bronchia and pleura. Julius Arnold has discovered 

 remarkable bell-shaped gangliouic corpuscles terminating the pulmonary 

 nerves of the frog. (Virchow's Archiv., vol. xxviii. p. 453.) 



DEVELOPMENT OF THE LUNGS AND TRACHEA. 



The lungs first appear as two small protrusions upon the front of the cesophageal 

 portion of the alimentary canal, completely hid by the rudimentary heart and liver. 

 These primitive protrusions or tubercles are visible in the chick on the third day of 

 incubation, and in the embryos of mammalia and of man at a corresponding stage of 

 advancement. Their internal cavities communicate with the oesophagus, and are lined 

 by a prolongation of its inner layer. At a later period they are connected with the 

 oesophagus by means of a long pedicle, which ultimately forms the trachea, whilst the 

 bronchia and air-cells are developed by the progressive ramification of the internal 

 cavity in the form of caecal tubes, after the manner of the ducts of glands. Ac- 

 cording to Kolliker, the human lung in the latter half of the second month presents 

 a granular appearance on the surface, produced by the primitive air-cells placed at the 

 extremities of ramified tubes, which occupy the whole of the interior of the organ ; 

 the ramification of the bronchial twigs and multiplication of air-cells goes on in- 

 creasing, and this to such an extent that the air-cells in the fifth month are only half 

 the size of those which are found in the fourth month. 



Fig. 633. Fig. 633. SKETCH ILLUSTRATING THE DEVE- 



LOPMENT OP THE RESPIRATORY ORGANS 

 A B c (from Rathke). 



A, oesophagus of a chick, on the fourth 

 day of incubation, with the rudimentary 

 lung of the left side, seen laterally ; 1, the 

 front, and 2, the back of the oesophagus ; 



3, rudimentary lung protruding from that 

 tube ; 4, stomach. B, the same seen in 

 front, so as to show both lungs. C, tongue 

 and respiratory organs of embryo of the 

 horse ; 1, tongue ; 2, larynx ; 3, trachea ; 



4, lungs seen from behind. 



For a long time the lungs are very small, and occupy only a limited space at the back 

 part of the chest. In an embryo, 16 lines in length, their proportionate weight to the 

 body was found by Meckel to be 1 to 25 ; in another, 29 lines long, it was 1 to 27 ; 

 in another, 4 inches in length, 1 to 41 ; and at the full period, 1 to 70. Huschke 

 found that the lungs of still-born male children were heavier in proportion to the 

 weight of the body than those of female children ; the ratio being, amongst females, 

 1 to 76, and in males, 1 to 55. 



CHANGES AFTER BIRTH. The lungs undergo very rapid and remarkable changes 

 after birth, in consequence of the commencement of respiration : these affect their 

 size, position, form, consistence, texture, colour, and weight, and should be carefully 

 studied, as furnishing the only means of distinguishing between a still-born child and 

 one that has respired. 



1. Position, aize, and form. In a foetus at the full period, or in a still-born child, the 

 lungs, comparatively small, lie packed at the back of the thorax, and do not entirely 

 cover the sides of the pericardium ; subsequently to respiration, they expand, and com- 

 pletely cover the pleural portions of that sac, and are also in contact with almost the 



