COMPARATIVE AND HISTORICAL. 205 



they were in a vacuum, or pure N. There is paralysis of the central nervous system, sometimes 

 preceded by convulsions. The heart ceases to beat (not the lymph hearts), while the excitability 

 of the motor nerves is lost at the same time, and lastly the direct muscular excitability disappears. 

 An excised frog's heart placed in under a very high pressure (13 atmospheres), scarcely beats 

 one-fourth of the time during which it pulsates in air. If the heart be exposed to the air 

 again, it begins to beat, so that compressed renders the vitality of the heart latent before 

 abolishing it. 



Phosphorus retains its luminosity under a high pressure in 0, but this is not the case with 

 the luminous organisms, e.g., Lampyris, and luminous bacteria. High atmospheric pressure is 

 also injurious to plants. 



140- COMPARATIVE AND HISTORICAL. Mammals have lungs similar to those of man. 

 The lungs of birds are spongy, and united to the chest-wall, while 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. The 

 diaphragm is absent. In 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 amphi- 

 bians (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 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. Insects and 

 centipedes respire by "trachea?," 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 tracheae and tracheal sacs, crabs by gills. The molluscs and cephalopods 

 have gills ; some gasteropods have gills and others lungs. Amongst the lower invertebrata some 

 breathe by gills, others by means of a special " water- vascular system," and others again by no 

 special organs. 



Historical. Aristotle (384 B.C.) regarded the object of respiration to be the cooling of the 

 body, so as to moderate the internal warmth. He observed correctly that the warmest animals 

 breathe most actively, but in interpreting the fact he reversed the cause and effect. Galen 

 (131-203 a. d.) thought that the "soot" was removed from the body along with the expired 

 water. The most important experiments on the mechanics of respiration date from Galen ; he 

 observed that the lungs passively follow the movements of the chest ; that the diaphragm is the 

 most important muscle of inspiration ; that the external intercostals are inspiratory ; and the 

 internal, expiratory. He divided the intercostal nerves and muscles, and observed that loss of 

 voice occurred. On dividing the spinal cord higher and higher, he found that as he did so the 

 muscles of the thorax lying higher up became paralysed. Oribasius (360 a. d. ) observed that in 

 double pneumothorax both lungs collapsed. Vesalius (1540) first described artificial respiration 

 as a means of restoring the beat of the heart. Malpighi (1661) described the structure of the 

 lungs. J. A. Borelli (f 1679) gave the first fundamental description of the mechanism of the 

 respiratory movements. The chemical processes of respiration could only be known after the 

 discovery of the individual gases therein concerned. Van Helmont (t 1644) detected C0 2 . 

 [Joseph Black (1757) discovered that C0 2 , or "fixed air," is given out during expiration.] In 

 1774 Priestley discovered 0. Lavoisier detected N (1775), and ascertained the composition of 

 atmospheric air, and he regarded the formation of C0 2 and H 2 of the breath as a result of a 

 combustion within the lungs themselves. J. Ingen-Houss (1730-1799) discovered the respira- 

 tion of plants. Vogel and others proved the existence of C0 2 in venous blood, and Hoffmann 

 and others that of in arterial blood. The more complete conception of the exchange of 

 gases was, however, only possible after Magnus had extracted and analysed the gases of arterial 

 and venous blood (36). 



