356 THE RESPIRATORY SYSTEM 



extra capacity required in inspiration is obtained by the 

 diaphragm, which is aided by the accessory muscles. The 

 part played by the latter group is well shown when an 

 emphysematous patient has a fit of coughing. The upper 

 limbs and their girdles are fixed so that those muscles which 

 pass between them and the chest wall may act on the latter. 

 In this way, the pectoral muscles elevate the ribs and the 

 sternum, the serratus anterior elevates the ribs, and its 

 digitations of origin stand out in relief on the medial wall of 

 the axilla. The latissimus dorsi elevates the lower ribs and 

 draws them outwards so as to increase the capacity of the 

 abdomen and lessen the resistance against which the dia- 

 phragm has to act. In addition to the fixation of the upper 

 limbs, the head is kept fixed in the middle line to enable the 

 sterno-mastoids to act on the manubrium sterni, and the 

 scalene muscles to act on the upper two ribs. 



NERVE-SUPPLY OF THE LUNGS. The lungs receive their 

 nerve-supply from the anterior and posterior pulmonary 

 plexuses, which lie in relation to the root. The vagus and the 

 sympathetic trunk share in the formation of both plexuses, and 

 their branches accompany the vessels and bronchi into the 

 lungs. It seems probable that the circular muscular coat of 

 the smaller bronchi is innervated through the vagus. In 

 spasmodic asthma, the circular muscle fibres become tonically 

 contracted, producing profound respiratory embarrassment. 

 The condition may be initiated reflexly by the stimulation of 

 the gastric branches of the vagus nerves. 



It is a matter of common experience that large areas of lung 

 tissue may be destroyed by disease and yet the process is 

 perfectly painless. Unfortunately, no satisfactory anatomical 

 explanation can be offered, for it is not sufficient to state that 

 lung tissue is insensitive to pain stimuli, as it might reasonably 

 be expected that the pain would be referred to the cutaneous 

 distribution of the intercostal nerves, since the sympathetic 

 fibres which supply the lung have their centres in the thoracic 

 region of the spinal medulla. 



