460 HANDBOOK OF PHYSIOLOGY. 



way to be distributed elsewhere, or through the sentient extremities 

 of nerves which are themselves distributed to the many trunks of 

 the nerves, the nervi nervorum. The latter is the more probable sup- 

 position. 



Conduction in the Nerves of Special Sense. The laws of conduction 

 in the olfactory, optic, auditory, gustatory, resemble in many respects 

 those of conduction in the nerves of common sensation, just described. 

 Thus the effect is always central; stimulation of the trunk of the nerve 

 produces the same effect as that of its extremities, and if the nerve be 

 severed, it is the central and not the peripheral extremity which re- 

 sponds to irritation, although the sensation is referred to the periphery. 

 There are, however, certain peculiarities in the effects. Thus the various 

 stimuli, which might cause, through an ordinary sensitive nerve, the 

 sense of pain, would, if applied to the optic nerve, cause a sensation as of 

 flashes of light; if applied to the olfactory, there would be a sense as of 

 something smelt. And so with the other two. 



Hence the explanation of so-called subjective sensations. Irritation 

 in the optic nerve, or the part of the brain from which it arises, may cause 

 a patient to believe he sees flashes of light, and among the commonest 

 troubles of the nerves of special sense, is the distressing noise in the 

 head (tinnitus aurium), which depends on some unknown stimula- 

 tion of the auditory nerve or centre quite unconnected with external 

 sounds. 



Conduction in Motor Nerves. Conduction in motor nerves presents 

 a remarkable contrast with the foregoing. Thus, the effect of applying 

 a stimulus to the motor nerve is always noticeable, at the peripheral ex- 

 tremity, in the contraction of muscles supplied by it. If a motor nerve 

 be severed, irritation of the distal portion causes contraction of muscle, 

 but no effect whatever is produced by stimulating that part of the nerve 

 which is still in direct connection with the nerve-centre. 



Contractions are excited in all the muscles supplied by the branches 

 given off by the nerve below the point irritated, and in those muscles 

 alone: the muscles supplied by the branches which come off from the 

 nerve at a higher point than that irritated, are not directly excited to 

 contraction. And it is from the same fact that, when a motor nerve 

 enters a plexus and contributes with other nerves to the formation of a 

 nervous trunk proceeding from the plexus, it does not impart motor 

 power to the whole of that trunk, but only retains it isolated in the fibres 

 which form its continuation in the branches of that trunk. 



NEEYE TERMINATIONS. 



A. Of Sensory Nerves. (1.) Pacinian Corpuscles. ThePacinian 

 bodies or corpuscles (Figs. 318 and 319), named after their discoverer 

 Pacini, also called Corpuscles of Vater, are little elongated oval bodies, 

 .situated on some of the cerebro-spinal and sympathetic nerves, especially 

 the cutaneous nerves of the hands and feet; and on branches of the 

 large sympathetic plexus about the abdominal aorta (Kolliker). They 

 often occur also on the nerves of the mesentery, and are especially well 

 seen even by the naked eye in the mesentery of the cat. They have been 

 observed also in the pancreas, lymphatic glands and thyroid glands, as 



