VERTEBRATES. 45 



connective tissue coat is very thick, as it frequently is. This coat may become closely 

 attached to parts of the vertebral column, or may itself become ossified in part. 



Apart from the respiratory function, which is only important in a few forms, the 

 air-bladder occasionally becomes subservient to the auditory organ, with which it may 

 be connected directly, or with the intervention of certain modified parts of the anterior 

 vertebrae. But its most important function is a hydrostatic one, in virtue of which a 

 fish possessed of an air-bladder is able to alter the amount of the contained air, and, 

 consequently, its own specific gravity, so as to be in equilibrium under varying pressures. 



As far as form goes, the air-bladder rarely presents a subdivision into right and 

 left sacs, like the lungs. It is more common to meet with a subdivision into anterior 

 and posterior chambers, an arrangement apparently serving to alter the centre of 

 gravity as occasion requires. 



The Lungs. 



In discussing the homology of the lungs of the higher vertebrates with the air- 

 bladder of fishes, a difficulty occurs at the outset, viz., that the latter organ is devel- 

 oped from the dorsal wall of the intestinal canal, while the lungs grow out fi"om the 

 ventral wall. Two ways out of this difficulty have been suggested. Either the luno-s 

 are not completely homologous with the air-bladder, and are merely similar outgrowths 

 from the same division of the intestinal tract, or they are comjjletely homologous, and 

 one or other organ has shifted its original place of development. That the latter is 

 the true explanation appears to be indicated by the similarity of tlie blood-supply of 

 the air-bladders in Amia and Pdlypterus, taken in conjunction with the diversity 

 in the position of the air-duct. The amphibious Dipnoan fishes lead us from the con- 

 dition in Pdlypterus to that in the Amphibia proper, while the Teleosts more closely 

 resemble Amia in this respect. It has been suggested that the ventral opening is the 

 more primitive position, and that the dorsal position has been secondarily acquired, 

 along with the hydrostatic function. 



As far as structure goes, the lungs in many of the lower vertebrates present little 

 advance over the air-bladder as met with in the gar-pike and bow-fin. But as we 

 advance to the higher forms, the tendency is towards greater complexity of the sur- 

 face, so that, instead of being simply bladder-like, the lining membrane becomes first 

 raised into folds, and eventually the whole organ becomes spongy by the increased 

 immber and subdivision of these partitions. The minute chambers bounded by these 

 folds are then known as air-cells, and the whole system of air-cells communicates with 

 the back of the mouth-cavity, or pharynx, by the wind-pipe, or trachea, which represents 

 the air-duct of the Physostomous fishes. It is often the case in the reptiles, that all 

 of the wall of the lung does not become complicated in this manner. Parts remain 

 thin and bladder-like, and this is the condition of affairs which is so exaggerated in 

 the birds, where the thin parts (air-sacs) are often extraordinarily developed, pushing 

 all yielding parts befoi-e them, displacing the marrow of the bones, filling up inter- 

 spaces between the intestines, or forming sacs beneath the skin. 



With the complication of the lungs in the higher forms, there go hand in hand 

 certain alterations in the wind-pipe, which are partly of resjjiratory importance and 

 partly subservient to the production of sound. To the former category belong 

 the cartilaginous rings, which prevent the trachea from collapsing, and which are con- 

 tinued to a greater or less extent into the subdivisions (bronchi) which go to each 

 lung. To the latter series of alterations are traceable the formation of the larynx, the 



