314 PULMONARY MUCOUS TISSUE. 



heart : it takes place chiefly in the right auricle. The effect 

 produced may be shown by exposing the heart of a frog, 

 and compressing it at the point of the opening of the vena 

 cava, and thus preventing the entrance of the blood: the 

 entire heart then ceases to beat, because the ventricle, as well 

 as the auricle, receiving no more blood, there is no longer on 

 the inner surface of these cavities any impression which may 

 se>ve as a point of origin of the reflex action which causes the 

 pulsation of the heart. If the man or animal, however, is in 

 a state of perfect health, it is not very likely that this mechan- 

 ism of compression will produce death. Indeed, though the 

 heart stops, the arteries by means of their elasticity drive 

 their contents into the veins, which become turgid, while the 

 summit of the venous cone quickly pours into the heart a 

 mass of blood, thus setting the heart in motion again. The 

 mechanism which we have described, however, explains the 

 so-called voluntary stoppage of the heart, of which some per- 

 sons have professed to be capable : the will acts upon the 

 heart, in this case, only through the medium of respiration. 



Respiration produces a similar effect on the general circula- 

 tion, the top of the two cones (the arterial and the venous) 

 being included in the thorax. We know that at the top of 

 the venous cone the pressure is so slight that it may be repre- 

 sented by or T J^ ; at the top of the arterial cone, on the 

 other hand, the contraction of the ventricle produces a pres- 

 sure which may be reckoned as -ffo (see p. 143). 



Let us suppose that, by means of a strong expiration, a 

 pressure of -pfc is produced in the cavity of the thorax : the 

 pressure at the top of the venous cone will then be -^^ an 

 enormous pressure for this part of the circulating system, an 

 essential feature of its working condition being the absence 

 of all pressure. The consequence will be a considerable reflux 

 into the veins; this reliux into the veins near the heart is 

 prevented by the numerous valves witli which they are fur- 

 nished, and.it is only at the top of the cone that the pressure 

 is made. As the blood continues to flow, and finds an 

 obstruction to its further progress, stagnation follows, accom- 

 panied by distention of the veins adjacent to the thorax. 

 This is chiefly seen in straining, and in those processes which 

 are accompanied by it, as parturition, defecation, &c.; the 

 signs of the stagnation of the blood are injection of the eyes, 

 redness of the face, cessation of the cerebral circulation, and, 

 finally, the suppression of the functions of the brain (vertigo 

 and even apoplexy) : a state of less entire stagnation, often 



