functions of the Medulla Oblongata. 611 



The reflex action of the eyelids in winking, when there is sudden ir- 

 ritation through the optic nerve and retina by an object coming close, 

 arises from the return stimulus along the ophthalmic branch of the 5th 

 pair. The contraction and dilation of the pupil is also automatic and 

 reflex through the optic nerve and the 3rd pair. The periodical wink- 

 ing which takes place with all several times a minute, is likewise reflex, 

 the stimulus probably being a result of the evaporation of the fluid 

 upon the eyeball. These movements may all take place consciously, 

 that is, when the attention is not occupied with stimuli of greater force. 

 But ordinarily they go on without arousing sensation, and they cannot 

 long be restrained or controlled by those other stimuli which constitute 

 the will. It is shown in certain cases of disease of the optic nerve cen- 

 ters, that while the possibility of a sensation of light is destroyed, yet 

 the pupil contracts through the stimulus of light upon the retina. 

 Sneezing is another reflex action arising from stimulation of the pituitary 

 membrane by irritating substances, the vehicle being nerves belonging 

 to the 5th pair. Coughing, too, is reflex from the stimulation of irri- 

 tating substances in the air passages ; the vehicle in this case being 

 branches of the pneumogastric pair. The act of swallowing is entirely 

 reflex, and is caused by the stimulation of the food upon the mucous 

 membrane of the fauces and root of the tongue. This stimulation is 

 carried to the medulla oblongata chiefly by the glosso pharyngeal, as- 

 sisted, however, by fibres of the trigeminum, or 5th pair. The return, 

 or motor, influence is conve} T ed to the muscles of the pharynx chiefly by 

 the branches of the pneumogastric ( 1 Oth pair ), assisted by branches 

 from the facial, the trigeminum and the hypoglossal pairs. The move- 

 ments in the pharynx may or may not be accompanied by sensation. 

 When' the food has passed through the pharynx and reached the esoph- 

 agus, it is propelled along that canal by its peristaltic action, which is 

 also purel}" reflex and devoid of sensation. Its action is caused by the 

 stimulation of the food upon its mucous lining, which stimulation is 

 carried to the ganglion in the mjedulla oblongata and returned to the 

 muscular layer of the gullet, which thereupon contracts rythmically 

 from top to bottom, driving the contents along and forcing them through 

 the cardiac sphincter into the stomach. The nerves concerned in this 

 action are branches of the pneumogastric. 



In vomiting, the action of the esophagus is reversed, the rythmic con- 

 traction beginning at the bottom instead of the top, the contraction of 

 the cardiac sphincter is relaxed, the stomach contracts, and its contents 

 are expelled upward into the gullet. This action is automatic, and is 

 brought about through the pneumogastric nerves, although not in all 

 cases caused by direct stimulus upon the mucous coats of the organs 

 themselves, since it is sometimes provoked through the emotions, as of 



