THE SPINAL ACCESSORY. 573 



experiment show that they are finally distributed, through the inferior 

 laryngeal branch, to the muscles of the larynx, where they preside over 

 its actions as a vocal organ. 



After the spinal accessory nerve has been torn away on both sides in 

 the manner above described, the most noticeable result is a loss of 

 power to produce vocal sounds. The other movements of the larynx 

 are not interfered with. Especially those of respiration go on in a natu- 

 ral manner. But the voice is completely lost, as much so as if the infe- 

 rior laryngeal nerves, or the pneumogastric trunks themselves, had been 

 divided. The difference between the two cases, however, is very impor- 

 tant. Section of the pneumogastrics, or of their inferior laryngeal 

 branches, paralyzes at once all the movements of the glottis, those of 

 respiration as well as those of phonation ; since these nerves contain all 

 the motor fibres distributed to the larynx, except those of the crico-thy- 

 roid muscles. On the other hand, section or evulsion of the spinal 

 accessory nerves paralyzes the movements of phonation alone, namely, 

 those in which the vocal chords are approximated and the rima glottidis 

 narrowed, while it leaves untouched the movements of respiration, in 

 which the vocal chords are separated and the rima glottidis opened. 



Thus the muscular apparatus of the lar} 7 nx, which is destined to per- 

 form separately two distinct functions, is supplied with motor nerves 

 from two different sources. Those which preside over the production 

 of vocal sounds originate exclusively from the spinal accessory ; those 

 which excite the movements of respiration are derived from the other 

 motor nerves (facial, hypoglossal, cervical) which also inosculate with 

 the pneumogastrics. 



The special function of the external or muscular branch of the spinal 

 accessory nerve is not so fully understood. The stern o-mastoid and 

 trapezius muscles, to which its fibres are distributed, also receive fila- 

 ments from the cervical spinal nerves ; and they still retain the power 

 of motion after division or evulsion of the spinal accessory on both 

 sides. The sterno-mastoid and trapezius muscles have no such peculiar 

 and easily recognizable mode of action as that of the larynx in the for- 

 mation of the voice ; and consequently it has not been easy to distin- 

 guish with certainty what special movement of these muscles is para- 

 lyzed by division of the spinal accessory, and what remains unaffected. 

 The most plausible conclusions are those derived by Bernard from 

 continued observation of animals preserved for some time after the 

 division of these nerves. 



According to this explanation, the fibres of the external branch of 

 the spinal accessory, like th6se of the internal branch, perform a func- 

 tion which is antagonistic to the movements of respiration. Respira- 

 tion is naturally suspended in all steady and prolonged muscular efforts. 

 In the acts of straining, lifting, pushing, and the like, respiration ceases, 

 the spinal column is made rigid, and the head and neck are placed in a 

 fixed position largely by the aid of the sterno-mastoid and trapezius 



