RESPIRATORY SYSTEM 



21 I 



of the vagus supplies in man apes, dogs, cats and rabbits the 

 cricothyroid muscle which opens the vocal cords and raises 

 the larynx (see Figs. 110 and in). All the other muscles of 

 the larynx are innervated by the recurrent laryngeal which 

 sends sensory fibres to the mucous membrane of the larynx, 

 trachea, bronchi and lungs. 



The action of the vagus nerves is not to stimulate respiratory 

 movements, only to inhibit and control them. There is a spot 

 in the floor of the fourth ventricle in the medulla oblongata 

 upon destruction of which respiration immediately stops. 



Respiratory movements are 

 excited simply by the CO 2 con- 

 tent of the blood ; all other im- 

 pulses have only a regulating 

 effect. The respiratory centre 

 is being constantly stimulated 

 by the CO 2 in the blood : hence 

 the act of inspiration. Expan- 

 sion of the lung is associated 

 with some stimulation of the 

 pulmonary terminations of the 

 vagus which checks inspira- 

 tion ; expiration then follows 

 passively. As the lung con- 

 tracts, the stimulus is with- 

 drawn from the vagus endings, 

 inhibition spontaneously ceases 

 and the act of inspiration starts 

 anew (auto-regulation of respiration by the vagus). 



In forced breathing the sterno-mastoid and posterior portion 

 of the trapezius muscles (both supplied by the eleventh, or spinal 

 accessory, nerve) come to the aid of the breast and rib muscles. 



Respiration not only provides the body tissues with the 

 oxygen necessary to their life but by means of the oxidation 

 processes is responsible for the vital heat. Even in cold- 

 blooded creatures the body heat, as the result of these pro- 

 cesses, always remains slightly higher than the surrounding 

 air or water, if only by the tenth part of a degree. The 

 temperature of such creatures rises and falls with the medium 



H* 



FIG. 109. Human respiratory move- 

 ments. (Munk - Schultz.) a, in 

 man ; b, in woman. 



