416 TEXT-BOOK OF PHYSIOLOGY. 



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 of any one of these nerves is followed by 

 paralysis of its related muscle. 



Inspiratory Center. 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 essential not only 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 purposes 

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

 of efficiency of the elastic forces. 



Expiratory Center. The simultaneous and coordinate activity of the 

 expiratory muscles in impeded expirations 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 discharged 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. ' ' 



The anatomic existence, however, of a definite group of cells which 

 initiates the respiratory movements has not as yet been demonstrated. 

 Nevertheless 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 de- 

 struction of which on one side is followed by a cessation of respiratory move- 

 ments 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 death of the animal. For this reason the term 

 "nceud vital" was applied to it. In this area the respiratory center was 

 located. It has, however, been shown by Gad that if this area be gradually 

 destroyed by cauterization the respiratory movements do not cease, but 

 continue until the cauterization has reached a point far forward in the 

 formatio reticularis, in which the respiratory center was assumed to lie. 



Though its existence has not been anatomically determined beyond 

 question, it is permissible to speak of the central mechanism as a "center" 

 located in the medulla oblongata. 



