292 ABSORPTION OF VAPOUR POISONOUS GASES. 



this channel. Of the water of the blood, from which this 

 exhalation is given-off, a small part is most probably formed 

 by the direct union of the hydrogen contained in the food 

 (especially when this is one of its predominating components, 

 153) with the oxygen absorbed. For it has been found by 

 careful experiment, that the proportion of inspired oxygen 

 which disappears (not being contained in the carbonic acid 

 expired, 305), is much greater in animals that are fed on a 

 flesh diet, than in those living on farinaceous food. Another 

 portion of such oxygen probably unites with the sulphur and 

 phosphorus of the food and tissues, to form sulphuric and 

 phosphoric acids, which are excreted through the kidneys in 

 combination with alkaline bases ( 367). 



344. Certain gases act as violent poisons, even when respired 

 in very small proportion. Thus, a Bird is speedily killed by 

 breathing air which contains no more than 1-1 500th part of 

 sulphuretted hydrogen ; and a Dog will not live long in an 

 atmosphere containing 1 -800th part of this gas. The effects 

 of carburetted hydrogen are similar ; but a larger proportion 

 is required to destroy life. Both these gases are given-off by 

 decomposing animal and vegetable matter ; the neighbourhood 

 of which is consequently very injurious to health. Several 

 cases of arsenical poisoning have occurred, from the accidental 

 inhalation of a small quantity of arseniuretted hydrogen, the 

 amount of arsenic contained in which must have been so 

 minute as to be scarcely appreciable. 



CHAPTER YIL 



OF EXCRETION AND SECRETION. 



General Purposes of the Excreting Processes. 



345. WE have seen that the Blood, in the course of its 

 circulation, not only deposits the materials that are converted 

 into the several fabrics of which the body is composed, but 

 also takes-up into itself the products of the decomposition 

 which is continually going-on in its various parts ; and it is 

 to replace this, that the constant Nutrition of the tissues is 

 required. In order that the blood may retain its fitness foi 



