214 PHYSIOLOGY FOR DENTAL STUDENTS. 



into the thorax by expanding the larger veins. The expansion 

 of the lungs at inspiration also dilates the capillaries and arteri- 

 oles imbedded in these tissues, hence a greater volume of blood 

 can pass through them in the same unit of time. If the heart 

 beat remains constant in strength and rate, the increased amount 

 of blood pumped during inspiration will cause the blood pres- 

 sure to rise. 



It is well to bear in mind that under abnormal conditions the 

 respiration may affect the blood pressure to a dangerous extent. 

 For instance, in the attempt to force air from the lungs under 

 pressure into a vessel, as in blowing up a football or testing the 

 strength of expiration on a machine made for the purpose, the 

 air pressure can be increased within the thorax to more than 

 equal the pressure in the vessels of the lungs, and the circulation 

 is temporarily stopped in the pulmonary vessels. The blood be- 

 comes dammed up in the venous system and forced out of the 

 lungs by the pressure of air. This experiment is dangerous in 

 one who has not a first-class heart and vascular system. The ef- 

 fects on the lungs and blood pressure of sucking, inspiration and 

 expiration can be conveniently reproduced on an artificial schema 

 which represents the thoracic cavity, lungs, heart and related 

 vessels, as shown in Fig. 31. 



Variations occur in the respiratory movements under various 

 emotional and physical conditions. Any foreign or irritating 

 body within the air passages will cause a cough. This consists 

 in a forced expiration, during the first portion of which the glot- 

 tis is closed. The irritating substance is likely to be expelled by 

 the sudden opening of the glottis. The presence of irritating 

 substances in the nasal cavity gives rise to sneezing, a sudden 

 and noisy expiration through the nasal passages preceded by a 

 rapid and deep inspiration. In crying, inspirations are short and 

 spasmodic, followed by prolonged expirations, whereas laughing 

 is quite the reverse. Yawning, the expression of drowsiness or 

 ennui, consists in long deep inspirations followed by a short ex- 

 piration. Hiccoughing is due to spasmodic contractions of the 

 diaphragm, the peculiar sound being due to sudden closure of 

 the glottis. 



