660 KEFLEX CENTRES OF THE MEDULLA OBLONGATA. 



branches of the facial, the efferent impulses are conveyed to the orbicularis 

 palpebrarum. The centre extends from about the middle of the ala cinerea 

 upwards to the posterior margin of the pons (Nickell). 



The refle* closure of the eyelids always occurs on both sides, but closure may be produced volun- 

 tarily on one side (winking). When the stimulation is strong, the corrugator and other groups of 

 muscles which raise the cheek and nose towards the eye may also contract, and so form a more 

 perfect protection and closure of the eye. Intense stimulation of the retina causes closure of the 

 eyelids [and in this case the shortest reflex known, the latent period, is 0*05 second ( Waller)]. 



2. Sneezing centre. The afferent channels are the internal nasal branches of 

 the trigeminus and the olfactory, the latter in the case of intense odours. The 

 efferent or motor paths lie in the nerves for the muscles of expiration ( 120, 3, 

 and 347, II.). Sneezing cannot be performed voluntarily, [but it may be inhibited 

 by compressing the nasal nerve at its exit on the nose]. 



3. Coughing centre. According to Kohts, it is placed a little above the 

 inspiratory centre ; the afferent paths are the sensory branches of the vagus 

 ( 352, 5, a). The efferent paths lie in the nerves of expiration and those that 

 close the glottis ( 120, 1). 



4. Centre for sucking and mastication. The afferent paths lie in the sensory 

 branches of the nerves of the mouth and lips (2nd and 3rd branches of the 

 trigeminus and glosso- pharyngeal). The efferent nerves for sucking are ( 152) : 

 Facial for the lips, hypoglossal for the tongue, the inferior maxillary division of the 

 trigeminus for the muscles which elevate and depress the jaw. For the movements 

 of mastication, the same nerves are in action ( 153) ; but when food passes within 

 the dental arch, the hypoglossal is concerned in the movements of the tongue, and 

 the facial for the buccinator. 



5. Centre for the secretion of saliva (p. 215) lies in the floor of the 4th 

 ventricle. Stimulation of the medulla oblongata causes a profuse secretion of 

 saliva when the chorda tympani and glossopharyngeal nerves are intact, a much 

 feebler secretion when the nerves are divided, and no secretion at all when the 

 cervical sympathetic is extirpated at the same time (Griitzner). 



6. Swallowing centre lies in the floor of the 4th ventricle ( 156). The afferent 

 paths lie in the sensory branches of the nerves of the mouth, palate, and pharynx 

 (2nd and 3rd branches of the trigeminus, glossopharyngeal, and vagus) ; the 

 efferent channels, iu the motor branches of the pharyngeal plexus (352, 4). 

 Stimulation of the glossopharyngeal nerve does not cause deglutition; on the 

 contrary, this act is inhibited (p. 228). 



According to Steiner, every time we swallow there is a slight stimulation of the respiratory 

 centre, resulting in a contraction of the diaphragm. [Kronecker has shown that if a glass of 

 water be sipped slowly, the action of the cardio-inhibitory centre is interfered with reflexly, so 

 that the heart beats much more rapidly, whereby the circulation is accelerated, hence probably 

 the reason why sipping an alcoholic drink intoxicates more rapidly than when it is quickly 

 waliuwed (p. 668).] 



7. Vomiting centre ( 158). The relation of certain branches of the vagus to 

 this act are given at 352, 2, and 12, d.\ 



8. The upper centre for the dilator pupillse muscle, the smooth muscles of the 

 orbit, and the eyelids lies in the medulla oblongata. The fibres pass out partly in 

 the trigeminus ( 347, I., 3), partly in the lateral columns of the spinal cord as far 

 down as the cilio-spinal region, and proceed by the two lowest cervical and the two 

 upper dorsal nerves into the cervical sympathetic ( 356, A, 1). The centre is 

 normally excited reflexly by shading the retina, i.e., by diminishing the amount of 

 light admitted into the eye. It is directly excited by the circulation of dyspnceic 

 blood in the medulla. (The centre for contracting the pupil is referred to at *58 345 

 and 392.) , . 



The centre may be excited reflexly by stimulation of sensory nerve, e.g., the sciatic. These 

 afferent fibres pass upwards through both lateral columns to their centre (Kowalewsky). 



