COMPARATIVE AND HISTORICAL. 277 



die when the pressure is reduced to 120 mm. Hg. ; mammals at 40 mm. Hg. ; frogs 

 endure repeated evacuations of the receiver, whereby they are much dis- 

 tended, owing to the escape of gases and water, but after the entrance of air they 

 become greatly compressed. The cause of death in mammals is ascribed by 

 Hoppe-Seyler to the evolution of bubbles of gas in the blood ; these bubbles stop 

 up the capillaries, and the circulation is arrested. Local diminution of the atmo 

 spheric pressure causes marked congestion and swelling of the part, as occurs 

 when a cupping-glass is used. 



Great Increase of the Atmospheric Pressure. The phenomena, which 



are, for the most part, the reverse of the foregoing, have been observed in pneumatic 

 cabinets and in diving bells, where men may work even under 4| atmospheres 

 pressure. The phenomena are : (1.) Paleness and dryness of the external sur- 

 faces, collapse of the cutaneous veins, diminution of perspiration, and mucous 

 secretions. (2.) The tympanic membrane is pressed inwards (until the air escapes 

 through the Eustachian tube, after causing a sharp sound), acute sounds are heard, 

 pain in the ears, and difficulty of hearing. (3. ) A feeling of lightness and freshness 

 during respiration, the respiration becomes slower (by 2-4 per minute), inspiration 

 easier and shorter, expiration lengthened, the pause distinct. The capacity of the 

 lungs increases, owing to the freer movement of the diaphragm, in consequence of 

 the diminution of the intestinal gases. Owing to the more rapid oxidations in the 

 body, muscular movement is easier and more active. The O absorbed and the C0 2 

 excreted are increased. The venous blood is reddened. (4. ) Difficulty of speaking, 

 alteration of the tone of the voice, inability to whistle. (5.) Increase of the urinary 

 secretion, more muscular energy, more rapid metabolism, increased appetite, sub- 

 jective feeling of warmth, pulse beats slower, and pulse-curve is lower (compare 

 p. 150). In animals subjected to excessively high atmospheric pressure, P. Bert 

 found, that the arterial blood contained 30 vols. per cent. O (at 760 mm. Hg. ); when 

 the amount rose to 35 vol. per cent., death occurred with convulsions. Compressed 

 air has been used for therapeutical purposes, but in doing so a too rapid increase 

 of the pressure is to be avoided. Waldenburg has constructed such an apparatus, 

 which may be used for the respiration of air under a greater or less pressure. 



140. Comparative and Historical. 



Mammals have lungs similar to those of man. The lungs of birds are spongy, 

 united to the chest- wall, and there are openings on their surface communicating 

 with thin- walled "air-sacs" which are placed amongst the viscera. The air-sacs 

 communicate with cavities in the bones, which give the latter great lightness 

 (Aristotle). The diaphragm is absent. Jn reptiles the lungs are divided into 

 greater and smaller compartments ; in snakes one lung is abortive, while the other 

 has the elongated form of the body. The amphibians (frog) possess two simple 

 lungs, each of which represents an enormous infundibulum with its alveoli. The 

 frog pumps air into its lungs by the contraction of its throat, the nostrils being 

 closed and the glottis opened. When young until their metamorphosis frogs 

 breathe like fishes by means of gills. The perennibranchiate amphibians (Proteus), 

 retain their gills throughout life. Amongst fishes, which breathe by gills and 

 use the O absorbed by the water, the Dipnoi have in addition to gills a swim-bladder 

 provided with afferent and efferent vessels, which is comparable to the lung. The 

 Cobitis respires also with its intestine (Erman, J80S). Insects and centipedes 

 respire by " tracheae," which are branched canals distributed throughout the body; 

 they open on the surface of the body by openings (stigmata) which can be closed. 

 Spiders respire by means of trachese and tracheal sacs, crabs by gills. The 

 molluscs and cephalopods have gills, some gasteropods have gills and others lungs. 



