322 FUNCTIONS OF THE NERVOUS SYSTEM. 



upon the Heart's action, it remains to inquire whether that influence is essen- 

 tial to its movements ; and whether these nerves form the channel, through 

 which they are affected by emotions of the mind, or by conditions of the 

 bodily system. In regard to the first point, no doubt can be entertained; 

 since the regular movements of the heart are but little affected by section of 

 the Vagi. With respect to the second, there is more difficulty ; since the 

 number of causes, which may influence the rapidity and pulsations of the 

 heart, is very considerable. For example, when the blood is forced on more 

 rapidly towards the heart, as in exercise, struggling, &c., the stimulus to its 

 contractions is more frequently renewed, and they become more frequent; 

 and when the current moves on more slowly, as in a state of rest, their fre- 

 quency becomes proportionably diminished. If the contractions of the heart 

 were not dependent upon the blood, and their number were not regulated by 

 the quantity flowing into its cavities, very serious and inevitably fatal dis- 

 turbances of the heart's action would soon result. That this adjustment takes 

 place otherwise than through the medium of the nervous centres, is evident 

 from the fact that, in a dog, in which the par vagum and sympathetic had 

 been divided in the neck on each side, violent struggling, induced by alarm, 

 raised the number of pulsations from 130 to 260 per minute. It is difficult 

 to ascertain, by experiment upon the lower animals, whether simple emotion, 

 unattended with struggling or other exertion, would affect the pulsation of the 

 heart, after section of the Vagi; but when the large proportion of the Sympa- 

 thetic nerves proceeding to this organ is considered, and when it is also re- 

 membered that irritation of the roots of the upper cervical nerves stimulates 

 the action of the heart through these, we can scarcely doubt that both may 

 serve as the channels of this influence, especially in such animals as the dog, 

 in which the two freely inosculate in the neck. 



417. In regard to the functions of the Spinal Accessory nerve, also, there 

 has been great difference of opinion; the peculiarity of its origin and course 

 having led to the belief, that some very especial purpose is answered by it. 

 The predominance of motor fibres in its roots, its inosculation with the Par 

 Vagum, and its probable reception of sensory fibres from the latter whilst 

 imparting to it motor filaments, have been already referred to ( 408). As its 

 trunk passes through the foramen lacerum, it divides into two branches; of 

 which the internal, after giving off some filaments that assist in forming the 

 pharyngeal branch of the Par Vagum, becomes incorporated with the trunk of 

 that nerve; whilst the external proceeds outwards, and is finally distributed to 

 the sterno-cleido-mastoideus and trapezius muscles, some of its filaments 

 inosculating with those of the cervical plexus. When the external branch is 

 irritated, before it perforates the sterno-mastoid muscles, vigorous convulsive 

 movements of that muscle, and of the trapezius, are produced; and the animal 

 does not give any signs of pain, unless the nerve is firmly compressed between 

 the forceps, or is included in a tight ligature. Hence it may be inferred, that 

 the functions of this nerve are chiefly motor, and that its sensory filaments are 

 few in number. Further, when the nerve has been cut across, or firmly tied, 

 irritation of the lower end is attended by the same convulsive movements of 

 the muscles; whilst irritation of the upper end, in connection with the spinal 

 cord, is unattended with any muscular movement. Hence it is clear that the 

 motions occasioned by irritating it are of a direct, not of a reflex character. 

 The same muscular movements are observed on irritating the nerve in the 

 recently-killed animal, as during life. 



a. According to Sir C. Bell, the Spinal Accessory is a purely Respiratory nerve, whose office 

 it is to excite the involuntary or automatic movements of the muscles it supplies, which 

 share in the a<-t of n-.-piration ; and he Mates that the division of it paralyzes the muscles to 

 which it is distributed, as muscles of respiration ; though they still perform the voluntary 



