CHAPTER XXXVII. 

 INNERVATION OF THE RESPIRATORY MOVEMENTS. 



The nervous supply to the respiratory muscles is received from 

 a number of nerves, the nervous machinery being widely dis- 

 tributed in the brain and cord. The most important of the motor 

 nerves of respiration is the phrenic, which supplies the diaphragm 

 and originates from the fourth and fifth cervical spinal nerves. 

 The spinal accessory and branches of the cervical and brachial 

 plexus innervate the muscles of the neck and shoulder which are 

 concerned in inspiration; the intercostals innervate the muscles of 

 the thorax and abdomen, while branches of the lumbar plexus send 

 fibers to the muscles of the groin. Moreover, the facial sends 

 motor branches to the muscles of the nose and the vagus supplies 

 the muscles of the larynx. All of these muscles belong to the 

 skeletal group and are under voluntary control. Under normal 

 conditions, however, this entire respiratory apparatus works 

 rhythmically without voluntary control, in alternate inspirations 

 and expirations, all the inspiratory muscles contracting together, 

 and all the expiratory muscles together in their turn when the 

 expirations are active. The co-ordinated activity of such an ex- 

 tensive mechanism is explained by the existence of a respiratory 

 center in the medulla oblongata. 



The Respiratory Center. The discovery of the location of the 

 respiratory center was due mainly to the experiments of two French 

 physiologists, Legallois and Flourens. The latter placed the 

 center in the medulla at the level of the calamus scriptorius, and 

 described it as a very small area or spot, which he designated at first 

 as the vital knot (nceud vital) under the mistaken impression that 

 it formed, as it were, a central or focal point of the motor system. 

 It has since been shown that this center, like the vasomotor 

 center, is bilateral. If the medulla is cut through in the mid- 

 line the respirations may proceed in a normal manner. The center 

 consists of two parts, each connected primarily with the muscula- 

 ture of its own side. Each half occupies an area that lies some 

 distance lateral to the mid-line and beneath the floor of the medulla 

 at the general level of the calamus. According to Gierke,* the area 

 extends in rabbits from a point 3 or 4 mms. in front of, to a point 



* Gierke, " Archiv f. die gesammte Physiologic," 7, 583, 1873; and "Cen- 

 tralblatt f. d. med. Wissenschaften, " No. 34, 1885. 



631 



