2 8o MECHANISM OF THE RESPIRATORY MOVEMENTS. 



partly to the drawing down of the lungs and trachea, in consequence of 

 the deepening of the thoracic cavity. The elevation during expiration 

 is due chiefly to the recovery of the lungs, partly to the contraction of 

 the thyro-hyoid muscles. 



2. Movements of the vocal cords. — During quiet breathing the rinia 

 glottidis is moderately open, and shows but slight movements. If, how- 

 ever, deep inspirations be made, the glottis is widely dilated with each 

 inspiration, by means of contraction of the posterior crico-aiytenoid 

 muscles. Even in quiet breathing, however, the muscles of the larynx 

 must be in a certain state of activity, since in total paralysis of all the 

 muscles the position of the glottis differs from that in the normal indi- 

 vidual. In the former case the vocal cords are placed rather obliquely, so 

 that their upper surface looks outwards and their lower inwards. The 

 effect of paralysis is to make the rinia into a valve-shaped opening, which 

 may close by the apposition of the two cords if any sudden inspiratory 

 effort be made. There will, however, be no impediment to expiration. 



3. Movements of aim nasi. — During each inspiration the nostril is 

 dilated by the action of the dilatator naris, contracting again during 

 expiration. This contraction is chiefly due to the elastic reaction of the 

 nasal cartilages, but may be assisted by the action of the compressor 

 naris (m. nasalis). The movements of the ala± nasi are more evident in 

 children than in adults, and become especially well marked in the 

 former when there is any obstruction to the entry of air into the lungs. 



Other accessory muscles of respiration are those which fix the head, 

 shoulder, spinal column, etc., and which only come into play during 

 laboured respiration. 



Different types of respiratory movements. — The amount to which 

 each diameter of the thorax is altered with respiration differs, not only 

 according to the depth of the respiratory movements, but also with the 

 sex. 1 



Thus in man, ordinary quiet respiration is effected almost entirely 

 by contraction of the diaphragm, so that the chief external sign is a 

 pushing out of the abdominal wall and the lower ribs. In woman, on 

 the other hand, inspiration is mainly carried out by means of the thorax, 

 so that the abdomen recedes with inspiration. 2 In laboured respiration 

 these differences almost disappear, the breathing being mainly thoracic 

 in both cases, and the abdomen recedes with each inspiration. Mosso 3 

 has shown that during sleep the respiration in the male sex is also 

 chiefly thoracic. 



Movements of the lungs. — Since the lungs in inspiration expand 

 all over, while the increase in the thoracic cavity is more marked in 

 certain diameters (especially the vertical), it follows that there must be 

 a movement of the lung with regard to the thoracic wall as it expands. 

 This movement of the lungs occurs in two directions from two fixed 

 points, being in the vertical direction from the apex towards the base of 

 the chest, and from the attachment of the lungs to the bronchi towards 

 the front and sides of the chest, and is rendered possible, as already 

 mentioned, by the existence of the pleural cavity. 



1 Cf. Hutchinson, loc. cit. 



- It is stated that the difference is mainly produced by the habit of wearing stays, and 

 that in women who have never adopted this habit, the ordinary respiration is abdominal 

 (Wilberforce Smith, Brit. Med. Jonm., London, 1890, vol. ii. p. 84:1). 



3 Arch. f. Physiol., Leipzig, 1878, S. 441. 



