234 THE MUSCLES OF RESPIRATION. 



and that it returned when respiration commenced. A few muscular contractions 

 may occur towards the end of the pause (rare). 



With regard to the causes of this phenomenon there is some doubt. According to 

 Rosenbach, the anomalous nutrition of the brain causes certain intracranial centres, 

 especially the respiratory centre, to be less excitable and to be sooner exhausted, 

 and this condition reaches its maximum during the respiratory pause. During the 

 pause these centres recover, and they again become more active. As soon as they are 

 again exhausted, their activity ceases. Luciani also regards variations in the ex- 

 citability of the respiratory centre as the cause of the phenomenon, which 

 he compares with the periodic contraction of the heart (p. 104). He observed 

 this phenomenon after injury to the medulla oblongata above the respiratory 

 centre, and after apnoea produced in animals deeply narcotised with opium. It 

 also occurs in the last stages of asphyxia, during respiration in a closed space. 

 Mosso found a similar phenomenon normally in the hybernating dormouse 

 (Myoxus.) 



Periodic Respiration Of Frogs. If frogs be kept under water, or if the 

 aorta be clamped, after several hours, they become passive. If they be taken out 

 of the water, or if the clamp be removed from the aorta, they gradually recover 

 and always exhibit the Cheyne- Stokes' phenomenon. In such frogs the blood- 

 current may be arrested temporarily, while the phenomenon itself remains 

 (Sokolow and Luchsinger). If the blood-current be arrested by ligature of the 

 aorta, or if the frogs be bled, the respirations occur in groups. This is followed by 

 a few single respirations, and then the respiration ceases completely. During the 

 pause between the periods, mechanical stimulation of the skin causes the discharge 

 of a group of respirations (Siebert and Langendorff ). Muscarln and digUalin cause 

 periodic respiration in frogs [which is not due to the action of these drugs on the 

 heart.] 



112. General View of the Respiratory Muscles. 



(A.) Inspiration. 



I. During Ordinary Inspiration are Active. 



1. The diaphragm (Nervus phrenicus.) 



2. The Mm. levatores costarum longi et breves (Rami posteriores 

 Nn. dorsalium). 



3. The Mm. intercostales externi et intercartilaginei (Nn. inter- 

 costales). 



II. During Forced Respiration are Active. 



(a.) Muscles of the Trunk. 



1. The three Mm. scaleni (Rami musculares of the plexus cervicalis et 

 brachialis). 



2. M. sternocleidomastoideus (Ram. externus N. accessorii). 



3. M. trapezius (R. externus N. accessorii et Ram. musculares plexus 

 cervicalis). 



4. M. pectoralis minor (Nn. thoracici anteriores). 



5. M. serratus posticus superior (N. dorsalis scapulae). 



6. Mm. rhomboidei (N. dorsalis scapulae). 



