RESPIRATION 21 1 



has been found, and in deep expiration a somewhat greater 

 positive pressure* (Practical Exercises, p. 274). 



But with ordinary breathing, the variations of pressure as 

 measured by this method do not exceed 5 to 10 mrn. of 

 mercury above or below the pressure of the atmosphere. 



When the external openings are not obstructed, as, for 

 example, when the lateral pressure is taken in the trachea 

 of an animal by means of a cannula with a side-tube con- 

 nected with a manometer, still smaller, and doubtless truer, 

 values have been found (2-3 mm. of mercury as the positive 

 expiratory pressure, and i mm. as the negative inspiratory 

 pressure in dogs). But since the respiratory passages are 

 abruptly narrowed at the glottis, the variations of pressure 

 must be greater below than above it, and in general they 

 must increase with the distance from that orifice, being 

 greater, for instance, in the alveoli than in the bronchi. 



Relation of Respiration to the Nervous System. Unlike the 

 beat of the heart, the respiratory movements are entirely 

 dependent on the nervous system ; and the ' centre ' which 

 presides over them is situated in the spinal bulb. It is a 

 bilateral centre that is, it has two functionally symmetrical 

 halves, one on each side of the middle line ; and each of 

 these halves seems to have to do more particularly with the 

 respiratory muscles of its own side, for destruction of one- 

 half of the spinal bulb causes paralysis of respiration only 

 on that side. Anatomically the respiratory centre has not 

 been sharply localized ; but it lies lower than the vaso-motor 

 centre, not far from the point of the calamus scriptorius. 

 It is brought into relation with the muscles of respiration by 

 efferent nerves. The phrenic nerves to the diaphragm, and 

 the intercostal nerves to the muscles which elevate the ribs, 

 are the most important of those concerned in ordinary 

 breathing. The circular muscles of the bronchi are also 

 supplied with motor fibres that run in the pneumogastric. 

 The bronchial tubes are narrowed by their artificial excitation, 

 but their function in respiration is unknown. The respiratory 



* The maximum negative pressure in deepest inspiration averaged for 

 49 students, 73 mm. (highest observation 137 mm.) of mercury ; the 

 maximum positive pressure in deepest expiration, +80 mm. .(highest 

 observation +uomm.). 



142 



