MECHANISM OF RESPIRATION IN MAN. 379 



attended with a decided elevation of the ribs, is usually made at every 

 fifth recurrence. The frequency of the respiratory movements, how- 

 ever, is liable to be greatly increased by various causes, such as violent 

 muscular exertion, mental emotion, or quickened circulation ; whilst it 

 may be diminished by the torpidity of the nervous centres, on whose 

 agency the movement depends, as we see in apoplexy, narcotic poison- 

 ing, &c. An acceleration seems very constantly to take place in dis- 

 eases, which unfit a part of the lung for the performance of its func- 

 tion ; and the rate bears a proportion to the amount thus thrown out of 

 use. Thus, the usual proportion between the respiratory movements and 

 the pulse being as 1 to 4J or 5, it may become in Pneumonia as 1 

 to 3, or even in severe cases 1 to 2 ; the increase in the number of 

 respiratory movements being much greater in proportion, than the aug- 

 mentation of the rate of the pulse. But it must be remembered by the 

 practitioner, that a simply hysterical state may produce, in young 

 females, an extraordinary acceleration of the respiration ; the number 

 of movements being sometimes no less than 100 per minute. There 

 will be a great increase, also, in the number of inspirations, when the 

 regular movements are prevented from being fully performed, by any 

 cause that affects their mechanism, even whilst the lungs themselves 

 are quite sound. Thus in inflammation of the pleura or pericardium, 

 or in rheumatic affections of the intercostal muscles, the full action of 

 the ribs is prevented by the pain which the movements produce ; and 

 the same is the case in regard to the diaphragm, when the peritoneum 

 or the abdominal viscera are affected with inflammation. Under such 

 circumstances, there is an involuntary tendency to make up for the de- 

 ficiency in the amount of the respiratory movements, by an increase in 

 their number. 



684. The combined actions of the respiratory muscles, which have 

 been now explained, belong to the group termed reflex ; being the result 

 of the operation of a certain part of the nervous centres, which does 

 not involve the will, or even sensation, and which may continue when 

 all the other parts of the nervous centres have been removed. In the 

 Invertebrated Animals, we commonly find a distinct ganglionic centre 

 set apart for the performance of the respiratory movements ; and the 

 division of the nervous centres in Vertebrated animals, which is the 

 seat of the same function, may be clearly marked out, although it is not 

 so isolated from the rest. It is, in fact, that segment of the Medulla 

 Oblongata and upper part of the Spinal Cord, which is connected with 

 the 5th, Yth, and 8th pairs of cephalic nerves, and with the phrenic. 

 The entire brain may be removed from above (by successive slicing), 

 and the whole spinal cord may be destroyed below ; and yet the respi- 

 ratory movements of the diaphragm will still continue, those of the 

 intercostal and other muscles being of course suspended, by the destruc- 

 tion of that portion of the cord from which their nerves arise. But if 

 the spinal cord be divided, between the point at which it receives the 

 5th and 8th pairs of nerves, and that at which it gives origin to the 

 phrenic, the movements of the diaphragm immediately cease ; and this 

 is the reason why deatfy is so instantaneous, in cases of luxation or frac- 

 ture of the higher cervical vertebrae, causing pressure upon the spinal 



