RESPIRATION 



665 



body, but we must not forget that it has not 

 always been be.ieved in, and even now is doubted 

 by some. The original theory was that the oxygen 

 was used, and the carbonic acid formed, in the 

 lungs only. This was disproved when it was 

 shown that there is more oxygen and less carbonic 

 acid in the blood coming from the lungs than in 

 that going to them. Next it was, and still is by 

 some, thought that the oxidations take place within 

 the blood ; the cells of the tissues were imagined 

 as pouring oxidisable matters into the Wood. 

 Usually very little matter capable of taking 

 oxygen away from a loose combination can be 

 found in the blood, but in that of asphyxiated 

 animals more of such matter was found ; this was 

 explained by supposing that in asphyxia the oxidis- 

 able excreta from the cells accumulated in the 

 blood through insufficiency of oxygen ; but it has 

 recently been shown that this reducing stuff only 

 exists in the reil blood-cells i.e. in the reduced 

 h;einoglobin while lymph, which we might expect 

 to lind rich in such matters, it being into the 

 lymph that most of the excreta of the cells are 

 poured, is totally devoid of it. Lastly, the supposi- 

 tion that the cells of the tissues use the oxygen 

 directly is so much in harmony with all our present 

 idea- of animal physiology and with the facts of 

 comparative respiration (one-celled animals breathe, 

 mid plants breathe, anil in these there is no cir- 

 culating blood) and of embryology (the embryo 

 mammal breathes though its blood-vessels are not 

 connected directly with those of its mother) that 

 one is disposed to believe it without further proof. 



The mode of interchange of gases between the 

 blood and the tissues must l>e the same as that 

 with which we are already familiar viz. the 

 diffusion from a place of high partial pressure 

 to one of lower partial pressure. The fact that 

 a low partial pressure of oxygen is constantly 

 maintained within the tissues is one of the pheno- 

 mena that constitute the mystery of life. We 

 have already seen that even in outer respiration 

 the living cells of the essential membrane of the 

 lungs may apparently do work against partial 

 pressure, absorbing more oxygen and excreting 

 more carbonic acid than the differences of pressure 

 will account for ; it is therefore extremely probable 

 that a similar state of activity is characteristic of 

 the cells of the other tissues. Taking the more 

 obvious facts first, we know that with any weight 

 of body i.e. with a given amount of tissue to be 

 supplied with oxygen the amount of oxygen taken 

 in and of carbonic acid excreted varies with the 

 activity of the organism and with the amount of 

 work that it is doing ; it is greater in youth than 

 in old age, in wakefulness than in sleep, during 

 the activity of secreting glands than when these 

 are at rest, during the performance of muscular 

 work than in repose ; in tins case it is the excretion 

 of carbonic acid rather than the intake of oxygen 

 which is especially marked. This last peculiarity 

 brings us face to face with a remarkable state of 

 .affairs. The partial pressure of oxygen within 

 muscular tissue is always practically zero i.e. 

 however low the external pressure of oxygen may 

 l>e, none will leave the muscle. The effect of this 

 of course will be, so far as ordinary diffusion is 

 concerned, that oxygen will always l>e leaving the 

 blood and entering the tissues. This oxygen is in 

 some way stored up within the muscle-cells, so 

 that a muscle will work for a consideralde time 

 without any fresh supplies of oxygen, even in an 

 atmosphere of nitrogen. This explains the fact 

 noted above, that during^ muscular work the excre- 

 tion of carbonic acid is in excess of the absorption 

 of oxygen. A supply of oxygen, however, is neces- 

 a*y for the maintenance of the irritability of the 

 muscle, which soon falls off without it, probably 



before the supply of stored oxygen used for the 

 performance or its work has been exhausted. This 

 is about all that is known of the chemical changes 

 connected with respiration within a cell. The 

 oxygen enters it by diffusion, possibly aided by 

 some vital activity ; the rapid storing away of the 

 oxygen and consequent readiness to absorb more is 

 in reality an example of such activity ; the. oxygen 

 is made use of within the cell for maintaining its 

 life, for producing heat, for producing rapid decom- 

 positions which supply the energy of muscular 

 contraction ; finally the carlxmic acid leaves the 

 cell and enters the blood, possibly aided in this 

 process by some process other than a simple diffu- 

 sion. The respiratory changes of other tissues are 

 probably similar to those of muscle ; within them, 

 within the lymph that bathes them, and within 

 their secretions there is practically no free oxygen, 

 while the pressure of carlionic acid, owing to its 

 constant production within the cells, is greater 

 within the cells, their secretions, and the lymph 

 that bathes them than it is in venous blood. 



There is another fact about respiration which is 

 still a puzzling matter, and, since it results from the 

 changes within the cells, is likely to remain so for 

 some time. All the food of a meal, or its equiva- 

 lent, is in about six hours oxidised into carbonic 

 acid, water, and urea. This is obvious without any 

 elaborate calculations from the fact that we nray 

 eat every six hours and yet not gain in weight, 

 while, apart from the indigestible parts of the food, 

 which do not affect the problem, the chief matters 

 that leave the body are those mentioned above. 

 Yet food-stuffs outside the body are not affected by 

 oxygen at the temperature of the body. Various 

 ,-t ions as to the possible reason for this have 



been made ; but, since the phenomenon is obviously 

 dependent upon the vital processes of cells, sug- 

 gestions in terms of the principles of ordinary 

 chemistry cannot carry us far. 



Further information regarding respiration will 

 be found in the last editions of the text-books 

 of physiology recommended at the end of the 

 article upon that subject. The diseases of the 

 respiratory organs are dealt with in separate 

 articles, BRONCHITIS, CONSUMPTION, PLEURISY, 



PNEUMONIA, TUBERCLE, &c. 



Jlistoricttl. Aristotle (384 B.C.) thought that 

 the object of respiration was to cool the body. 

 He observed that the warmer the animal the 

 more rapid the breathing, arrd transposed cause 

 and effect. Galen (131-203 A.D.) experimented 

 upon the mechanics of respiration, and knew some- 

 thing of the nervous mechanism. He believed 

 that ' soot ' and water were excreted from the body 

 by the lungs. Malpighi ( 1661 ) described the struc- 

 ture of the lungs. Van Helmont ( 1664) discovered 

 carlionic acid ; Black ( 1757) observed that carbonic 

 acid is breathed out of the body. Priestley (1774) 

 discovered oxygen. Lavoisier (1775) discovered 

 nitrogen, found the composition of the air, and 

 taught that the formation of carbonic acid and 

 water resulted from the combustion that took place 

 in the lungs. Vogel proved the existence of car- 

 Iwnic acid in the venous blood ; Hoffmann found 

 oxygen in arterial blood. Magnus extracted and 

 analysed the gases of the blood in both states. 



Comparative. Most of tlie Protozoa, all the 

 sponges and stinging animals, and many simple 

 worm-types live in water, which washes their 

 surface and saturates their substance, the oxygen 

 dissolved in the water serving the same purpose 

 as that mixed with the air. While many worms 

 breathe simply through their skin, many of the 

 aquatic forms have structures specialised for res- 

 piration modifications of the legs or tentacles or 

 vascular outgrowths of the body-wall. In Echino- 

 derrns respiration is effected by the tube-feet, and 



