CH. XXXV.] DEGLUTITION 527 



swan. If we suppose the bolus to be at one particular place in the 

 tube, it acts stimulatingly on the circular muscular fibres behind it, 

 and inhibitingly on those in front ; the contraction therefore squeezes 

 it into the dilated portion of the tube in front, where the same pro- 

 cess is repeated, and this travels along the whole length of the tube. 

 The second and third parts of the act of deglutition are involuntary. 

 The action of these parts is more rapid than peristalsis usually is. 

 This is due to the large amount of striated muscular tissue 

 present. It serves the useful purpose of getting the bolus as quickly 

 as possible past the opening of the respiratory tract. 



Nervous Mechanism. The nerves engaged in the reflex act of 

 deglutition are : sensory, branches of the fifth cranial nerve supplying 

 the soft palate and tongue; glosso-pharyngeal, supplying the tongue 

 and pharynx ; the superior laryngeal branch of the vagus, supplying 

 the epiglottis and the glottis ; while the motor fibres concerned are : 

 branches of the fifth, supplying part of the digastric and mylo-hyoid 

 muscles, and the muscles of mastication; the bulbar part of the 

 spinal accessory through the pharyngeal plexus, supplying the levator 

 palati, probably by rootlets which are glosso-pharyngeal in origin ; the 

 glosso-pharyngeal and vagus, and possibly the bulbar part of the spinal 

 accessory, supplying the muscles of the pharynx through the phar- 

 yngeal plexus; the vagus, in virtue of its spinal accessory roots, 

 supplying the muscles of the larynx through the inferior laryngeal 

 branch ; and the hypo-glossal, the muscles of the tongue. The nerve- 

 centre by which the muscles are harmonised in their action, is situated 

 in the medulla oblongata. Stimulation of the vagi gives rise to peri- 

 stalsis of the oesophagus. The cell stations of these fibres are in the 

 ganglion trunci vagi. Division of both pneumogastric nerves gives 

 rise to paralysis of the oesophagus and stomach, and firm contraction 

 of the cardiac orifice. These nerves therefore normally supply the 

 oesophagus with motor, and the cardiac sphincter with inhibitory 

 fibres. If food is swallowed after these nerves are divided, it accumu- 

 lates in the gullet and never reaches the stomach. 



In discussing peristalsis on a previous occasion (p. 158), we 

 arrived at the conclusion that it is an inherent property of muscle 

 rather than of nerve; though normally it is controlled and influenced 

 by nervous agency. This nervous control is especially marked in the 

 oesophagus; for if that tube is divided across, leaving the nerve 

 branches intact, a wave of contraction will travel from one end to the 

 other across the cut. 



Swallowing of Fluids. The swallowing both of solids and 

 liquids is a muscular act, and can, therefore, take place in 

 opposition to the force of gravity. Thus, horses and many other 

 animals habitually drink up-hill, and the same feat can be 

 performed by jugglers. 



