506 SPECIAL PHYSIOLOGY. 



voluntary movement, or at least one which, by trifling practice, may 

 be voluntarily performed. The second stage is, however, wholly in- 

 voluntary and automatic, and is performed through the intervention of 

 a reflex action, though it may be partly imitated by the will. No 

 sooner has the food reached a certain part of the fauces, than it ex- 

 cites afferent nerves distributed to that part, the impressions on the 

 fibres of which, being conveyed to a certain nervous centre, are re- 

 flected through efferent fibres of other nerves, to the various and nu- 

 merous muscles required to contract ; and, by the simultaneous action 

 of these, this stage of deglutition is rapidly performed. Whilst, then, 

 the first stage, which involves no obstacle to respiration through the 

 nose and pharynx, is voluntary and deliberate, the second stage, 

 during which respiration must be suspended, is involuntary and rapid, 

 and, moreover, is not intrusted to movements requiring practice, habit, 

 or attention, to insure their perfect co-operation, but is performed as 

 promptly, efficiently, and safely, the first time by the new-born infant, 

 as at any after period of life. The accidental passage of food or drink 

 into the air-passages, with its accompanying inconveniences, inciden- 

 tally proves the advantage of the perfect performance of this movement. 

 The afferent nerves concerned in this important reflex act, are those 

 supplying the mucous membrane of the fauces arid neighboring parts 

 of the pharynx, viz., the palatal branches of the fifth pair, and, chiefly, 

 the pharyngeal branches of the glosso-pharyngeal and pneumogastric 

 nerves; the efferent or motor fibres are contained, some in the former, 

 but mostly in the latter nerves, being, however, derived partly from 

 the spinal accessory nerves (p. 267). Some also belong to the hypo- 

 glossal, which governs the movements of the tongue, and certain muscles 

 of the neck; to the facial nerve, which supplies the digastric and 

 stylo-hyoid muscles; and perhaps a few to the cervical spinal nerves. 

 The reflex nervous centre is situated in the medulla oblongata, and 

 upper part of the spinal cord. The third stage of deglutition is also 

 entirely involuntary, and chiefly, if not wholly, reflex. The afferent 

 fibres concerned, are contained in the cesophageal branches of the 

 pneumogastric nerves, and the efferent fibres are included in the same 

 branches, derived partly, however, from the spinal accessory nerves. 

 It is supposed by many, that the non-striated muscular fibres of the 

 oesophagus, may be directly stimulated by the substances swallowed, 

 without the intervention of any reflex nervous action. 



Movements of the Stomach. 



The stomach, Figs. 13, 89, s, the dilated part of the alimentary canal, 

 into which the oesophagus opens above, and from which the small 

 intestine leads below, is a musculo-membranous bag, of a peculiar shape, 

 extending across the abdominal cavity, from left to right, in front of 

 the vertebral column, just below the diaphragm and liver, immediately 

 behind the anterior wall of the abdomen, and above the transverse 

 colon. It is somewhat pear-shaped, the wider end, fundus or cardiac 

 end, Fig. 89, 0, being turned to the left side, and the smaller or pyloric 

 end, p, which ends in the small intestine, being turned to the right 



