244 ANATOMY FOR NURSES [Chap. XIII 



(1) The fibres from the pneumogastric nerve are of two kinds : 

 (a) inspiratory fibres which tend to increase the rate of respiration, 

 and (6) expiratory fibres which check the action of the inspiratory 

 set. The inspiratory fibres are stimulated to action when the 

 lung collapses ; the expiratory when the lung expands. 



(2) The respiratory centre shows a specific irritabihty for carbon 

 dioxide, and an increased amount of carbon dioxide in the blood acts 

 as a stimulus, increasing the rate and depth of the respirations, so 

 that the lungs are more thoroughly ventilated. Increased activ- 

 ity, or any abnormal condition that increases the oxidation of the 

 tissues, naturally results in an increased production of carbon 

 dioxide, and an increase in the rate and depth of the respirations. 

 On the other hand, an excess of oxA'gen in the blood may cause a 

 condition known as physiological apncea, i.e. where the blood is 

 so rich in oxj'gen and poor in carbon dioxide that a respiratory 

 act is unnecessary. 



Reflex stimulation of the respiratory centre. — Every one must 

 have noticed that the respiratory movements are affected by 

 stimulation of the sensory nerves. Strong emotion, sudden pain, 

 or a dash of cold water on the skin produce changes in the rate of 

 the respirations. It is assumed, therefore, that the respiratory 

 centre is in connection with the sensory fibres of all the cranial 

 and spinal nerves. 



Cause of the first respiration. — The immediate cause of the 

 first respiratory effort is closely connected with the cause of the 

 activity of the respiratory centre during life. The stimulus is 

 supposed to come from (1) the increased amount of carbon 

 dioxide in the blood, due to the cutting of the cord ; and 

 (2) the sensory nerves of the skin, due to cooler air, handling, etc. 

 During intrauterine life the foetus receives its supply of oxj'gen 

 from the blood-vessels of the umbilical cord, which connect with 

 the placenta. The lungs are in a collapsed condition and contain 

 no air. The walls of the air-sacs are in close contact, and the walls 

 of the smaller bronchial tubes, or bronchioles, touch one another 

 When the chest ex-pands with the first breath taken, the inspired 

 air has to overcome the adhesions existing between the walls of the 

 bronchioles and air-sacs. The force of this first inspiratory effort, 

 spent in opening out and unfolding, as it were, the inner recesses 

 of the lungs, is considerable. In the succeeding expiration, most of 



