520 A MANUAL OF VETERINARY PHYSIOLOGY 



sympathetic, from which results, in the horse and most other 

 animals, a single cord, which passes down the neck in company 

 with the carotid artery. As it enters the chest, the vagus 

 separates from the sympathetic, and the right and left trunks 

 proceed on their way. They both give off important branches 

 known as the ' recurrent laryngeals,' fibres to the heart, trachea, 

 bronchi, and oesophagus, and then each divides into two branches, 

 dorsal and ventral, which come together from opposite sides 

 and form the dorsal and ventral oesophageal branches. These 

 penetrate the diaphragm, after running above and below the 

 oesophagus. The dorsal branch, composed mainly of fibres 

 from the right vagus, passes to the cardia of the stomach, gives 

 off many fibres to that organ, and, continuing its course back- 

 wards, joins the anterior mesenteric ganglion (solar plexus) of 

 the sympathetic (Fig. 81) . The ventral branch of the oesophageal 

 nerve proceeds to the lesser curvature of the stomach, while fibres 

 pass on to the duodenum and liver. In this way the vagus 

 supplies fibres to the larynx, trachea, bronchi, lungs, heart, 

 oesophagus, stomach, duodenum, liver and, through the sympa- 

 thetic ganglion, fibres to the small and a portion of the large 

 intestines. In the chapters dealing with the Heart, Circulation, 

 Respiration, and Digestion, the special functions of this nerve 

 have received full consideration. Nevertheless, there are some 

 features of the vagus which may be conveniently dealt with 

 here. 



Through the jugular ganglion the vagus is brought into rela- 

 tion with the facial, glossopharyngeal and spinal accessory nerves, 

 and it is intimately connected, both in the neck, thorax, and 

 abdominal cavity, with the sympathetic system. In the neck 

 it gives off a pharyngeal branch, which forms a plexus with the 

 pharyngeal branch of the ninth pair ; and from this plexus 

 motor fibres proceed to the middle and posterior constrictor 

 muscles of the pharynx, and finally distribute themselves on the 

 cervical portion of the oesophagus, to which they furnish motor 

 power. The vagus next gives off the superior laryngeal, a mixed 

 nerve, which supplies acute sensation to the mucous membrane 

 of the epiglottis and larynx, and inhibitory fibres to the respiratory 

 centre. It is this nerve which is reflexly excited in the act of 

 coughing, and is capable of arresting inspiration, a very necessary 

 provision at the moment of swallowing. The motor fibres in 

 this nerve are the external laryngeal, given off before the main 

 trunk enters the larynx, and supplying motor power to the 

 crico-pharyngeus, crico-thyroid muscles, and in part to the 

 oesophagus. Anatomists are not agreed as to the innervation 

 of the crico-thyroid muscle in the horse.' Moeller regards its 

 motor supply as being derived from the first cervical pair ; 



