^84 RESPIRATION. 



of the heart in such cases is not less arterialized, but, if anything, more 

 arterialized than usual. The increased respiratory movements induced by 

 the changed blood soon prove sufficient, or even more than sufficient, to give 

 the blood the extra quantity of oxygen and to remove the extra quantity of 

 carbonic acid. Obviously the blood corning from the tetanized muscles 

 affects the respiratory centre by virtue of some quality which, unlike that 

 due to the deficiency of oxygen or excess of carbonic acid, is not imme- 

 diately affected by the passage through the lungs. Whether the quality in 

 question be dependent on an excess of sarcolactic acid, or on some other 

 product or products of muscular metabolism, we do not as yet know. But 

 the fact that substances in the blood may so affect the respiratory centre is 

 interesting, since it shows by how many safeguards the working of the re- 

 spiratory centre is carefully adapted to the needs of the economy. 



Thus a change in the circumstances surrounding an animal body, or a 

 change in the body itself, may, in one or more of several ways by acting as 

 a stimulus to some afferent nerves and so sending up afferent nervous im- 

 pulses to the respiratory centre, or by interfering with the interchange of 

 gases in the lungs, or by otherwise altering the proportion of the gases pres- 

 ent in the blood reaching the respiratory centre, or by generating or increas- 

 ing in that blood some substance or substances tending to affect the nutrition 

 of the respiratory centre affect the working of the all-important breathing 

 mechanism. And the affection so wrought has generally an adaptative cha- 

 racter; it generally tends to protect the organism against the evil effects of 

 the change. 



317. Apncea, When we attempt to hold our breath, we find that we 

 can do this for a limited time only ; sooner or later a breath must come ; but, 

 as is well known, the time during which we can remain without breathing 

 may on occasion be much prolonged, if we first of all take a series of deep 

 breaths. It is probable, though perhaps not distinctly proved, that when 

 we breathe voluntarily, or when by an act of the will we hold the respiratory 

 apparatus in any one respiratory phase, the nervous impulses, generated by 

 the will, do not pass down by a direct and independent course to the respira- 

 tory muscles, but that the will makes use or modifies the activity of the 

 medullary and spinal nervous respiratory mechanisms. The breath sooner 

 or later inevitably follows because at last the natural impulses proceeding 

 from the respiratory centre become too imperious to be any longer held in 

 check by the impulses of volition passing down to the centre from the brain. 

 The fact that a series of deep breaths, a thorough ventilation of the lungs, 

 postpones the victory of the unconscious centre, shows that such a ventila- 

 tion in some way delays the development of the natural respiratory impulses. 

 A similar but still more marked delay may often be seen in an animal 

 under artificial respiration. If in a rabbit artificial respiration is carried on 

 very vigorously for a while, and then suddenly stopped, the animal does not 

 immediately begin to breathe. For a variable period no respiratory move- 

 ments at all take place, and breathing when it does begin occurs gently and 

 normally, only passing into dyspnoea if the animal is unable to breathe of 

 itself; a'nd even then the transition is quite gradual. Evidently during this 

 period the respiratory centre is in a state of complete rest, no explosions are 

 taking place, no respiratory impulses are being generated, and the quiet 

 transition from this condition to that of normal respiration shows that the 

 subsequent generation of impulses is attended by no great disturbance. Not 

 only is the centre at rest, but it is less irritable than the normal ; impulses 

 along t\ie vagus or other nerves which otherwise would produce respiratory 

 explosions are now ineffectual. This state of things is known as that of 

 apncea, the converse of dyspnoea ; and the longer pause in breathing men- 



