212 MUSCULAR MECHANISM IN INSPIRATION AND EXPIRATION. 



aorta be clamped, they lose the power of reaction in a few hours. When taken 

 out of the water, or when the clamp is removed, they recover immediately, and 

 they invariably exhibit the phenomenon of Cheyne-Stokes' respiration. In such 

 frogs the circulation may be interrupted for a time, during which this form of 

 breathing still continues. 



Curtailment of the blood-supply in frogs by blood-letting results in periodically 

 grouped respirations. These are followed by a stage of single, infrequent respira- 

 tions, and finally the breathing stops completely. During the pauses between the 

 periods, each mechanical irritation of the skin will give rise to a series of respirations. 

 Periodic respiration, without variations in the depth of the separate respirations 

 (so-called Biot's respiration), also occurs normally in sleep. While the nervous 

 centers are endeavoring to obtain rest, they forget, to a certain extent, to send 

 out respiratory impulses, and the organism takes no notice of these short pauses. 

 Periodic irregularities in respiration also are frequently of reflex origin. Muscarin, 

 digitalin, curare, chloral, hydrogen sulphid, and the toxins of some infectious dis- 

 eases (typhoid fever, diphtheria, scarlet fever) are likewise capable of exciting 

 periodic respirations. 



SUMMARY OF THE MUSCULAR MECHANISM CONCERNED IN 

 INSPIRATION AND EXPIRATION. 



A. INSPIRATION. 



I. During quiet inspiration the following muscles are active: 



1. The diaphragm (phrenic nerve, from the third and fourth cervical nerves). 



2 . The external intercostal and intercartilaginous muscles (intercostal nerves) . 



3 . Long and short elevators of the ribs (posterior branches of the dorsal nerves) . 

 In a state of rest the elastic traction of the lungs appears to draw the chest 



together somewhat with tension on all sides. Accordingly the elastic force thus 

 exerted would act as an aid to the beginning of inspiration. Also Landerer con- 

 siders the thorax at rest to be an apparatus tending toward the attitude of inspira- 

 tion, by means of the elasticity in an upward direction of the six upper ribs. 



II. During forced inspiration the following muscles are active: 



(a) Trunk-muscles. 



1. The three scalene muscles (muscular branches of the cervical and brachial 

 plexuses) . 



2. Sterno-cleido-mastoid (external branch of the spinal accessory nerve). 



3. Trapezius (external branch of the spinal accessory, and muscular branches 

 of the cervical plexus) . 



4. Lesser pectoral (anterior thoracic nerves). 



5 . Posterior superior serratus (dorsal nerve of the scapulas) . 



6. Rhomboids (dorsal nerve of the scapulae) . 



7. Extensor muscles of the vertebral column (posterior branches of the dorsal 

 nerves). 



The assumption that the greater anterior serratus (long thoracic nerve) and the 

 subclavius (brachial plexus) are accessory muscles of inspiration is unwarranted. 



(b) Laryngeai muscles. 



1 . Sterno-hyoid (descending branch of the hypoglossus) . 



2 . Sterno-thyroid (descending branch of the hypoglossus) . 



3. Posterior crico-arytenoid (inferior laryngeal branch of the vagus). 



4. Thyro-arytenoid (inferior laryngeal nerve). 



(c) Facial muscles. 



1. Anterior and posterior dilators of the nares (facial nerve). 



2 . Levator of the ala nasi (facial nerve) . 



3. The muscles that separate the lips and open the mouth during extreme 

 forced respiration gasping (facial nerve). 



(d) Muscles of the palate and pharynx. 



1 . Elevator of the veil of the palate (facial nerve) . 



2. Azygos of the uvula (facial nerve). 



3. According to Garland, the pharynx is always narrowed. 



B. EXPIRATION. 



I. During quiet expiration the size of the thoracic cavity is reduced essentially 

 by the weight of the chest-walls, together with the elasticity of the lungs, costal 

 cartilages, and abdominal muscles. 



