CHAP, ii.] RESPIRATION. G03 



left side of the he-art in such cases, is not less arterial izcd 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 coming 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 immediately 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 shews by how many safeguards the working of the respiratory 

 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 impulses to the respiratory centre, 

 or by interfering with the interchange of gases in the lungs, or by 

 otherwise altering the proportion of the gases present in the blood 

 reaching the respiratory centre, or by generating or increasing 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 character, it generally tends to protect 

 the organism against the evil effects of the change. 



374. 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 respiratory muscles, but that the will 

 makes use or modifies the activity of the medullary and spinal 

 nervous respiratory mechanisms. The breath sooner or later in- 

 evitably 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, shews that such a ventilation 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 



