THE NERVOUS SYSTEM 529 



way as the accelerator and inhibitory nerves of the heart — viz., 

 as anabolic or builders, and katabolic or users up. The cell- 

 stations, generally speaking, of anabolic nerves are in the pre- 

 vertebral or terminal ganglia ; the cell-stations of the katabolic 

 nerves are, with the same reservation, in the vertebral or else in 

 the prevertebral ganglia. 



The pilomotor nerves issue from the cord with the ventral 

 roots of the spinal nerves, have their cell-stations in the vertebral 

 ganglia, return to the motor spinal nerve as a grey ramus, and 

 with it are issued to the skin. Stimulation of the grey ramus 

 causes erection of the hair above the vertebra to which it belongs ; 

 stimulation of the white ramus causes erection of hairs over three 

 or four vertebrae. 



Sensory Phenomena in the Autonomic System. — It has already 

 been stated that the majority of fibres in the sympathetic 

 system are efferent ; afferent fibres are known to exist, but there 

 is very little information concerning them. As a rule, the parts 

 supplied by the sympathetic system are devoid of ordinary 

 sensibility, so that the existence of afferent fibres must be very 

 limited. The intestines can be handled and cut without the 

 animal evincing pain ; the heart can be injured in many ways, 

 and shows no sign of sensibility ; the liver, spleen, and kidneys 

 are equally devoid of touch sensibility, and ordinarily free from 

 painful impressions. It is the absence of afferent fibres which 

 renders a true reflex action in the autonomic system impossible ; 

 yet stimulation of the central ends of sympathetic nerves gives, 

 as we have seen, great reflex effects on blood pressure, and causes 

 much pain. 



It is a remarkable fact that the intestines can be handled, 

 pinched, and douched with hot or cold water, without causing 

 pain. In man the part is not entirely devoid of feeling, but the 

 sensibility is so low that no pain is caused by measures which 

 applied outside the body would cause pain. It is no wonder 

 that the interior of the body is non-existent as a subjective 

 sensation to the majority of people. In spite of this, the most 

 acute pain experienced by the horse is referred to the abdominal 

 cavity. There is no pain to equal that caused by a twisted 

 condition of the intestine, and an attack of acute colic comes 

 next in order of intensity. It is by no means clear why tissues 

 normally insensitive become the seat of such acute pain under 

 disease. This class of pain is not referred to the viscera, but 

 to the abdominal wall, and the explanation which has been 

 offered of this fact is that the sensory cutaneous nerves which 

 are stimulated belong to the segment of cord in which the 

 afferent nerves from the viscera end, and the pain thus referred 

 to the skin. 



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