OH. XXIV.] TISSUE-RESPIRATION. 381 



oxygen is certainly far in excess of anything that can be explained 

 by mere diffusion. The storage of oxygen, moreover, ceases when 

 the vagus nerves which supply the swim-bladder are divided. 



Tissue-Respiration. Before the processes of respiration were 

 fully understood the lungs were looked upon as the seat of 

 combustion ; they were regarded as the stove for the rest of the 

 body to which effete material was brought by the venous blood to 

 be burnt up. When it was shown that the venous blood going 

 to the lungs already contained carbonic acid, and that the 

 temperature of the lungs is not higher than that of the rest 

 of the body, this explanation had of necessity to be dropped. 



Physiologists next transferred the seat of the combustion to 

 the blood ; but since then numerous facts and experiments have 

 demonstrated that it is in the tissues themselves, and not in the 

 blood, that combustion occurs. The methylene-blue experiments 

 already described (p. 378) show this ; and the following experiment 

 is also quite conclusive. A frog can be kept alive for some time 

 after salt solution is substituted for its blood. The metabolism 

 goes on actively if the animal is kept in pure oxygen. The 

 taking up of oxygen and giving out of carbonic acid must there- 

 fore occur in the tissues, as the animal has no blood. 



Ventilation. It is necessary to allude in conclusion to this 

 very important practical outcome of our consideration of 

 respiration. 



Some Continental observers have stated that certain noxious 

 substances are ordinarily contained in expired air which are much 

 more poisonous than carbonic acid, but researches in this country 

 have failed to substantiate this. If precautions be taken by 

 absolute cleanliness to prevent admixture of the air with exhala- 

 tions from skin, teeth, and clothes, the expired air only contains 

 one noxious substance, and that is carbonic acid. 



Absolute cleanliness is however not the rule ; and the air of 

 rooms becomes stuffy when the amount of expired air in them is 

 just so much as to raise the percentage of carbonic acid to o'i 

 per cent. An adult gives off about o - 6 cubic feet of carbonic 

 acid per hour, and if he is supplied with 1,000 cubic feet of fresh 

 air per hour he will add o'6 to the 0*4 cubic feet of carbonic 

 acid it already contains ; in other words the percentage of that 

 gas will be raised to 0*1. An hourly supply of 2,000 cubic feet of 

 fresh air will lower the percentage of carbonic acid to 0*07, and 

 of 3,000 cubic feet to o'oo, and this is the supply whicn is usually 

 .recommended. In order that the air may be renewed without 

 giving rise to draughts, each adult should be allotted sufficient 

 space in a room, at least 1,000 cubic feet. 



