LUNGS OF VERTEBKATA. 



5G9 



b) 



Lungs. 



§ 425. 



As the pneumatic organ appended to, and differentiated from, 

 the primitive wall of the enteron takes on its respiratory form, we 

 meet with a gradual differentiation of that portion of it which cor- 

 responds to the ductus pneumaticus, and which gives rise to air- 

 passages. These are the means of communication between the 

 lung and the pharynx, and are divided into several portions which 

 take on new functions ; the most important 

 of these is an organ for the production of 

 voice. The lungs themselves are now paired 

 organs. This paired condition, however, is 

 less due to a further development of that 

 division, which is indicated in many air- 

 bladders, or even to the completion of such 

 division, than to the development of the 

 organ in a manner which is adapted to its 

 position. As it is always connected with 

 the ventral surface of the pharynx, it is 

 easy to see how it is that an air-carrying 

 organ developed from this point would 

 grow out on either side. When filled 

 with air the two halves must be blown 

 out into a dorsal position, the result of 

 which must necessarily be the formation 

 of two completely separate lungs, which 

 are only connected together by the ventral 

 air-passage. 



We have as the first point in the diffe- 

 rentiation of the trachea a short wide 

 canal which connects the two lungs with 

 the pharynx. As this canal increases in 

 length it develops, in its wall, cartila- 

 ginous organs of support, and is divided 

 into two branches, which pass to the 

 lungs. A paired and an unpaired portion 

 can therefore be distinguished in the air- 

 passages. The supporting organs of these 

 canals, which are generally very short in 

 the Amphibia, are formed by two lateral 

 bands of cartilage (Fig. 322, A a), which 

 are continued as far as the commence- 

 ment of the lungs (b) (Proteus); in others 



(B) the upper ends (a) of these two pieces are separated off and 

 form the groundwork of a special portion, which is henceforward 



Fig. 322. Cartilage of the 

 larynx in the Amphibia and 

 Keptilia. A Of Proteus. 

 B Of Salamandra. C Of 

 Eana. D Of Python. 

 a Arytenoid cartilage. 

 b Supporting cartilage, 

 which in A B and C forms 

 the skeleton of the paired 

 and unpaired portions of the 

 trachea 5 in D it is merely 

 represented by the com- 

 mencement of the unpaired 

 portion of the trachea (after 

 Henle). 



