To 



442 PHYSIOLOGY CHAP. 



paralysed, including those of the diaphragm, and only the 

 rhythmical movements of the glottis and the muscles of the lips 

 and nose persist, which, as we have seen, not infrequently accom-^ 

 pany the rhythmical movements of the thorax. These muscles 

 are served by nerves, which emerge from the medulla oblongata ; 

 the muscles of the lips and nose receive branches from the facial 

 nerve, and the muscles of the larynx are innervated by branches 

 of the vagus (the crico-thyroid muscle from the superior laryngeal, 

 and the rest of the laryngeal muscles from the inferior laryngealjf 

 The vagus also provides the motor nerves to the smootn 

 muscles of the bronchial tubes. This""was lirst demonstrated by\ 

 LSnget"(i^42); it was subsequently contested, and finally confirmed 

 by the later experiments of Bert, Schiff, Gerlach and others. The 

 fact that stimulation of the peripheral trunk in certain kinds of 

 animals reduces the volume of the lung, which can only be due 

 to the contraction of the smooth muscles of the bronchi, is very 



FIG. 202. Effect of dividing phrenics in dog. (Lnciani.) To, intrathoracic pressure ; Ca, carotid 

 pressure. A-B, previous to section of phrenic ; B-C, after section. The tracing shows that 

 both inspiratory and expiratory movements are exaggerated after section. 



striking. Eoy, with Brown, and Sandmann claimed to have also 

 discovered dilator bronchial fibres in the vagus, the action of 

 which is expressed by pulmonary dilatation, when the peripheral 

 end of the vagus is excited with strong currents. It is not 

 improbable that the presence of these dilator fibres in many cases 

 weakens or nullifies the effect of the simultaneous excitation of 

 the constrictor fibres, which would account for the negative result 

 obtained by some observers. 



Division of the vagi in the horse causes a considerable increase 

 of volume in the lung, a proof that the constrictor fibres of the 

 bronchi in these animals are in constant or tonic excitation. In 

 dogs, on the contrary, section of the vagi produces a scarcely 

 perceptible dilatation of the lung, showing that there is only weak 

 tonic excitation. 



According to some interesting observations of Fano and Fasola, 

 the lungs of the marsh tortoise are capable of very extensive 

 active movements, due partly to the smooth muscle cells which 



