494 HEALTH AND DISEASE 



the wound than through the trachea, in consequence of which the lungs 

 become compressed and cease to perform theii- office. 



The Nerves and Nerve - Centres of Respiration. — The nerve- 

 centres which are implicated in quiet respiration are situated in that 

 part of the brain termed the medulla oblongata, at the point where the 

 vagi nerves take their origin. The destruction of this very small spot 

 causes immediate death by arrest of the respiratory acts. It was accord- 

 ingly named the " noeud vital", or vital spot, by Flourens. Nerve fibres 

 run to and from this point. Amongst those which convey nervous im- 

 pulses to it are the fibres of the vagus nerve which pass up to it. Those 

 which conduct impulses away from it are fibres which pass down the 

 spinal cord for some distance, and then emerge at the lower part of the 

 neck and along the thorax to form the phrenic and intercostal nerves 

 distributed to the diaphragm and the intercostal muscles. 



When at rest, the horse breathes ten or twelve times per minute, and 

 there is a general relation between the number of respirations and the 

 number of the beats of the heart, the proportion being about 1:4 or 1:5, 

 but the frequency with which the acts of respiration are performed is 

 subject to great variation. At rest, or during sleep, the number is about 

 ten or twelve in the minute, but after vigorous exercise, such as galloping 

 for ten minutes, it may rise to fifty, sixty, or more in the same time, 

 gradually subsiding as the animal becomes quiescent. In two horses, 

 after a run of about 7 miles, the pulse was observed to rise from 40 

 to 132, and the respirations from 12 to 102, in the minute. After being 

 at rest for three-quarters of an hour the pulse had fallen to 66 and to 

 54, whilst the respirations in each animal were 60 per minute (Arloing). 

 The depth of the respirations greatly augments with increased frequency, 

 so that a much larger volume of air enters, and is expelled from, the 

 lungs at each inspiration and expiration. This constitutes forced respira- 

 tion. 



ASPHYXIA 



This term, which, etymologically speaking, signifies pulselessness, has 

 been erroneously applied to that condition in which there is great 

 deficiency of oxygen, and, as a rule, increase of carbon dioxide in the 

 blood. The amount of oxygen entering the lungs may be reduced 

 rapidly by strangulation, or by closure of the mouth and nostrils, as in 

 smothering, or by sudden closure of the glottis, as by choking, or by 

 exposure to irrespirable gases, such as carbon dioxide and chlorine, or to 

 such a gas as carbon monoxide, which, though respirable, replaces oxygen 

 in the blood corpuscles, or by plunging the animal into another medium, 



