CHAP.XLII.] OF LUNGS. G05 



Anterior Venous Trunks. At an early period of development of 

 the human embryo, the veins entering the heart from the upper 

 part of the body are symmetrical; and in many of the lower 

 animals they preserve this arrangement throughout life. As the 

 development of the human embryo advances the large venous 

 trunk on the left side diminishes, and subsequently disappears 

 entirely, leaving the right trunk only as a persistent vessel. In 

 a valuable paper published in the Philosophical Transactions, 

 Mr. J. Marshall shows, that the dilated portion of the coronary 

 vein, the coronary si?ius, is the persistent lower portion of the left 

 anterior vein. It is interesting, that in animals which have a left 

 superior cava, the great cardiac coronary vein opens into it. 

 Even in the adult human heart there are certain structures which 

 are obviously the remains of the upper portion of the left primitive 

 venous trunk. These observations are very interesting, as they 

 serve to explain^ certain unusual arrangements in the great anterior 

 veins. Cases are recorded in which the two symmetrical trunks, 

 usually only found at a very early period of development, remain 

 persistent in the adult an abnormal condition which receives 

 explanation from the investigation of the nature of the early 

 embryonic changes. 



Development of the Lungs. The lungs are first seen in the form 

 of two small masses of cells, at the lower part of the oesophagus. 

 These masses gradually increase in rig 2ss. 



size, and a cavity is formed within. 

 They coalesce at the upper part 

 which ultimately becomes the 

 trachea. At this period of their 

 development, the respiratory or- 

 gans appear in the form of vesicles, 

 appended to the lower part of the 

 trachea. 



Reichert has shown that in the 



Development of respiratory organs, showing 



Chick the lunffS appear about the origin of lungs from upper part of alimentary 



\ r canal, after Kathke. A. (Esophagus of a chick 



Same time as the liver, and States, on the fourth day of incubation. The rudiments 



of the trachea and lungs of the left side. 1. 



that although they Seem tO take Inferior wall of the oesophagus; 2. superior wall; 



. . i .3. rudimentary lung; 4. stomach. B. The 



their rise from the metnbrana in- same seen from below. C. Tongue and respira- 



. tory organs of the embryo of ahorse. 1. Tongue; 



termedia (lying beween the rudl- 2. larynx ; 3. trachea; 4. lungs seen from behind. 



mentary nervous centres and the mucous membrane, p. 582), the 

 upper portion of the visceral tube, according to his observations, is 

 the real seat of their origin. 



Thyroid Glands. The first traces of the thyroid are observed in 



