Ttte Nervous System. 280 



produced. The inferior laryngeal gives motor branches to 

 the cesophagus and part of the pharynx, and exercises a 

 controlling or inhibitory action over the respiratory centre. 

 The cardiac branches of the vagus contain the inhibitory 

 fibres of the heart, so that division of the nerve causes 

 the heart to beat more quickly and with increased strength. 

 The depressor nerve in rabbits is given off by the superior 

 laryngeal ; it is an afferent nerve of the heart ; if irritated, 

 and one vagus be intact, it slackens the heart, but only 

 reflexly through the cardio-inhibitory centre ; further it 

 dilates the arteries, and causes a fall in blood pressure 

 (see p. 72). The pulmonary branches supply both sensory 

 and motor branches to the trachea, and motor fibres to the 

 bronchi ; they transmit to the medulla impressions which 

 stimulate the respiratory and vaso-motor centres, causing 

 through the latter a fall in blood pressure. Division of both 

 pneumogastrics causes the breathing to become deeper and 

 more prolonged, but does not produce a sense of suffocation ; 

 further, it causes the lungs to become gorged with blood, 

 and produces a low form of pneumonia. The oesophageal 

 branches supply the cardiac end of the cesophagus with 

 motor power, the cervical portion of the tube being supplied 

 by the pharyngeal branches ; division of them causes the 

 food to accumulate in the lower end of the tube. The 

 gastric plexus supplies branches to the oesophagus, stomach, 

 and liver. According to Colin's experiment on horses, 

 division of both vagi does not influence either the secretion 

 of gastric juice or the process of digestion, but owing to the 

 paralysis of the lower end of the cesophagus, food is likely 

 to accumulate in the stomach, cesophagus, and pharynx.* 



The functions of the vagus may be summarised as follows : 

 The vagus supplies (1) motor influences to the pharynx, 

 (esophagus, stomach and small intestines, the larynx, 

 trachea, bronchi, and lungs : (2) sensory and in part 

 (8) vaso-motor influences to the same regions; (4) in- 

 hibitory influence to the heart ; (5) inhibitory afferent 



* I have to acknowledge Chauveau's experiments on the Yagus of 

 the horse, published in the Edinburgh Veterinary Review, 1864. 



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