178 THE BODY AT WORK 



all accessible nerves have been cut, including even the posterior 

 roots of the cervical nerves. Probably it is a mistake to look 

 for definite afferent channels in the medulla and the rest of the 

 brain. All parts of the body need aerated blood. From all 

 parts, including nerve-tissue itself, arises the demand for 

 respiration. Possibly nerve-centres have the power, as it were, 

 of storing impulses, and discharging them after the stream of 

 fresh arrivals has ceased to flow. They may acquire a habit. 



The resistance in the centre is profoundly affected by the 

 condition of the blood. As the blood becomes more venous, 

 impulses pass across the nerve connections with ever-increasing 

 force. Kept in the first instance to definite channels, they 

 spread as the centre becomes more excitable farther and 

 farther afield, reaching one group of muscles after another, 

 and pressing them into the service of respiration. When, in 

 dyspnoea, every muscle which can in any way help the move- 

 ments of the chest is doing its best, others which are useless 

 for this purpose receive the reflected impulses and join in, 

 producing general convulsions. The increased activity of the 

 respiratory centre which is produced by slight venosity of the 

 blood is shown in the rapid and deep inspirations which are 

 caused by violent exercise. Perhaps it is justifiable to go a 

 step farther, and to assert that there is something in blood 

 which has been rendered venous by muscular activity which 

 is specially exciting to the respiratory centre. If the blood 

 from a limb be prevented from returning to the general cir- 

 culation, by compressing or tying its great veins, and if the 

 muscles of the limb be strongly stimulated by an electric 

 current, their activity, so long as the -passage through the 

 veins is blocked, has no influence upon respiration. But, on 

 relaxation of the pressure on the veins, respiration may 

 become twice as deep and twice as frequent as it was before 

 the muscles were stimulated, although the limb is now in a 

 condition of perfect rest. 



What is the special action of the vagus nerve ? Its superior 

 larnygeal branch checks inspiration and induces expiration, as 

 already said. The impulses which pass up its main trunk 

 bring about ordered movements. They are not dependent for 

 their generation upon the condition of the blood in the lungs. 

 When the chest is filled with nitrogen, inspiration and expira- 



