640 journal of Agriculture, [10 Oct., 1908, 



largely due to the increase to the carbon-dioxide in the blood. We have 

 here a beautiful automatic mechanism whereby the carbon-dioxide in arterial 

 blood can be kept almost constant. Even a trifling rise in the carbon- 

 dioxide content of the blood brings about a big increase in lung ventilation 

 and in consequence the gas can be got rid of at a quicker rate. 



2. Low oxygen content of the blood. If an animal breathes air con- 

 taining less than 13 per cent, of oxygen the deficit in the blood excites- 

 the centre and respiration is quickened and deepened. Thus animals at 

 an altitude of 12,000 feet and over display a marked increase in the ampli- 

 tude of respiration. In strangulation the violent respiratory efforts which 

 are made are produced not only by oxygen deficiency but by the heaping 

 up of carbon-dioxide. 



3. When the temperature of the blood rises above the normal the 

 respiratory centre is excited, particularly in the non-sweating animals. The 

 excitation, however, is never so intense as that due to either of the fore- 

 mentioned factors. 



4. There is some evidence that in very violent exertion and in asphyxia 

 such as occurs in strangulation, drowning, &c., the centre is excited, not 

 only by carbon-dioxide excess and oxygen loss, but also by the entry into- 

 the blood of waste products from the muscles which would under normal 

 conditions be oxidised. 



The centre can be influenced by nervous channels as well, witness the 

 limited voluntary control of the higher brain over the respiratory move- 

 ments. Then there is also the action of the vagus to consider. When the 

 lung is inflated and stretched the vagus sends up messages which stop 

 further inspiration ; when the lung is partly deflated, and the stretch some- 

 what relieved, the vagus sends up messages which start inspiration. The 

 vagus mechanism is therefore for regulating the rhythm of the respiratory 

 centre. 



RESPIRATORY HYGIENE. 



If the air be too dry the mucous membranes are liable to suffer, if too 

 moist the temperature regulation of the body is impeded. Rarely does 

 the oxygen of a badly ventilated house or stable or pen fall to an extent 

 that would be harmful. But the carbon-dioxide of the air should not be 

 allowed to mount up. In the case of the human being, house-ventilatioo 

 aims at 0.06 per cent, as the maximum, and the same figure may be given 

 for the domestic animals. When animals are herded too closely together 

 in a building a number of harmful factors are produced. The air is 

 rendered too moist and too warm ; the carbon- dioxide is increased in per- 

 centage ; poisonous gases voided from the intestine are breathed; more- 

 over the warm moist air allows bacteria to grow luxuriantly on the debris 

 of the body (sweat, shed cuticle, and, in the case of the lower 

 animals, ordure) and so poisonous products of putrefaction are added to 

 the air. The opportunity given to infectious diseases to spread where over- 

 crowding exists is obvious. 



