DIGESTION. 245 



agus, and food accordingly accumulates in the tube. The second and 

 third parts of the act of deglutition are involuntary. 



Nerve Mechanism. The nerves engaged in the reflex act of degluti- 

 tion are: sensory, branches of the fifth cerebral supplying the soft pal- 

 ate; 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 masti- 

 cation; the facial, supplying the levator palati; the glosso-pharyngeal, 

 supplying the muscles of the pharynx; the vagus, supplying the muscles 

 of the larynx through the inferior laryngeal branch, and the hypoglossal, 

 the muscles of the tongue. The nerve-centre by which the muscles are 

 harmonized in their action, is situated in the medulla oblongata. In the 

 movements of the oesophagus, the ganglia contained in its walls, with 

 the pneumo-gastrics, are the nerve-structures chiefly concerned. 



It is important to note that the swallowing both of food and drink 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. 



THE STOMACH. 



In man and those Mammalia which are provided with a single stom- 

 ach, it consists of a dilatation of the alimentary canal placed between 



FIG. 184. Stomach of a sheep, ce, oesophagus; Ru, rumen; Ret, reticulum; Ps, psalterium, or 

 manyplies; A, abomasum; Z>u, duodenum; g, groove from oesophagus to psalterium. (Huxley.) 



and continuous with the oesophagus, which enters its larger or cardiac 

 end on the one hand, and the small intestine, which commences at its 

 narrowed end or pylorus, on the other. It varies in shape and size ac- 

 cording to its state of distention. 



The Ruminants (ox, sheep, deer, etc.) possess very complex stom- 

 achs; in most of them four distinct cavities are to be distinguished (Fig. 

 184). 



1. The Paunch or Rumen, a very large cavity which occupies the 

 cardiac end, and into which large quantities of food are in the first in 

 stance swallowed with little or no mastication. 2. The Reticulum, or 



