HYPOGLOSSAL. 477 



movements of the voice alternate with each other, but are never 

 simultaneous ; so that the internal branch of the spinal accessory is 

 antagonistic to the motor fibres of the larynx de^'Wl from other 

 nerves. 



It is thought by M. Bernard, that the fibres of the external 

 branch of the spinal accessory have also a function which is anta- 

 gonistic to respiration. For respiration is naturally suspended in 

 all steady and prolonged muscular efforts. In these efforts, such as 

 those of straining, lifting, and the like, the movements of respira- 

 tion cease, the spinal column is made rigid by the contraction of 

 its muscles, and the head and neck are placed in a fixed position, 

 principally by the contraction of the sterno-mastoid and trapezius 

 muscles. The function of the spinal accessory, in both its branches, 

 is therefore regarded as destined to excite movements which are 

 incompatible with those of respiration ; and which accordingly come 

 into play only when the ordinary movements of respiration have 

 been temporarily suspended. 



HYPOGLOSSAL. The hypoglossal nerve originates from the ante- 

 rior and lateral portions of the medulla oblongata, and passing out 

 by the anterior condyloid foramen, is distributed exclusively to the 

 muscles of the tongue. Irritation of its fibres in any part of their 

 course produces convulsive twitching in this organ. Its section 

 paralyzes completely the movements of the tongue, without affect- 

 ing directly the sensibility of its mucous membrane. This nerve, 

 accordingly, is the motor nerve of the tongue. If irritated at its 

 origin, the hypoglossal nerve, according to the experiments of 

 Longet, is entirely insensible ; but if the irritation be applied in the 

 middle of its course, signs of pain are immediately manifested. Its 

 sensibility, like that of the facial, is consequently derived from its 

 inosculation with other sensitive nerves, after its emergence from 

 the skull. 



