THE MEDULLA OBLONGATA. 507 



diameter, caused immediate stoppage of the respiration; while this 

 effect was not produced by similar injuries inflicted either above or 

 below. This spot, which contains the nervous centre of the movements 

 of respiration, corresponds in level, in front of the medulla oblongata, 

 with the upper end of the decussation of the anterior pyramids, or the 

 lower extremity of the olivary bodies, and is somewhat below the 

 apparent origin of the pnenmogastric nerves. 



Respiration accordingly is an act consisting of various associated 

 movements, which have their nervous centre in the medulla oblongata. 

 The respiratory movements themselves are completel3 r involuntary in 

 character ; for although those of the chest and abdomen may be for a 

 short time increased in frequency, the surplus movements thus per- 

 formed are not necessary to respiration, and soon produce a fatigue which 

 prevents their continuance. Respiration goes on with its natural rhythm, 

 and entirely unaccompanied by fatigue, under the influence of the me- 

 dulla, from the first moment of birth and without any necessary con- 

 sciousness of its existence. If arrested by a voluntary effort, the 

 internal stimulus which prompts the movement grows gradually 

 stronger, until the will is no longer capable of resisting its demands. 

 As soon as the voluntary resistance is overcome or discontinued, the 

 respiratory movements recommence by the independent action of the 

 medulla oblongata. 



The action of the medulla in respiration is one of a reflex nature. 

 The impression by which it is called out has its origin in the partial 

 want of arterialization of the blood, and especially in the commencing 

 accumulation of carbonic acid in the lungs and in the tissues. In 

 normal respiration, this impression is sufficient to excite the reflex 

 action of the medulla without producing a conscious sensation ; and on 

 the renewal of the air in the lungs by inspiration, the impulse is satisfied, 

 the muscles relax, and expiration is accomplished by the passive col- 

 lapse of the lungs and thorax. In a few seconds the previous condition 

 recurs and the actions are repeated as before, causing in this way the 

 regularly alternating movements of inspiration and expiration. 



Since the acts of inspiration are performed partly by the diaphragm 

 and partly by the intercostal muscles, they will be differently modified 

 by injuries or lesions of the nervous system, according to the spot at 

 which they are situated. If the spinal cord be divided or compressed 

 in the lower cervical region, all the intercostal muscles are necessarily 

 paralyzed, and respiration is then performed only by the diaphragm. 

 If the phrenic nerve, on the other hand, be divided, the diaphragm alone 

 is paralyzed, and respiration is performed altogether by the rising and 

 falling of the ribs. If the injury inflicted upon the spinal cord be above 

 the origin of the third cervical nerve, both the phrenic and intercostal 

 nerves are paralyzed, and death takes place from suffocation. The at- 

 tempt at respiration, however, still continues in these cases, showing 

 itself by ineffectual inspiratory movements of the mouth and nostrils. 

 Finally, if the medulla itself be broken up at the situation of the 



