THE MUSCULAR AND NERVOUS MECHANISM OF 

 THE RESPIRATORY MOVEMENTS. 



By E. H. Starling. 



Contents. — Mechanical Conditions, p. 274 — Inspiration, p. 275— Diaphragm, p. 

 276— Intercostal Muscles, p. 277 — Expiration, p. 279 — Accessory Respiratory 

 Movements, p. 279 — Types of Respiration, p. 280 — Movements of the Lungs, 

 p. 280— Respiratory Rhythm, p. 281— Pressure in Lungs, p. 281— Methods of 

 Registering Movements,' p. 282— Nervous Mechanism, p. 283 — Respiratory 

 Centre, p. 284 — Spinal Centres, p. 286 — Automatism of Centres, p. 288— 

 Influence of Afferent Nerves, p. 292— Action of A T agus, p. 292 — Normal Move- 

 ments and Effects of Division of Vagi, p. 296 — Effects of Distension and Col- 

 lapse of Lung, p. 298 — Apnoea, p. 298 — Electrical Stimulation of Vagus, p. 301 

 — Influence of other Sensory Nerves, p. 304 — Influence of Brain, p. 305— 

 Asphyxia, p. 306 — Modified Respiratory Movements, p. 308. 



The Mechanical Conditions of Kespiration. 



Since the lungs are contained in a closed cavity, their outer surface 

 being in direct contact with the inner wall of the thorax, it is evident 

 that any changes in the capacity of the thorax must lead to corre- 

 sponding changes in the volume of the lung. In inspiration, the cavity 

 of the thorax is enlarged in all directions, in consequence of the action 

 of certain muscles. The lungs must therefore expand to take up the 

 increased space in the thorax, and air is drawn into them. The existence 

 of the pleural cavity insures the uniform expansion of the lung, even 

 although some parts of the thoracic wall move more freely than others. 

 The inspiratory enlargement of the lungs not only acts against gravity 

 in raising the ribs, but also stores up potential energy in consequence 

 of a stretching of the rib cartilages and of the elastic lungs. As soon, 

 therefore, as the muscles relax, these forces — gravity and elastic reaction 

 of thoracic wall and pulmonary tissue — come into play, bringing the 

 thorax back to its original size, and thus causing an expulsion of air 

 from the lungs — an expiration. On the other hand, the cavity of 

 the thorax can be diminished below that corresponding to its position 

 of rest by contraction of certain expiratory muscles. In this case, 

 relaxation of the muscles would be followed by a return to the normal 

 size of the thorax, and therefore by an inspiration. It is evident, 

 therefore, that respiration might be carried out in one of two ways — by 

 an active inspiratory effort, followed by a passive respiration due to 

 elastic reaction ; or an active expiratory effort, followed 1 >y an elastic 

 inspiratory reaction. We find that in all mammalia the former mode 

 of breathing is the normal one, inspiration being due to the contraction 

 of muscles, whereas expiration is largely passive. 



