RESPIRATION 113 



the alveoli and so continually keep the movements of the lungs delicately 

 in touch with the means of their ventilation. 



The respiratory center is a portion of the gray matter of the brain 

 which controls the actions of the respiratory mechanism. It was 

 discovered by Flourens. He observed that puncture or destruction in a 

 certain small spot in the medulla oblongata was followed immediately -by 

 cessation of respiration, which could not be recovered from. This 

 switch-board for incoming and outgoing respiratory nervous impulses is 

 now known to be bilateral, below the vaso-motor center, and near the 

 point of the calamus scriptorius in the floor of the fourth ventricle in 

 the medulla. It appears also that one part of this minute knot of 

 neurones is concerned in inspiration, and another part with expiration. 

 It is in close connection with the center of the vagus. Some authorities 

 doubt the precisely definite location which used to be described, and 

 say that we cannot locate it more closely than in the lower portion of 

 the medulla. 



This center is unique in the body so far as known, in that it is controlled 

 by certain definite substances present in the blood-stream which flows 

 through it and around its nerve-cells. This fact makes it in technical 

 terms an "automatic center." There has been much discussion as to 

 whether it is the lack of oxygen or the excess of carbon dioxide in the 

 blood which stimulates it. The fact seems to be that it may be both 

 of these; but the excess of carbon dioxide stimulates it much more 

 actively it is probable than does a deficiency of oxygen. Pfltiger has 

 suggested that some easily oxidized substance may be the actual 

 stimulant, this being usually absent from well-oxygenated blood. Recent 

 work by Plavec and by Laulanie makes it still more probable that the 

 immediate stimulant is carbon dioxide. The center has an intimate 

 association with about all the other important nuclei in the medulla. 

 One sees this readily in the great sensitivity of respiration to almost all 

 conditions, emotional, morbid, etc. 



The efferent nerves of respiration are chiefly the phrenics, certain of 

 the intercostals, and the vagus. Of these, the phrenics are perhaps most 

 essential because they actuate the diaphragm, which is the essential 

 respiratory motor organ. 



The Process and the Mechanism of Internal Respiration. The means 

 by which internal respiration is carried on, the essential portion of the 

 whole process, is in part physical without being mechanical, and in part 

 the mechanism of another functional system namely, the circulation. 

 Internal respiration is the interchange of the two respiratory gases 

 between the blood and the tissue-protoplasm. The mechanism of the 

 blood's movement is one part of this process and the passage through 

 the animal membranes intervening between the interior of a capillary 

 and the interior of a tissue-cell is the other part. The circulatory 

 mechanism will be described in a chapter by itself. (See page 278.) 

 First, then, we must here inquire by exactly what carrying agents the 

 oxygen and the carbon dioxide are conveyed between the alveoli and the 

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