RESPIRATORY ORGANS. 



27 



two epithelia run numerous blood-vessels, — arteries, veins, and 

 capillaries, — conveying blood to and from the lungs. 



In the lower amphibia the lungs develop scarcely beyond 

 the condition of simple sacs with respiratory ducts.^ In other 

 forms, however, there is increase of surface by a folding of the 

 internal wall, to be described later ; and in those still higher 

 there is a division of the primary bronchi into bronchi of sec- 

 ondary and tertiary orders, each of which connects with a 

 separate division of the pulmonary organ. 



Fig, 29. Alimentary tract of human embryo, A at four weeks, B at five 

 weeks, after His. a, allantois stalk ; b, bile duct; c, caecum; e, epiglottis; k, kid- 

 ney; /, lung; Im, lower jaw; /, pancreas; r, Rathke's pocket; s, stomach; sp, 

 Seessel's pocket ; /, thymus ; tg, tongue ; iv. Wolffian duct. 



In the dipnoi the trachea and bronchi are without skeletal 

 supports in their walls ; but in all other forms cartilaginous parts 

 are present, which tend to keep the tube from collapse. In the 

 lower air-breathers these consist of separate pieces of cartilage 

 on either side of the trachea ; but from the reptiles upwards 

 they consist of rings of cartilage, incomplete in mammals, the 

 gap on the dorsal surface of such being crossed by membrane 

 so as to allow the passage of food through the overlying 



1 A considerable number of salamanders have recently been shown to be lungless ; even 

 the trachea has disappeared, and respiration is carried on by the skin. 



