202 PHYSIOLOGY CHAP. 



longer exists. Many persons eventually become accustomed to 

 these sensations, and cease to notice them ; but they surge up 

 again when attention is focussed upon them, and are often felt 

 distinctly in the fingers, sole of the foot, or hand. The sensation 

 is more acute when pressure is exerted on the stump. 



The symptoms of anaesthesia dolorosa are no less important 

 to the demonstration of the peripheral projection of sensations. 

 Traumatic paralysis from compression or section of a nerve trunk, 

 in which more or less extensive cutaneous areas become totally in- 

 sensitive to the strongest stimuli, though the patient still complains 

 of intense pain in them owing to the irritable state of the nerve 

 trunk, is not infrequent. In surgery, division of the nerve may 

 fail to cure neuralgia, as it merely interrupts the conduction of 

 external peripheral excitations to the centre, but cannot suppress 

 the conduction of central irritation in the nerve, which gives 

 origin to sensations projected to the periphery similar to those 

 produced by extrinsic local stimulation. 



The phenomenon of the peripheral projection of sensations can 

 easily be demonstrated under normal conditions by mechanical 

 excitation of one's own ulnar nerve in the groove of the internal 

 condyle at the elbow, where it is accessible ; this produces a prick- 

 ing in the palm and back of the hand, and in the third and fourth 

 fingers. Pressure on the infraorbital nerve, where it issues from 

 its foramen, produces pricking at many points of the cheek and 

 upper lip. 



III. Johannes Miiller in 1844 declared the problem of the 

 velocity of nerve conduction to be insoluble, and compared it with 

 that of light. " The time," he writes, " in which a sensation 

 passes from the exterior to the brain and spinal cord, and thence 

 back to the muscle so as to produce a contraction, is infinitely 

 small and immeasurable." Only six years later, in 1850, 

 Helmholtz was able by exact physical methods to determine the 

 rate of propagation in a frog's nerve, and to demonstrate that it 

 is infinitely slow in comparison with the propagation of physical 

 energy. Electricity traverses a space of 464 million metres in a 

 second, light 300 million, sound 332 metres; the excitatory 

 impulse in nerve, on the contrary, is transmitted at a rate so 

 much lower that it may be compared with the speed of a loco- 

 motive or the flight of an eagle. 



The first exact measurement of the velocity of conduction in 

 nerve was made by Helmholtz on a frog's nerve-muscle preparation 

 (Fig. 3). If the time-interval between the stimulation of the 

 nerve and the contraction of the muscle (latent period) is measured, 

 it is found to be greater when the motor nerve is stimulated at a 

 point remote from the muscle than when it is stimulated near the 

 muscle. The difference in the time-interval is also, carteris paribus, 

 proportional to the length of nerve between the two points excited. 



