206 ANIMAL PHYSIOLOGY. 



151. Functions of the Encephalon. The medulla 

 oblongata is principally remarkable for its connection with 

 respiration. Respiration is a reflex act in which, a stimulus 

 is apparently furnished by the unaerated blood, an impression 

 is conveyed to a nervous centre, and an impulse proceeds 

 thence, producing a co-ordinated movement of the muscles of 

 the chest. No doubt these movements are capable of con- 

 siderable control by the will, but they are continued in con- 

 ditions of unconsciousness; and, although by an effort they 

 may be delayed for a moment, the impulse soon becomes 

 imperative, and breaks through all restraint. The centre 

 engaged in this reflex action is the medulla oblongata; and 

 that part of the brain is, therefore, of the utmost importance 

 for the continuance of life. The whole of the rest of the 

 brain may be gradually removed without any interference 

 with respiration; also the spinal cord may be divided at 

 different levels; and only when the section is made high up 

 in the neck, above the origin of the (phrenic) nerves which 

 supply the diaphragm, is respiration materially affected; but 

 when that portion of the medulla oblongata is removed from 

 which the vagus nerves take origin, respiration ceases at 

 once, and the animal dies. This does not arise from mere 

 interference with the functions of the vagus ; for that pair of 

 nerves may be divided, and respiration continues, although, 

 no doubt, the entrance of air into the windpipe is interfered 

 with by paralysis of the larynx, and even when that incon- 

 venience is remedied by an artificial opening into the trachea, 

 death results after a time from the irritation of foreign bodies 

 entering the lungs. The sudden death which follows removal 

 of the upper part of the medulla oblongata is, therefore, only 

 to be accounted for by that part being the centre from which 

 the respiratory movements receive their impulse. It is like- 

 wise the centre engaged in the act of swallowing, which, like 

 respiration, continues to be performed after removal of the 

 rest of the brain. 



Being the centre which governs respiration, the medulla 

 oblongata is likewise to be regarded as the centre engaged in 

 various spasmodic actions of an occasional kind. In cough- 

 ing, an irritation of the pneumogastric nerve excites first a 

 spasmodic closure of the glottis, and afterwards a convulsive 



