RESPIRATION 181 



holds up the lower jaw. The masseter goes off duty for a 

 moment, allowing the jaw to fall. A reflex contraction of the 

 muscles which open the mouth immediately follows. Muscles 

 of the neck and head also come into play. Not improbably 

 the yawn ends in a general stretch. If the origin of this reflex 

 is obscure, its usefulness is marked. The circulation is 

 quickened, the blood is changed, nervous system and muscles 

 again become alert. 



" Apncea " is the condition of arrested respiration. If a man 

 about to dive into the water breathe deeply and rapidly half a 

 dozen times, he abolishes for a while the desire to breathe. 

 One is naturally inclined to explain this as due to a surplus of 

 oxygen taken into the blood, but a moment's reflection shows 

 that this cannot be the cause. In the first place, as we have 

 already pointed out, the blood which leaves the lungs in tran- 

 quil respiration is very nearly saturated with oxygen. It can 

 take up but little more. Again, the deep inspirations do not 

 change the air in the air-chambers ; time is required for 

 the renewal by diffusion of their gaseous contents. It is 

 improbable that the constitution of the air in the alveoli is 

 sensibly altered by a few deep breaths. Probably the explana- 

 tion is to be found in the effect upon the nerve-centre of dis- 

 tention of the chest. Stretching the nerve-endings of the 

 vagus in the lungs inhibits inspiration. If the stimulation be 

 excessive, inspiration is inhibited for a considerable time. 

 That this is the right theory of apncea is proved by repeatedly 

 inflating the lungs of an anaesthetized animal with a pair of 

 bellows. The same arrest of inspiration is induced whether 

 the lungs are inflated with air or with a neutral gas, such as 

 nitrogen, so long as the vagus nerve is intact. If this be cut, 

 inflation with a neutral gas no longer produces apncea. 



" Dyspnoea " is the term applied to the complex conditions 

 and movements which result from deficient aeration of the 

 blood, or, rather, from the distribution of insufficiently aerated 

 blood to the centres in the medulla oblongata. The blood of 

 the rest of the body may be in a satisfactory condition, but if, 

 owing to ligature of the carotid and vertebral arteries or other 

 causes, the blood supplied to the brain be inadequate to its 

 proper nutrition, the phenomena of dyspnoea are as marked 

 as they are when air is prevented from entering the lungs. 



