380 RESPIRATION. 



as we have seen, the superior laryngeal, and, it is said, the splanchnic nerves, 

 the effects are exclusively, or, at least, chiefly inhibitory of inspiration and 

 augmentative of expiration, that is, expiratory, and in others, perhaps, chiefly 

 augmentative of inspiration, or inspiratory, in the case of most nerves the 

 effect may be, according to circumstances, either in the one direction or the 

 other. Perhaps, as a rule, weak stimuli tend to augment, and strong to in- 

 hibit inspiration ; but the effects of artificially stimulating sensory nerves 

 are complicated and often confused, because powerful afferent impulses by 

 giving rise to pain may, through impulses generated by the pain itself and 

 descending to the medulla from the brain, act in an indirect as well as in a 

 direct manner ; and the prominence of the indirect painful impulses will, in 

 any experiment, depend on the anaesthetic used. We may say, however, that in 

 all cases the effect is very largely determined by the condition at the time 

 being of the respiratory centre itself; and that, in turn, is determined not 

 only by things which affect its nutrition, such as the character of the blood 

 circulating in it, but also by the nature and amount of the other afferent 

 impulses which are playing upon it at the same time. Thus, as we shall pres- 

 ently see, the effect of a stimulus applied to the vagus, when the respiratory 

 centre is inadequately supplied with arterial blood, is not the same when the 

 centre has its normal supply of normal blood. So also a stimulus which, 

 applied to the vagus or to another nerve in an intact animal, simply 

 quickens inspiration, applied in an animal whose cerebral hemispheres 

 have been removed will call forth a prolonged tetanic inspiratory gasp. 

 The respiratory centre responds, in fact, in the most intricate and varied 

 manner to nervous impulses proceeding from all parts of the body, and 

 thus delicately adjusts the working of the respiratory pump to the needs 

 of the economy. 



312. The complicated nature of the respiratory centre is further shown 

 by the fact that it appears to consist of two lateral halves which normally 

 work in unison and yet may be made to work independently. If the me- 

 dulla oblongata be carefully divided in the middle line respiration may con- 

 tinue to go on in quite a normal fashion. If, however, one vagus be then 

 divided, the respiratory movements, both costal and diaphragmatic, on the 

 side of the body on which division of the vagus has taken place, become 

 slower than those on the other side, so that the two sides are no longer 

 synchronous ; and a stimulus confined to one vagus affects the respiratory 

 movements of that side of the body only. So, also, a section of a lateral 

 half of the cord below the medulla stops the respiratory movements on that 

 side alone. 



313. Besides these nervous influences, however, there is another cir- 

 cumstance which, perhaps, above all others affects the respiratory centre, and 

 that is the condition of the blood in respect to its respiratory changes ; the 

 more venous (less arterial) the blood, the greater is the activity of the respira- 

 tory centre. When by reason either of any hindrance to the entrance of air 

 into the chest or other interference with the due interchange between the 

 blood and the pulmonary air or of a greater respiratory activity of the tissues, 

 as during muscular exertion, the blood becomes less arterial, more venous, 

 i. e., with a smaller charge of oxygen and more heavily laden with carbonic 

 acid, the respiration from being normal becomes labored. We may speak of 

 normal breathing as eupncea, and say that this, when the blood is insufficiently 

 arterialized, passes into dyspnoea, an intermediate stage in which the respira- 

 tory movements are simply exaggerated being known as hyperpncea. The 

 modifications of breathing thus caused by deficient arterialization of blood 

 are especially characterized by an increase in the total energy of the respira- 

 tory impulses generated, and in this respect differ from the modifications 



