THE SPECIAL NEEVOUS MECHANISMS 109 



olfactory nerve are still more delicate — even in man, in whom 

 the sensation of smell is degenerate — and chemical substances 

 existing in such small quantity in the air that they cannot be 

 detected by any known methods of analysis are easily distin- 

 guished by smell. Still more incredible is the delicacy of the 

 sense in such an animal as a bloodhound. 



Heat receptors in the skin are nerve terminations that are 

 stimulated by a rise of temperature above a certain limit, but 

 are not affected by a fall below that. Cold receptors are, con- 

 versely, stimulated by a decrease, but not by an increase of 

 temperature. Pressure receptors, or muscular sense receptors, 

 are stimulated mechanically — that is, by something pressing on 

 the skin, or by the degree of tension of a muscle, but not (or 

 at least not much) by chemical changes in the skin or muscle, 

 and not by changes of temperature. Equilibrium receptors, 

 which are present in the " vestibular " part of the internal ear, 

 are stimulated by changes in the position of the body with 

 respect to its surroundings. Thus a man who is blindfolded 

 has his ears and nose stopped, and who is lying immobile on a 

 turn-table, can appreciate a noiseless, frictionless change in the 

 position of his body, and can even roughly estimate the magni- 

 tude of the angle through which he is turned. 



Thus the first step in the development of -sensation consists 

 in a refinement of the general irritability, or susceptibility to 

 external changes, which we regard as one of the essential pro- 

 perties of living tissues. The " refinement " means that speciali- 

 ties of reaction are evolved: one kind of tissue, or arrangement 

 of nerve terminations, becomes more sensitive to one kind of 

 physical stimulus and less sensitive to all others. These 

 specialised receptors then localise themselves in appropriate 

 parts of the body, and become served by separate nerves. It is 

 improbable that the fibres themselves are different in different 

 sensory nerves — that is, could we transplant the auditory nerve 

 fibres to the optic tracts, they would probably conduct optic 

 stimuli just as well as they conducted auditory ones, and vice 

 versa. The nerve fibres are conductors, and nothing more. 



The next step is to place the fibres that transmit impulses 

 from a receptor in connection with fibres that transmit impulses 

 to a muscle or other effector organ, and that is done in the spinal 

 cord, the lower brain, and the^cerebellum. The sensory (or 

 afferent) fibres enter into synapses with motor' (or efferent) 



