DIRECT STIMULATION OF THE CAEDIO-INHIBITORY CENTRE. 667 



in the foetus than in the newly born, and it increases from day to day after birth. Amongst 

 the causes of the diminished excitability are the small amount of in foetal blood, and the slow 

 velocity of the circulation. If an inspiration is discharged in the foetus, it is at once inhibited 

 by fluid passing into the nostrils and inhibiting the act reflexly. The chief cause of the first 

 respiration after birth, is undoubtedly the increasing venosity of the blood, and also the dis- 

 appearance of the above-named reflex inhibitory process.] Death of the mother acts like 

 compression of the umbilical cord. In the former case, the maternal venous blood robs the 

 foetal blood of its 0, so that death of the foetus occurs more rapidly (Zuntz). If the mother be 

 rapidly poisoned with CO ( 17), the foetus may live longer, as the CO-hsemoglobin of the 

 maternal blood cannot take any O from the foetal blood ( 16 Hogyes). In slow poisoning the 

 CO passes into the foetal blood (Grthant and Quinquand). 



In many cases, especially in cases of very prolonged labour, the excitability of the respiratory 

 centre may be so diminished, that after birth, the dyspnceic condition of the blood alone is not 

 sufficient to excite respiration in a normal rhythmical manner. In such cases stimulation of 

 the skin also acts, e.g., partly by the cooling produced by the evaporation of the amniotic fluid 

 from the skin. When air has entered the lungs by the first respiratory movements, the air 

 within the lungs also excites the pulmonary branches of the vagus (Pfiiiger), and thus the 

 respiratory centre is stimulated reflexly to increased activity. According to v. Preuschen's 

 observations, stimulation of the cutaneous nerves is more effective than that of the pulmonary 

 branches of the vagus. In animals which have been rendered apnceic by free ventilation of 

 their lungs, respiratory movements may be discharged by strong cutaneous stimuli, e.g., dashing 

 on of cold water. The mechanical stimulation of the skin by friction or sharp blows, or the 

 application of a cold douche, excites the respiratory centre. When the placental circulation is 

 intact, cutaneous stimuli do not discharge respiratory movements {Zuntz and Gohnstein), (Arti- 

 ficial respiration, 134). 



[Action of Drugs on the Respiratory Centre. Ammonia, salts of zinc and copper, strychnin, 

 atropin, duboisin, apomorphin, emetin, the digitalis group, and heat increase the rapidity and 

 depth of the respirations, while they become frequent and shallower after the use of alcohol, 

 opium, chloral, chloroform, 'physostigmin. The excitability of the centre is first increased and 

 then diminished by caffein, nicotin, quinine, and saponin (Brunton).'] 



369. CENTRE FOR THE INHIBITORY NERVES OF THE HEART 

 (CARDIO-INHIBITORY). The fibres of the vagus, when moderately stimulated, 

 diminish the action of the heart ; when strongly stimulated, however, they arrest 

 its action and cause it to stand still in diastole (352, 7) ; they are supplied to the 

 vagus through the spinal accessory nerve, and have their centre in the medulla 

 oblongata ("~353). 



[Gaskell has shown that stimulation of the vagus not only influences the rhythm 

 of the heart's action, but modifies the other functions of the cardiac muscle. 

 Stimulation of the vagus influences (a) the automatic rhythm, i.e., the rate at 

 which the heart contracts automatically ; (b) the force of the contractions, more 

 especially the auricles, although in some animals, e.g., the tortoise, the ventricles 

 are not affected; (c) the power of conduction, i.e., the capacity for conducting the 

 muscular contractions. According to Gaskell, the vagus acts upon the rhythmical 

 power of the muscular fibres of the heart.] 



This centre may be excited directly in the medulla, and also reflexly, by stimu- 

 lating certain afferent nerves. 



Many observers assume that this centre is in a state of tonic excitement, i.e., that there is 

 a continuous, uninterrupted, regulating, and inhibitory action of this centre upon the heart 

 through the fibres of the vagus. According to Bernstein, this tonic excitement is caused 

 reflexly through the abdominal and cervical sympathetic. 



I. Direct Stimulation of the Centre. This centre may be stimulated directly, 

 by the same stimuli that act upon the respiratory centre. (1) Sudden ancemia of 

 the oblongata, ligature of both carotids or both subclavians, or decapitating a 

 rabbit, the vagi alone being left undivided, cause slowing and even temporary 

 arrest of the action of the heart. (2) Sudden venous hypercemia acts in a similar 

 manner, e.g., by ligaturing all the veins returning from the head. (3) Increased 

 venosity of the blood, produced either by direct cessation of the respirations (rabbit), 

 or by forcing into the lungs a quantity of air containing much C0 2 (Traube). As 

 the circulation in the placenta (the respiratory organ of the foetus) is interfered 





