RESPIRATION. [CH. XXIV. 



the pulmonary artery, the greater distensibility increases the flow 

 of blood as inspiration proceeds, whilst during expiration, except 

 at its beginning, this property of theirs acts in the opposite 

 direction, and diminishes the flow. Thus, at the beginning of 

 inspiration the diminution of blood-pressure, which commenced 

 during expiration, is continued, but after a time the diminution 

 is succeeded by a steady rise ; the reverse is the case with expira- 

 tion at first a rise and then a fall. 



The nervous system also plays a part in producing rhythmical 

 alterations quite independent of the mechanically caused undula- 

 tions of the blood-pressure. It will be noticed that the heart 

 beats more rapidly during the rise* of blood-pressure than during 

 the fall which accompanies expiration. This will be seen in 

 the tracing (fig. 327, p. 365 ; see also fig. 277, p. 289). This effect 

 disappears after section of the vagi (fig. 328, i). It has been 

 found that it is reflexly caused. Stimulation of the pulmonary 

 branches of the vagus by electrical stimuli, or of their terminations 

 in the alveoli by certain irritating vapours like chloroform or 

 bromine, causes a reflex inhibition of the heart ; great distension 

 of the alveoli has a similar effect, but moderate distension, such 

 as occurs during ordinary inspiration, has the opposite reflex effect, 

 causing the heart to beat more rapidly. The afferent fibres from 

 the pulmonary alveoli enter the bulb by the upper set of the 

 rootlets of the combined glossopharyngeal-vagus-spinal accessory 

 nucleus (the a group, see p. 252). Sometimes the rootlets of 

 this group are three in number, sometimes two ; when there 

 are two the lower rootlet, when there are three the lower two 

 rootlets, contain the fibres in question. (Cadman.) 



Asphyxia. 



Asphyxia may be produced in various ways : for example, by 

 the prevention of the due entry of oxygen into the blood, either 

 by direct obstruction of the trachea or other part of the respi- 

 ratory passages, or by introducing instead of ordinary air a gas 

 devoid of oxygen, or by interference with the due interchange of 

 gases between the air and the blood. 



The symptoms of asphyxia may be divided into three groups, 

 which correspond with the stages of the condition which are 

 usually recognised ; ( i ) the stage of exaggerated breathing ; 

 (2) the stage of convulsions ; (3) the stage of exhaustion. 



In the first stage the breathing becomes more rapid and at 

 the same time deeper than usual, the inspiration at first 

 being especially exaggerated and prolonged. The muscles of 



