THE NERVOUS MECHANISM OF RESPIRATION. 369 



We cannot trace the oxygen through its sojourn in the tissue. We only 

 know that sooner or later it comes back combined in carbonic acid (and 

 other matters not now under consideration). Owing to the continual pro- 

 duction of carbonic acid, the pressure of that gas in the extra-vascular 

 elements of the tissue is always higher than that in the blood ; the gas 

 accordingly passes from the tissue into the blood, and the venous blood 

 passes on not only with its haemoglobin more or less reduced, i. e., with its 

 oxygen-pressure decreased, but also with its carbonic acid pressure increased. 

 Arrived at the lungs, the blood finds the pulmonary air at a lower carbonic 

 acid pressure than itself. The gas accordingly streams through the thin 

 vascular and alveolar walls until the pressure without the bloodvessel is 

 equal to the pressure within. At the same time the blood finds in the air of 

 the pulmonary alveoli a supply of oxygen, more than adequate to convert, 

 not entirely but nearly so, the reduced haemoglobin back again to oxy-hsemo- 

 globin. Thus the air of the pulmonary alveoli, having given up oxygen to 

 the blood and taken up carbonic acid from the blood, having in consequence 

 a higher carbonic acid pressure and a lower oxygen pressure than the tidal 

 air in the bronchial passages, mixes rapidly with this by diffusion. The 

 mixture is further assisted by ascending and descending currents ; and the 

 tidal air issues from the chest at the breathing out poorer in oxygen and 

 richer in carbonic acid than the tidal air which entered at the breathing in. 



THE NERVOUS MECHANISM OF RESPIRATION. 



304. Breathing is an involuntary act. Though the diaphragm and all 

 the other muscles employed in respiration are voluntary muscles, i. e., mus- 

 cles which can be called into action by a direct effort of the will, and though 

 respiration may be modified within very wide limits by the will, yet we 

 habitually breathe without the intervention of the will ; the normal breath- 

 ing may continue, not only in the absence of consciousness, but even after 

 the removal of all the parts of the brain above the medulla oblongata. 



We have already seen how complicated is even a simple respiratory act. 

 A very large number of muscles are called into play. Many of these are 

 very far apart from each other, such as the diaphragm and the nasal muscles ; 

 yet they act in harmonious sequence in point of time. If the lower inter- 

 costal muscles contracted before the scaleni, or if the diaphragm contracted 

 alternately with the other chest-muscles, the satisfactory entrance and exit of 

 air would be impossible. These muscles, moreover, are coordinated also in 

 respect of the amount of their several contractions ; a gentle and ordinary 

 contraction of the diaphragm is accompanied by gentle and ordinary con- 

 tractions of the intercostals, and these are preceded by gentle and ordinary 

 contractions of the scaleni. A forcible contraction of the scaleni, followed 

 by simply a gentle contraction of the intercostals, would perhaps hinder 

 rather than assist inspiration, and at all events would be waste of power. 

 Further, the whole complex inspiratory effort is often followed by a less 

 marked but still complex expiratory action. It is impossible that all these 

 so carefully coordinated muscular contractions should be brought about in any 

 other way than by coordinate nervous impulses descending along efferent 

 nerves from a coordinating nervous centre. By experiment we find this to 

 be the case. 



When in a rabbit the trunk of a phrenic nerve is cut, the diaphragm on 

 that side remains motionless, and respiration goes on without it. When both 

 nerves are cut, the whole diaphragm remains quiescent, though the costal 

 respiration becomes excessively labored. 



When an intercostal nerve is cut, no active respiratory movements are 



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