324 RESPIRATION 



second to the seventh cervical roots; those to the levatores costarum and the 

 abdominal muscles by the thoracic nerves; those to the diaphragm chiefly by 

 the third and fourth cervical roots and the phrenic nerve. According to 

 Luschka and Cavalie, the edge of the diaphragm receives some fibers also 

 from the lowermost intercostal nerves. 



If the spinal cord be sectioned below the exit of the last intercostal nerves, 

 the operation evidently has no direct influence on the respiratory movements. 

 But if the section be made in the thoracic cord, those muscles whose nerves 

 emerge from the spinal cord below the section are paralyzed. After section 

 above the first intercostal nerves, for example, the movements of the 

 ribs, with the exception of those provided for by the scaleni muscles, cease 

 entirely and the animal now breathes only with the diaphragm and the 

 scaleni. 



With still higher section of the spinal cord all the muscles above named 

 are paralyzed and there remain only the movements of the glottis, the mouth 

 and the nose (Galen, Le Gallois, Flourens). 



When the diaphragm is paralyzed by bilateral section of the phrenics, vari- 

 ous disorders in respiration appear, especially in animals which breathe mainly 

 by the help of the diaphragm. These may be accounted for partly by the fact 

 that the rib-lifting muscles now have all the work to do, and partly by the fact 

 that since the diaphragm is now relaxed, the abdominal viscera are sucked into 

 the thorax with each inspiration. However, no real danger to life is occasioned, 

 if one is dealing with grown animals, which have a rigid chest wall and strong 

 muscles. Young animals die after bilateral section of the phrenics, because the 

 yielding chest wall and the immature muscles make it impossible to dilate the 

 chest, once it has become narrowed by paralysis of the diaphragm. 



Observations on men have shown that when all the muscles except the dia- 

 phragm are paralyzed, as well as when the diaphragm alone is paralyzed, life 

 may be still maintained. In the latter case the respiratory frequency becomes 

 greater than normal and breathing goes on, without any participation of the 

 accessory muscles, under the cooperation of the levatores, the intercostals and 

 the scaleni. Great bodily exertion, however, results in severe respiratory distress. 



The motor nerves for the muscles of the larynx and bronchi run in the 

 trunk of the vagus. Among the laryngeal muscles the cricothyroid is inner- 

 vated from the superior laryngeal, the others from the inferior laryngeal. 



It was asserted by Longet (1842) that the bronchial muscles also are 

 under the influence of the vagus. This statement was often disputed by later 

 authors, but it has been established by the newer, much improved technique 

 that the vagi do in fact produce contraction of the bronchial muscles, and, 

 especially in the cat, contain inhibitory fibers also for these muscles. 



The bronchial muscles of the dog are under weak tonic stimulation, those 

 of the horse under a strong tonus; they are influenced, feebly as a rule, 

 by various afferent nerves, both contraction and relaxation appearing as the 

 results of stimulation. The most important broncho-constrictor reflexes ap- 

 pear to be started from the mucous membrane of the respiratory passages 

 (nose, larynx), and may possibly be regarded as protective reflexes, for the 

 narrower the bronchi become the more likely is the dust of the air to adhere 

 to their walls. 



