LUNGS OF VEETEBRATA. 573 



widened out anteriorly, but always end in an enlargement. The 

 ploxiform processes in the walls of the lung of Cryptobranchus are 

 more considerable, but they are very small in Triton. This is 

 frequently the case also in the other Salamandriua, but in the 

 Anura they are divided into smaller spaces by a close-set network. 

 The lung is thereby enabled to expose a larger quantity of blood for 

 the exchange of gas. This relation is still more complete in the 

 Reptilia. Although many (most Saurii) have very simple lungs, yet 

 in the Ophidii, as well as in the Crocodilini and Ohelonii, each lung 

 is divided into a number of large cavities, which again are divided 

 into smaller ones of various kinds. In the Ophidii the lungs may 

 be seen to be adapted to the elongated form of body and to be 

 themselves elongated; the more or less atrophied condition of one 

 lung is a similar adaptation. The elongation of the lung is accom- 

 panied by a peculiarity of the last portion of the lung, which is 

 generally a good deal enlarged, gets simpler in structure and loses 

 its respiratory function. Similar portions, which have lost their 

 respiratory function, may be seen in the Saurii. As in the Ophidii, 

 the most anterior portion, or that which is above the point at 

 which they are connected with, the air-passages, carries a more 

 closely-set meshwork, while the hinder end has its internal surface 

 less largely increased. In the Chamaeleons special cascal tubes 

 are given off from this portion, and project far into the coelom. 

 They are rudiments of an arrangement which has other functional 

 relations in the Birds. 



In Birds conical prolongations are developed on the surface of 

 the lung during the embryonic period, and these become con- 

 nected with other organs and form cavities for the passage of 

 air. In the adult this pneumatic apparatus is formed of mem- 

 branous sacs embedded between the viscera, or of tubes which 

 extend into parts of the skeleton. In the latter case air-cavities 

 take the place of the medulla, which disappears, and so diminish 

 permanently the specific gravity of the animal. In the same way, 

 the specific gravity of the animal may be diminished at will by 

 the blowing out of the sacs between the viscera, and this, like the 

 other arrangement, is of assistance in flight. 



As to their minute structure, the lungs of Birds have their finest 

 cavities connected with one another. The parenchyma of the lung 

 is spongy. In the Mammalia the lobate arrangement is continued 

 into the smallest portions of the lungs, while larger lobes may be 

 made out externally. 



There are greater peculiarities in the position of the lungs. In 

 the Amphibia, and in the Saurii and Ophidii, they project into the 

 ccelom. In the Chelonii and Aves they are placed on the dorsal 

 wall of the thorax, and are invested anteriorly by the peritoneum. 

 In the Crocodilini each lung is placed in a serous sac, by which it is 

 invested. The same is the case in the Mammalia, where the lungs, 

 which are covered by a pleural investment, occupy the lateral halves 

 of the thoracic cavity. 



