RESPIRATION. 41 



lated by reflex influence. The nerves which convey to the muscles 

 of inspiration their impulse are the phrenics (to the diaphragm) 

 and the intercostals (to the intercostal muscles). The reflex stimuli 

 are conveyed by the pneumogastric nerve, and are twofold : by the 

 main trunk of the nerve the inspiratory efforts are excited, and by 

 the superior laryngeal branch expiratory reflexes are obtained. 

 Thus stimulation of the superior laryngeal nerve may be said to 

 inhibit the inspiratory centre' and to excite the expiratory centre, 

 while the contrary may be said of the main trunk of the nerve. 



How is the vagus excited to this reflex activity? 



While the respiratory centres seem to act automatically, yet in 

 certain conditions, in which oxygen is wanting in the blood, the 

 absence of oxygen (not the presence of carbonic acid) stimulates 

 the respiratory centres, through the pneumogastric filaments in the 

 lungs, to increased activity. This reflex activity results in forcible 

 respiratory efforts which we call dyspnoea. It is not known whether 

 this reflex arises from the absence of oxygen from the air of the 

 alveoli or from the blood of the pulmonary capillaries. 



What are the symptoms of lack of oxygen in the blood ? 



If the need be only moderate, there will be increased effort of 

 both expiration and inspiration, and the respirations will be rapid 

 a condition known as hyperpncea. As the oxygen becomes less and 

 less abundant the symptoms become more severe, and the condition 

 is known as dyspnoea. The dyspnoea increasing, the respiratory 

 efforts become very violent, and the condition of asphyxia is seen. 

 In this the face is blue, eyes starting, face anxious, and respirations 

 very rapid and strident. Then follows a convulsive condition which 

 is brief, the convulsions being very violent and involving the entire 

 body. After this the patient lapses into a state of exhaustion, in 

 which the respirations are slow, very feeble, and the general con- 

 dition is one of collapse. Death ensues very soon. 



How is air vitiated by lack of ventilation ? 



In ill-ventilated rooms the air of the room is used repeatedly, 

 and, besides becoming partially deprived of its oxygen, is charged 

 with carbonic acid and with putrescible nitrogenous organic mat- 

 ters. This causes an atmosphere which is intolerable to one who 

 enters from fresh air. That such a condition is unsanitary needs 

 no argument. 



