SECBETION8. 77 



149. Seat of the Exhalation. Exhalations are either 

 external, that is, on the surface of the body ; or internal, 

 that is, on the surface of the internal cavities. 



150. External exhalation must not be confounded 

 with perspiration, or production of sweat ; it takes place on 

 the surface of the lungs as well as on that of the body, and is 

 called insensible transpiration, because, being evaporated by 

 the air, it escapes our notice. Men and animals lose much 

 daily by insensible transpiration, which, of course, is as con- 

 stantly restored ; according to Sanctorius, the loss by in- 

 sensible transpiration in man amounts to |ths of the whole 

 daily loss. 



The evaporation from the surface of the body varies with 

 many circumstances, as climate, &c. ; the escape of carbonic 

 acid gas from the lungs is by means of exhalation merely. 



151. The serous membranes found in the interior of the 

 large cavities of the body are the seat of internal exhalations, 

 consisting chiefly of water, and a few salts, mixed with a 

 small quantity of animal matter. The exhalation which 

 takes place on these surfaces has an exact counterpoise in 

 the absorption going on at the same time ; when this is dis- 

 turbed the fluid accumulates, and dropsies take place, 

 which receive names according to their" localities : hydro- 

 cephalus, in the head; ascites, in the abdomen; hydrothorax, in 

 the chest and pleurae ; and hydrops pericardii when the accu- 

 mulation takes place within the cavity of the pericardium. 



SECEETIONS. 



152. The secretions are special humours, formed at the 

 expense of the blood, in and by organs destined especially to 

 eliminate them from the blood. 



153. They may take place on the surface of mem- 

 branes, as on the skin and mucous membranes ; but they are 

 chiefly found in connexion with certain bodies termed glan- 

 dular, whose essential structure seems to consist in small 

 cavities, extremely minute, or pouches or canals of extreme 

 tenuity ; these receive bloodvessels and nerves, the latter no 

 doubt playing an important part in the phenomena of secre- 

 tion. The glands have been divided into perfect and imper- 

 fect, according as they are provided or not with a tube or 

 duct intended to carry away the product of the secretion. 



154. CHands, properly so called, may all be arranged, 



