PART SEYENTH. 



PULMONARY MUCOUS TISSUE. RESPIRATION. 

 ANIMAL HEAT. 



I. RESPIRATION. 



THE surface of the respiratory mucous 1 is that which, 

 next to the epithelial surface of the digestive tract, most 

 readily yields to interchanges of nutrition; these inter- 

 changes are, however, in the normal condition, chiefly 

 gaseous. As the absorption of the substances, called ali- 

 mentary, occurs slightly over all the 

 surfaces, and as we have seen that 

 the reabsorption of fat happens in all the 

 tissues, although these phenomena 

 have their special seat at the level of 

 the epithelium of the digestive tract, 

 so the gaseous interchanges take place 

 over many surfaces, for instance, in 

 ths skin, and the gases may be reab- 

 sorbed in the most interior portion of . 

 the tissues (as in sub-cutaneous em- 



, Fig. 72. Ramification of the 



physema) ; yet these phenomena are pulmonary pouch in the 

 connected chiefly, in the superior ani- yff^SSSKlS3S^ 

 ma Is, with the respiratory mucous. 



The respiratory mucous may, from an embryological point 

 of view, be considered as an offshoot of the sub-diaphrag- 

 matic part of the digestive tract ; indeed, the first appearance 

 of the lungs in the foetus exhibits the form of a growth of 

 the epithelium, of the anterior wall of the pharynx. This 



1 It may have been noticed that the word ' ' mucous ' ' has been 

 used frequently as referring to the mucous coat, tissue, or mem- 

 brane. 



