172 THE MUSCLES OF FORCED RESPIRATION. 



Causes. Luciani and Rosenbach regard variations in the excitability of the respiratory centre 

 as the cause of the phenomenon, which they compare with the periodic contraction of the heart 

 ( 58). The excitability of the respiratory centre is lowest during the pause. They observed this 

 phenomenon after injury to the medulla oblongata above the respiratory centre, and after apncea 

 produced in animals deeply narcotised with opium, and in the last stages of asphyxia, during 

 respiration in a closed space. During hybernation, this mode of respiration is normal in Myoxus, 

 the hedgehog, and the caiman. 



Periodic Respiration. 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). 



Action of Drugs. Muscarin, digitalin, curara, chloral, sulphuretted hydrogen, and the poison 

 of many infectious diseases (typhus, diphtheria, scarlet fever) may also cause periodic respiration 

 in frogs [which is not due to the action of these drugs on the heart]. 



Periodic respiration without any variation in the size of the individual respirations the so- 

 called "Biot's respiration" occurs normally during sleep. While the nervous system as it 

 were strives to rest, and thus forgets the respiration, the organism does not observe the short 

 pauses (Mosso). [There is a periodic increase or decrease in the depth of the respiration, 

 especially in old people and children, even to the extent of the respiration becoming "remit- 

 tent," or even " intermittent," for a period of 30 sec. during sleep. During periodic-respiration 

 the action of the several respiratory muscles does not coincide. As a rule, one respires more than 

 is required by the organism. Mosso calls this "luxus-respiration."] Periodic irregularities 

 in the respiration are often of reflex origin {Knoll). 



112. GENERAL VIEW OF THE RESPIRATORY MUSCLES. 



(A) Inspiration. 

 I. During Ordinary Inspiration. 



1. The diaphragm (Nervus phrenicus). 



2. The Mm. levatores costarum longi et breves {Rami posterior 'es Nn. dorsalium). 



3. The Mm. intercostales externi et intercartilaginei (Nn. inter costales). 



II. During Forced Respiration. 

 (a) Muscles of the Trunk. 

 1. The three Mm. scaleni {Rami rnusculares of the plexus cervicalis et brachialis). 

 '2. M. stemocleidomastoideus {Ram. extemus N. accessorii). 



3. M. trapezius {R. extemus N. accessorii et Ram. rnusculares plexus cervicalis). 



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



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



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



7. Mm. extensores columnar vertebralis {Ram. posteriores nervorum dorsalium) 

 [8. Mm. serratus anticus major (N. thoracicus longus). 1 1] 



(b) Muscles of the Larynx. 



1. M. sternohyoideus {Ram. descendens hypoglossi). 



2. M. sternothyreoideus {Ram. descendens hypoglossi). 



3. M. crieo-arytaenoideus posticus (N. laryngeus inferior vagi). 



4. M. thyreo-arytaenoideus (N. laryngeus inferior vagi). 



(c) Muscles of the Face. 



1. M. dilatator narium anterior et posterior (N. facialis). 



2. M. levator alae nasi (N. facialis). 



3. The dilators of the mouth and nares, during forced respiration, [ u gasping 

 for breath "] (N. facialis). 



{d) Muscles of the Pharynx. 



1. M. levator veli palatini (N. facialis). 



2. M. azygos uvulae (N. facialis). 



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



