RESPIRATION. 599 



nervous system. It lias been pointed out that the mechanical operations 

 of respiration require the associated action of a large number of different 

 muscles, different in function and in locality. It has been mentioned 

 that during respiration muscular movements occur in the face, neck, 

 thorax, and abdomen, and it is found that the character of the contrac- 

 tion of these muscles is the same in each group. Thus, in gentle inspi- 

 ration we notice a gentle contraction of the diaphragm, of the scalene 

 muscles, the external intercostals, and the elevators of the ribs. In 

 gentle expiration we notice but a gentle contraction of the abdominal 

 muscles. The degree of contraction of each of these muscles in such a 

 respiratory movement is identical. When any one muscle or group of 

 muscles is the seat of an especially violent contraction, the normal 

 rhythm of respiration is departed from. Again, in forced expiration the 

 same state of affairs holds; the degree of contraction of each muscle is 

 proportionate to that of all the other muscles concerned in producing 

 the respiratory movement. These facts indicate, as in the case of other 

 complex associated muscular movements, the domination of some part 

 of the central nervous system. In the case of respiration such a state 

 of affairs will also be noted. In other words, the movements of respira- 

 tion are controlled by a single co-ordinating centre, located in the me- 

 dulla oblongata. The proof of this statement may be reached in quite a 

 number of wa3^s. 



If one of the phrenic nerves is divided, the corresponding half of the 

 diaphragm is paralyzed, and, although motions of inspiration still are 

 possible, it will be noticed that the excursions of the non-paralyzed half 

 of the diaphragm will be greater than when the phrenic nerve has not 

 been cut, and that the share in inspiration borne by the other respiratory 

 muscles will be more marked than is normally the case. 



Section of both phrenic nerves emphasizes this point still further ; 

 the diaphragm is then completely paralyzed, but inspiration is accom- 

 plished solely by the muscles which elevate the ribs. 



Again, if one or more of the intercostal nerves be divided the mus- 

 cles supplied by those nerves will be paralyzed, and the act of inspira- 

 tion will then be pi-oduced by calling in the aid of an increased degree of 

 contraction of other inspiratory muscles. 



If the spinal cord be divided below the level of the seventh cervical 

 nerve, consequently below the origin of the phrenic nerves, all thoracic 

 and abdominal movement will cease, but respiration will still be possible, 

 and inspiration will be accomplished solely by the contraction of the 

 diaphragm, while expiration will be dependent entirely upon the recoil 

 of the elastic tissue of the lungs and thorax. In this state of affairs the 

 transmission of impulses to the muscles of respiration, with the excep- 

 tion of the diaphragm, has been interrupted. The thorax will then be 



