GENEKAL AND SPECIAL SENSIBILITY. 837 



system, through the cerebro-spinal nerves, so as to inhibit or modify 

 the function of certain organs. As an illustration of this may be men- 

 tioned the action of the sympathetic upon the pupil. According to 

 Budge, the fibres which are concerned in producing dilatation of the 

 pupil arise from the spinal cord and run from the upper two dorsal and 

 the lowest two cervical nerves into the cervical sympathetic, which 

 conveys them to the head. The influence of the sympathetic in 

 governing the movements of the iris will be given more in detail in the 

 consideration of the e} T e. 



Among other branches arising from the cervical part of the sym- 

 pathetic are found motor branches going partly to the external rectus 

 muscle of the eye. vaso-motor branches to the ear, the side of the face, 

 the conjunctiva, the iris, the choroid, and to the vessels of this portion 

 of the alimentary and respiratory tract. Secretory and vaso-motor fibres 

 are distributed to the salivary glands and to the sweat-glands of the 

 integument ; while, according to certain authorities, the lachrymal glands 

 receive sympathetic secretory fibres from this portion of the sympathetic. 



Of the thoracic and abdominal sections of the sympathetic the car- 

 diac plexus, which receives accelerator fibres for the heart, occupies the 

 most important position. The influence of the coeliac plexus of the 

 sympathetic on the heart has already been given. Of the abdominal 

 sympathetic, the coeliac and mesenteric plexuses and the splanchnic 

 nerves are the most important. They also have been already described. 



It is thus seen that the fibres of the sympathetic nervous system 

 act as conductors of both afferent and efferent impressions. Impressions 

 traveling from the periphery to the centre through the sympathetic 

 nerve do not produce an impression upon the sensorium. In other 

 words, the brain is not capable of taking cognizance of afferent im- 

 pulses traveling through the sympathetic. Nor, on the other hand, can 

 voluntary motor impulses pass through this nerve. 



IX. GENERAL AND SPECIAL SENSIBILITY. 



Sensation, or general sensibility, is that function of the brain by 

 which it perceives or becomes conscious of impressions which are made 

 upon the surface of the body or upon the nerves running from the 

 peripher}* to the nerve-centres. Ity perception is meant that faculty by 

 which sensations are referred to certain external causes. It is important 

 to understand that all sensations take place, not at the point of contact 

 of the irritant with the periphery, but in the brain itself, and we have 

 evidence of this in the fact that if the brain be in a state of torpor no 

 sensation occurs. Again, if a ligature be passed around an afferent nerve 

 at some point between its origin on the periphery and its termination in 

 the nerve-centre no sensation occurs, no matter how severe be the 



