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TEXT-BOOK OF PHYSIOLOGY. 



the floor of the fourth ventricle; the diaphragm and intercostal 

 muscles involve respectively the activity of the phrenic and inter- 

 costal nerves, the centers of origin of which lie in the anterior horn 

 of the gray matter of the spinal cord at a level, for the phrenic, of the 

 fourth, fifth, and sixth cervical nerves, and for the intercostals at the 

 level of the upper thoracic nerves. Division ofany one of these nerves 

 is followed by paralysis of its related muscle^ 



The coordinate contraction of the inspiratory muscles implies 

 a practically simultaneous discharge of nerve impulses from each of 

 the foregoing nerve-centers, accurately graduated in intensity in 

 accordance with inspiratory needs. This has been supposed to 

 necessitate the existence in the central nerve system of a single 

 group of nerve-cells from which nerve impulses are rhythmically 

 discharged and conducted to the previously mentioned nerve-centers 

 in the medulla oblongata and spinal cord, by which they are in 

 turn excited to activity. To this group of cells the term " inspiratory 

 center" has been given. For the free exit of air from the lungs it is 

 not only essential that the air-passages be open, but that the air in 

 the lungs be compressed until its pressure rises above that of the 

 atmosphere. This is accomplished by the recoil of the elastic tissue 

 of the lungs and thorax, the return of the displaced abdominal organs 

 aided by atmospheric pressure, and the contraction of the expiratory 

 muscles. In how far muscle action is necessary for expiratory pur- 

 poses will depend on the resistance offered to the outflow of air and 

 on the degree of efficiency of the elastic forces. The simultaneous and 

 coordinate activity of the expiratory muscles also involves the action 

 of motor nerves and nerve- centers. The simultaneous and coordinate 

 discharge of nerve impulses, also graduated in intensity for expiratory 

 needs, apparently implies the existence in the central nerve system 

 of a single center from which nerve impulses are rhythmically dis- 

 charged which excite and coordinate the lower nerve-centers. To 

 this group of cells the term "expiratory center" has been given. 

 The two centers taken together constitute the so-called " respiratory 

 center. " 



XThe ^existence, however, of a definite group of cells which. initiates 

 the respiratory movements has not as yet been demonstrated. 1 *^ Never- 

 theless there is in the dorsal portion of the medulla oblongata, at the 

 level of the sensory end-nucleus of the vagus nerve, a region the sudden 

 destruction of which on one side is followed by a cessation of respira- 

 tory movements on the corresponding side, though they continue on 

 the opposite side, a fact which indicates that the area, though acting 

 as a unit, is bilateral >^The bilateral character of the area is also 

 shown by the continuance of the respiratory movements on both sides 

 after longitudinal division of the medulla. Destruction of the entire 

 region is followed by a complete cessation of respiratory activity and 



