P2 THE ANATOMY OF VERTEBUATED ANIMALS. 



chiaj project freely from the visceral arches to which they are 

 attached, on the exterior of the body; and in some Amphibia, 

 such as the Axolotl [Siredon], they retain their form of exter- 

 nal plume-like appendages of the neck throughout life. But 

 in the adult life of most Fishes, and in the more advanced con- 

 dition of the Tadpoles of the higher Amphibia, the branchiae aro 

 internal, being composed of shorter processes, or ridges, which 

 do not project beyond the outer edges of the branchial clefts ; 

 and, generally, become covered by an operculum developed 

 from the second visceral arch. 



The lungs of vertebrated animals are sacs, capable of being 

 filled with air, and developed from the ventral wall of the 

 phar3rnx, with which they remain connected by a shorter or 

 longer tube, the trachea, the division of this for each luug 

 being a bronchus. Venous blood is conveyed to them directly 

 from the heart by the pulmonary arteries, and some * or all 

 of the blood which they receive goes back, no less directly, to 

 the same organ by the pulmonary veins. 



The vascular distribution thus described constitutes an es- 

 sential part of the definition of a lung, as many fishes possess 

 hollow sacs filled with air; and these sacs are developed, oc- 

 casionally, from the ventral, though more commonly from the 

 dorsal, wall of the pharynx, oesophagus, or stomach. But 

 such air-sacs — even when they remain permanently connected 

 with the exterior by an open passage or pneumatic duct — are 

 air-bladders, and not lungs, because they receive their blood 

 from the adjacent arteries of the body, and not direct from the 

 heart, while their efferent vessels are connected only with the 

 veins of the general circulation. 



The wall of each pulmonic air-sac is at first quite simple, 

 but it soon becomes cellular by the sacculation of its parietes. 

 In the lower pulmonated Vertehrata, the sacculation is more 

 marked near the entrance of the bronchus ; and when the lung- 

 sac is long, as in many Amphibia and in Snakes, the walls of 

 the posterior end may retain the smooth condition of the em- 

 bryonic lung. In Chelonia and Crocodilia, the lung is com- 

 pletely cellular throughout, but the bronchi do not give off 

 branches in the lungs. In Birds, branches are given off at 

 right angles ; and, from these, secondary branches, which lie 

 parallel with one another, and eventually anastomose. In 

 Mammalia, the bronchi divide dichotomously into finer and 

 finer bronchial tubes, which end in sacculated air-cells. 



* Generally all. hut in some AmpJdhia, siioh as Proteus, part of the blood 

 •uppUeJ to Slio lungs enters tlie general cireuliition. 



