RESPI RA TOR Y CEN TRE 1 .'to 



of the diaphragm on the corresponding side. Section of the 

 cord just below the fifth cervical nerve stops the costal move- 

 ments, but does not affect the diaphragm, because the nuclei 

 of origin of the phrenics lie just above the section. If the 

 section is placed somewhat higher, respiration ceases entirely, 

 but the associated movements of the larynx and face continue. 

 During forced breathing the facial, hypoglossal, and spinal 

 accessory nerves are called into action. During uterine life 

 the respiratory centre is in an apneic condition, on account 

 of a low irritability of the respiratory centre. 



Cases have been seen in which the child has made respi- 

 ratory efforts while within the intact fetal membranes. Such 

 an attempt draws some of the amniotic fluid into the nose, 

 causing inhibition of all further efforts. After birth, when sponta- 

 neous respiration is about to take place, it is well to remove all 

 mucus or other matter from the nose, to avoid inspiration of them. 



Normal, quiet respiration may be regarded as consisting 

 of an active inspiration and a passive expiration. It is the 

 coordinated activity of the inspiratory muscles that is char- 

 acteristic of respiration, and the expiratory muscles come 

 into action only occasionally under special conditions. Period- 

 ically the respiratory centre becomes active as the result 

 of the stimulating action of the carbon dioxide of the blood. 

 The inspiratory act follows and continues until the inhibitory 

 fibers in the vagus, stimulated by the expansion of the lungs, 

 brings inspiration to an end. The expiration which follows is, 

 in quiet respiration, a passive return of the chest to its original 

 condition. In other words, the expiratory centre is not, under 

 ordinary conditions, automatic. Its activity is, in some way, 

 dependent upon that of the inspiratory centre. Under special 

 conditions it becomes active: (1) In reflexes like coughing; 

 (2) voluntarily, as in straining; (3) as the result of the stimula- 

 tion of pain fibers; (4) by the action of substances, CO 2 and 

 others, in the blood. 



The bronchial musculature is supplied through the vagus 

 with motor and inhibitory fibers, or, as they are called, broncho- 

 constrictor and bronchodilator fibers. An artificial tonus of 

 the constrictor fibers can be produced by the administration 

 of a number of drugs such as muscarin, pilocarpin, and physo- 

 stigmine. They are supposed to stimulate the endings of the 



