26 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



shape it varies greatly ; it may be unpaired, or it may consist 

 of two paired lobes. It may be a simple sac, or it may be 

 subdivided into two or several successive chambers. Its in- 

 ternal walls are usually smooth, but they may be considerably 

 convoluted, thus greatly increasing the surface. Occasionally 

 its walls are calcified. Its chief function is that of a hydro- 

 static apparatus. It is not respiratory, as it receives arterial 

 blood and returns venous blood. In some fishes (ostariophysiae) 

 it is connected with the ear by aWeberian apparatus, consisting 

 of a chain of small bones. According to the latest conclusions 

 this apparatus seems to be for a recognition of variations in 

 hydrostatic pressure. The swim-bladder is absent in some bot- 

 tom-living fishes (pleuronectids, etc.). 



In the pharyngeal region of the elasmobranchs are caeca, 

 which may be the structures from which the swim-bladder has 

 developed. The swim-bladder, in turn, is usually regarded as 

 having given rise, by substitution of functions, to the lungs. 

 On the other hand, there are some who regard the lungs as new 

 formations in the air-breathing vertebrates, and as having arisen 

 by modification of a pair of gill pouches which have grown 

 backwards instead of outwards, and consequently have failed to 

 form connection with the ectoderm. The method of origin of 

 the lungs and the relations of the cartilages of the larynx, 

 shortly to be described, favor the latter view. 



Lungs. The lungs arise as an outgrowth from the ventral 

 wall of the pharynx, just posterior to the last gill pouch. The 

 outgrowth almost immediately divides into right and left halves, 

 which grow back, laterally to the heart, into the anterior part 

 of the body cavity, and the distal portions enlarge into thin 

 walled sacs, the lungs proper. The proximal portions of the 

 paired outgrowths form the bronchi, while the unpaired por- 

 tion gives rise to the windpipe or trachea, the opening by 

 which the trachea communicates with the pharynx being the 

 glottis. 



In this backward growth there is added to the entodermal 

 epithelium of these organs mesenchyme tissue, while the lungs, 

 invading the coelom, become covered externally with a thin 

 layer of epithelium (peritoneum, see coelom). Between the 



