476 THE CRANIAL NERVES. 



a part of the motor fibres of the inferior laryngeal or recurrent 

 branches of the pneumogastric ; being finally distributed to the 

 muscles of the larynx, which they supply with those nervous influ- 

 ences which are required for the formation of the voice. 



The special function of the external or muscular branch of the 

 spinal accessory is not so fully understood. This branch, as we 

 have seen, is distributed to the sterno-mastoid and trapezius mus- 

 cles. But these muscles also receive filaments from the cervical 

 spinal nerves ; and, accordingly, they still retain the power of mo- 

 tion, to a certain degree, after the external branches of the spinal 

 accessory have been divided on both sides. 



The spinal accessory is, accordingly, a nerve of very peculiar 

 distribution. For it partly supplies motor fibres to the pneumo- 

 gastric nerve, and is partly distributed to two muscles, both of 

 which also receive motor nerves from another source. Sir Charles 

 Bell, noticing the close connection between this nerve and the 

 pneumogastric, regarded the two as associated also in their func- 

 tion, as nerves of respiration. He considered, therefore, the exter- 

 nal branch of the spinal accessory as destined to assist in the 

 movements of respiration, when these movements become unusu- 

 ally laborious, by bringing into play the sterno-mastoid and trape- 

 zius muscles, in aid of the action of the intercostals. He therefore 

 called this nerve the " superior respiratory nerve." 



But the most satisfactory explanation of this peculiarity is that 

 proposed by M. Bernard. According to this explanation, whenever 

 a muscle, or set of muscles, derive their nervous influence from two 

 different sources, this is not for the purpose of assisting them in the 

 performance of the same function, but of enabling them to perform 

 two different functions. We have seen this already exemplified in 

 the muscles of the larynx. For these muscles perform certain 

 movements of respiration for which they receive indirectly filaments 

 from the facial hypoglossal, and cervical nerves. But they also 

 perform the movements necessary to the formation of the voice, the 

 nervous stimulus for which is derived altogether from the spinal 

 accessory. 



The internal branch of the spinal accessory, accordingly, excites, 

 in the parts to which it is distributed a function which is incompa- 

 tible with respiration. For the movements of respiration cannot 

 go on while the voice is sounded; and a necessary preliminary 

 to the production of a vocal sound, is the temporary stoppage of 

 respiration. The movements of respiration, therefore, and the 



