552 THE MECHANICAL PROCESSES OF DIGESTION [CH. XXXYII. 



stimulatingly on the circular muscular fibres behind it, and inhibit- 

 ingly on those in front ; the contraction therefore squeezes it into the 

 dilated portion of the tube in front, where the same process 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. 



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 uphill, and 

 the same feat can be performed by jugglers. 



In swallowing liquids the two mylo-hyoid muscles form a 

 diaphragm below the anterior part of the mouth. The stylo-glossi 

 draw the tongue backwards and elevate its base ; the two hyo-glossi 

 act with these, pulling the tongue backwards and downwards. The 

 action of these muscles resembles that of a force-pump projecting the 

 mass of fluid down into the oesophagus ; it reaches the cardiac orifice 

 with great speed, and the pharyngeal and cesophageal muscles do not 

 contract on it at all, but are inhibited during the passage of the fluid 

 through them (Kronecker). 



This is proved in a striking way in cases of poisoning by corro- 

 sive substances, such as oil of vitriol; the mouth and tongue are 

 scarred and burnt, but the pharynx and oesophagus escape serious 

 injury, so rapidly does the fluid pass along them; the cardiac 

 orifice of the stomach is the next place to show the effects of the 

 corrosive. Kronecker's view has also been confirmed in man by 

 the X-ray method. 



There is, however, no hard-and-fast line between the swallowing 

 of solids and fluids : the more liquid the food is, the more does the 

 force-pump action just described manifest itself. 



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 



