204 MECHANISM OF THE RESPIRATORY MOVEMENTS. 



in the entire bronchial tree is to offer resistance within the air-passages to the 

 increased pressure that occurs in all forced expirations, as in speaking, singing, 

 blowing, straining. According to the testimony of many investigators the vagus 

 is the motor nerve ; upon it depends the so-called pulmonary tone when the tension 

 within the air-passages is increased. Irritation of the vagus, or of the lung directly, 

 does not induce sudden, expiratory movements (as can be seen by fastening a 

 manometer in the trachea) . The only result of irritation of the vagus is an increase 

 in the resistance of the air passing through the small bronchi that have been nar- 

 rowed by the irritation. Section of the vagus also is said to increase the volume 

 of the lungs. Atropin paralyzes, pilocarpin stimulates, the bronchial muscles of 

 the dog, while reflex stimulation takes place through sensory branches of the 

 vagus. During deepest inspiration the unstriated muscles of the air-passages con- 

 tract, and during forced expiration they are relaxed. 



Pathological. Irritation of the unstriated muscles, causing spasmodic narrow- 

 ing of the smaller bronchi, may give rise to asthmatic attacks. If the escape of 

 air from the alveoli is thus made difficult or obstructed, an acute inflation of the 

 lungs acute emphysema may result. 



According to Sandmann a reflex effect may be produced upon the bronchial 

 muscles from the mucous membrane of the nose and the larynx. This would explain 

 the occurrence of asthma attending nasal affections, such as polypoid growths of 

 the mucous membrane. In addition to the elements of the connective, elastic, 

 and muscular tissues, and of the mucous membrane, the lungs contain lecithin, 

 inosite, uric acid (taurin and leucin in the ox; guanin (?), xanthin, hypoxanthin in 

 the dog), also sodium, potassium, calcium, magnesium, iron oxid, considerable 

 phosphoric acid, also chlorin, sulphuric acid, silicic acid, and carbon. In cases of 

 diabetes sugar has been found; in the presence of purulent infiltration glycogen 

 and sugar; in that of renal degeneration urea, oxalic acid, and ammonium-salts; 

 in that of autointoxications leucin and tyrosin. 



MECHANISM OF THE RESPIRATORY MOVEMENTS. 

 ABDOMINAL PRESSURE. 



The mechanism of breathing consists in an alternating dilatation and 

 contraction of the thoracic cavity. The dilatation of the cavity is termed 

 inspiration, and the narrowing expiration. The whole outer surface of 

 both elastic lungs is, by means of its smooth, moist covering of pleura, 

 intimately and hermetically applied to the inner surface of the chest- 

 wall, which in its turn is covered by the parietal pleura. Hence, it is 

 evident that every expansion of the thorax is accompanied by a corre- 

 sponding expansion of the lungs, and every contraction compresses 

 those organs. These movements of the lungs are, therefore, wholly 

 passive, being dependent on the thoracic movements. 



By reason of their complete elasticity the lungs are able to follow 

 every change in the capacity of the thorax, without causing the two 

 layers of the pleura ever to separate. The cavity of the unexpanded 

 thorax is greater than the volume of the collapsed lungs when removed 

 from the body; therefore, the lungs in their natural position within the 

 chest must be stretched, and they are, to a certain degree, in a state of 

 elastic tension. This tension varies directly with the size of the thoracic 

 cavity. If the pleural cavity be opened by a perforation from without 

 or by a wound of the lungs from within, the elasticity of the lungs causes 

 them to collapse, and there arises an air-space between the outer surface 

 of the lungs and the inner surface of the thorax (pneumothorax). The 

 affected lung is incapacitated for respiration. Double pneumothorax 

 is accordingly fatal. 



The degree of the elastic traction of the stretched lung may be measured by 

 introducing a manometer through an intercostal space into the pleural cavity of 

 a dead body. The elastic tension here is the same as that in the living body dur- 

 ing a state of quiet expiration, and is equal to 6 mm. of mercury. In a patient 



