3 o8 MECHANISM OF THE RESPIRATORY MOVEMENTS. 



oxygen, begins to fail. Each beat becomes more and more feeble, the 

 quantity of residual blood left in the cavities at the end of systole 

 increasing at each beat, until finally the heart is enormously dilated, 

 and each contraction is marked only by a feeble flutter of the cardiac 

 walls. In consequence of this heart failure, the pressure sinks steadily 

 until circulation is at an end and the pressure is the same all over the 

 vascular system (about 10 mm. Hg). A vigorous animal, with well- 

 marked vasomotor reaction, will die of heart failure, before any relaxa- 

 tion of the vessels has taken place. This is shown by the fact that if 

 artificial respiration be suddenly employed, when the blood pressure is 

 approaching its lowest point, the access of oxygen benefits the heart 

 earlier than the vasomotor centre, and the pressure may rise to a point 

 even higher than that reached during the primary asphyxial rise. 



Failure of the circulation, however, gradually impairs the activity of 

 the medullary centres, as is shown by the gradual decline in arterial 

 and venous pressures after all circulation has ceased (in consequence of 

 increased capacity of vascular system). 



At the end of asphyxia, therefore, all cavities of the heart are dis- 

 tended with blood, the distension being most pronounced in the cavities 

 with thin walls, i.e. auricles and right ventricle. The difference thus 

 occasioned between right and left ventricles is generally increased after 

 rigor mortis has set in, and it is supposed that the post-mortem con- 

 traction of the left ventricle expels the contents of that cavity, so that 

 it may be found almost empty unless examined immediately after death. 



Modified Eespiratory Movements. 



Coughing. — This consists in the first place of a deep inspiration, 

 followed by closure of the glottis. This is succeeded by a sudden 

 powerful expiratory effort, with total relaxation of the inspiratory 

 muscles. The air is forced suddenly through the resistance offered 

 by the glottis, and the blast of air thus produced serves to expel 

 through the upper air-passages and mouth any foreign body lodged in 

 the larynx. Coughing is usually excited by stimulation of the termina- 

 tions of the superior laryngeal nerve in the mucous membrane of the 

 larynx. But it may also be clue to stimulation of certain of the vagus 

 endings in the lungs, or of the nerves supplied to the pleura. 



Sneezing is very allied in its general characters to coughing. 

 In each case an inspiration is followed by a powerful expiratory 

 effort. In sneezing, the resistance to the outburst of air seems to be 

 afforded in the first place by the approximation of the dorsum of the 

 tongue to the soft palate, so that the first part of the expired air passes 

 through the nasal passages. The greater part of the air is, however, 

 expelled through the mouth. Sneezing is usually excited reflexly by 

 stimulation of the nasal branch of the fifth nerve ; when it is pro- 

 duced by sudden exposure to a bright light, the afferent impulses 

 probably travel up the optic nerve. 



In laughing and crying, a deep inspiration is followed by a series 

 of short spasmodic expirations. The glottis is freely open during the 

 whole time, and the vocal cords being tense, are thrown into vibration, 

 so occasioning the characteristic sounds of the two actions. The 

 chief difference between the two actions is in the accompanying facial 

 expressions. 



