258 



CHORDATE ANATOMY 



remain separated by the pericardium. With the development of the 

 diaphragm, the pleural cavity containing the lungs becomes separated 

 from the more posterior peritoneal cavity. According to Broman the 

 diaphragm arises from four sources, the septum transversum anterior to 

 the liver, the pleuroperitoneal membranes, the body-wall, and the dorsal 

 mesentery. 



The nasal passages of lower vertebrates, such as the Dipnoi and 

 Amphibia, develop from nasobuccal grooves similar to those seen in some 

 adult elasmobranchs. In the embryo an ectodermal groove extends from 

 each olfactory pit to the corner of the mouth. Later the groove deepens, 



Fig. 239.^ — Stages in the development of the trachea, bronchi and lungs in the pig. 

 The pulmonary arteries are shown in black; the veins are cross hatched. Ep, bud of 

 eparterial bronchus. (From Patten's "Embryology of the Pig," after Flint.) 



its edges meet and fuse together and convert the groove into a tubular 

 passage which connects the pit with the mouth cavity (Fig. 352). 



The development of the nasal passage in the human embryo is 

 slightly different. In the month-old embryo a similar nasobuccal groove 

 makes its appearance. The nasal passage, however, is not formed by 

 the closure of this groove, but by the backward extension of the epithehum 

 of the olfactory pit, which thus acquires a secondary connexion with the 

 mouth (Fig. 353). 



History of the Pulmonary System. Invertebrates have no organs 

 comparable with the human pulmonary system, the so-called lungs of 

 pulmonate molluscs being modifications of the mantle and not out- 

 growths from the alimentary canal. Opinions are divided as to the origin 

 of the lungs. According to some, a pair of gill pouches which failed to 

 reach the skin have been converted into lungs. Others suppose that lungs 

 have evolved from the air bladder of fishes. Some seek to reconcile these 



