RESPIRATION: THE MECHANISM OF BREATHING 363 



Influence of the Respiration on the Lymph-Flow. During 

 inspiration, when intrathoracic pressure is lowered, lymph is 

 pressed into the thoracic duct from the abdominal lymphatics. 

 In expiration, when thoracic pressure rises again, the extra lymph 

 cannot flow back on account of the valves in the lymphatic ves- 

 sels, and it is consequently driven on to the cervical ending of the 

 thoracic duct. The breathing movements thus pump the lymph 

 on. 



The Respiratory Center. The respiratory movements are to a 

 certain extent under the control of the will ; we can breathe faster 

 or slower, shallower or more deeply, as we wish, and can also " hold 

 the breath " for some time but the voluntary control thus exerted 

 is limited in extent; no one can commit suicide by holding his 

 breath. In ordinary quiet breathing the movements are quite in- 

 voluntary; they go on perfectly without the least attention on our 

 part, and, not only in sleep, but during the unconsciousness of 

 fainting or of an apoplectic fit. The natural breathing movements 

 arc therefore either reflex or automatic. 



The muscles concerned in producing the changes in the chest 

 which lead to the entry or exit of air are of the ordinary striped 

 kind; and these, as we have seen, only contract in the Body under 

 the influence of the nerves going to them; the nerves of the dia- 

 phragm are the two phrenic nerves, one for each side of it ; the ex- 

 ternal intercostal muscles are supplied by certain branches of the 

 thoracic spinal nerves, called the intercostal nerves. If the phrenic 

 nerves be cut the diaphragm ceases its contractions, and a similar 

 paralysis of the external intercostals follows section of the inter- 

 costal nerves. 



Since the inspiratory muscles only act when stimulated by 

 nervous impulses reaching them, we have next to seek where these 

 impulses originate; and experiment shows that it is in the medulla 

 oblongata. All the brain of a cat or a rabbit in front of the medulla 

 can be removed, and it will still go on breathing; and children are 

 sometimes born with the medulla oblongata only, the rest of the 

 brain being undeveloped, and yet they breathe for a time. If, on 

 the other hand, the spinal cord be divided immediately below the 

 medulla of an animal, all breathing movements of the chest cease 

 at once. We conclude, therefore, that the nervous impulses calling 

 forth contractions of the respiratory muscles arise in the medulla 



