COMPARATIVE. HISTORICAL. 



2 55 



into the blood-filled body-cavities; while in the lung-breathing vertebrates the 

 blood conducted through tubes is brought from the whole body to the respiratory 

 organ. The stigmata on the outer surface of the body, constituting the entrances 

 to the tracheas, are provided with peculiar contrivances for closing, and can be 

 employed for the emission of sounds. Arachnids respire by means of tracheas and 

 lung-like air-sacs (tracheal pouches) ; crabs, by means of gills. Mussels and cephalo- 

 pods possess fully developed gills; snails have partly gills, partly lungs. Among 

 the lower animals, gill-like formations are still found among the round worms and 

 in the echinoderms ; intestinal respiration occurs in the tunicates and many of the 

 mites. Respiration by means of a water- vascular system, a system of canals through 

 which water flows, is peculiar to the medusas and the flat worms. The lowest animal 

 forms protozoa, sponges, polyps do not possess a special respiratory organ; in 

 them the surfaces in contact with water carry on the respiratory interchange of 

 gases. 



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

 cooling of the body, in order to moderate the internal heat. He observed cor- 

 rectly that the warmest animals also respire most actively, but in the interpretation 

 he reversed cause and effect; for the warm-blooded animals do not respire on 

 account of their heat (for cooling purposes) , but they are warm as a result of their 

 more active respiration (combustion). 



Galen (203-131 B. C.) already observed the purifying action of the respiratory 

 organ, assuming that the "soot" was removed from the body with the expired 

 air, together with the expired water. The most important experiments concerning 

 the mechanics of respiration date from Galen. He maintained that the lungs 

 passively follow the movements of the thorax, that the diaphragm is the most 

 important respiratory muscle, that the external intercostals are inspiratory mus- 

 cles, and the internal intercostals expiratory. He divided the intercostal nerves 

 and muscles, and observed that loss of voice followed. After dividing the spinal 

 cord at progressively higher levels, he found that successively higher thoracic 

 muscles became paralyzed. Theophilus Philaretus taught that the circulation 

 could be improved by loud crying, singing, or speaking. Oribasius (360 A. D.) 

 observed that both lungs collapsed in the presence of double pneumothorax. 

 Vesalius (1540) first described artificial respiration as a means of reanimating and 

 stimulating the heart's action. Malpighi (1661) described the peculiar structures 

 of the lungs. Lower (1669). saw the blood become bright red in the lungs. 

 Borelli (died 1679) first explained most thoroughly the mechanism of the respira- 

 tory movements. 



The chemical processes attending respiration were already suspected by 

 Mayow (1679): "Ignis et vita iisdem particulis aereis sustinetur." However, 

 more accurate knowledge could be obtained only after the discovery of the several 

 gases coming under observation. J. B. van Helmont (died 1644) discovered car- 

 bon dioxid, and found that the air was vitiated by respiration; but Black (1757) 

 first discovered the excretion of carbon dioxid during respiration. In 1774 Pristley 

 and Scheele discovered oxygen. Lavoisier, in 1775, found the nitrogen, and at 

 the same time ascertained the composition of the atmosphere. The same investi- 

 gator also represented the formation of carbon dioxid and water during respiration 

 as being the result of combustion within the lungs. J. Ingenhousz (1779) dis- 

 covered the respiration of plants the absorption of carbon dioxid and the giving 

 off of oxygen during that process. Senebier (1785) found that this exhaled oxygen 

 arose from decomposition of the carbon dioxid. Vogel and others definitely proved 

 the existence of carbon dioxid in venous blood. Hoffmann and others demon- 

 strated the presence of oxygen in arterial blood. Lavoisier with Seguin, in 1789, 

 made the first communication concerning the quantitative absorption of oxygen 

 and excretion of carbon dioxid during respiration. More complete insight into the 

 interchange of gases during respiration could be obtained only after Magnus ex- 

 tracted and analyzed the gases from arterial and venous blood. 



