NERVOUS MECHANISM OF RESPIRATION 169 



tant in arresting respiration at any stage whatever during the 

 act of deglutition. 



Stimulation of the sensory fibers of the trigeminal in the nose, 

 as by irritating vapors, may arrest respiration. 



Irritation of the cutaneous nerves in general, as by cold or hot 

 water, slapping, etc., stimulates respiratory movement. 



There are, of course, running from the cortex to the respira- 

 tory center intracranial fibers whereby the organ of the will 

 makes its presence felt in respiration. 



But when all the afferent nerve connections are severed, respi- 

 ration continues with modified rhythm and rate, at least for a time. 

 It is thought that, under these conditions, it is the circulation 

 through the center of blood deficient in oxygen which causes the 

 cells to discharge ; that is, after every inspiration and subsequent 

 expiration there is not another inspiration until the blood has 

 become sufficiently deoxygenated, or charged with carbon 

 dioxide, to irritate the respiratory center. 



We may conclude that "the rhythmical discharges from the 

 center are due primarily to an inherent quality of periodic ac- 

 tivity of the nerve cells constituting the respiratory center, and 

 maintained by the blood, and that the rhythm, rate, and other 

 characters of these discharges may be affected by the will and 

 the emotions, by the composition, supply and temperature of 

 the blood, and by various afferent impulses. The chief factors 

 are the quantities of O and CO 2 in the blood, and the impulses 

 conveyed from the lungs by the fibers of the pneumogastric 

 nerves.'* (Am. Text-Book.) 



The efferent nerves of respiration control the muscular move- 

 ments of that act. They are chiefly the facial, hypoglossal and 

 spinal accessory controlling the respiratory movements about the 

 face and throat; the pneumogastric going to the larynx; the 

 phrenic to the diaphragm certain of the spinal nerves. 



To the lungs proper fibers are distributed by the vagus, the 

 dorsal spinal and the sympathetic nerves. Besides the expiratory 



