108 THE MECHANISM OF LIFE 



This is a very complicated instance, but it is the type of an 

 essential sense receptor. There is always a terminal nerve cell, 

 and the dendrites of this receive the physical stimulus and trans- 

 mit it through synapses to other neurones, and, finally, via the 

 sensory tract, to the cerebral centres. These terminal dendrites, 

 or sensory nerve terminations, are peculiarly modified in each 

 case, and their threshold is lowered, so that they are susceptible 

 to one form of physical stimulus rather than others. 



The non-nervous parts of the sense organ are concerned in 

 transmitting the particular stimulus and in excluding others. 

 Thus the retina is enclosed in the ball of the eye, and the front 

 part of the latter (the cornea) is transparent, so that light can 

 pass through it. Behind the cornea is the lens with the 

 mechanisms for altering its focus, and the changing diaphragm, 

 or iris, which regulates the amount of light that penetrates it. 

 These dioptric parts of the eye are in every way comparable with 

 a camera, lens, and diaphragm, which focus the external light 

 upon a sensitive plate, making a picture there. 



In the same way the ear consists of the outer, middle, and 

 internal parts. Externally there is a stretched membrane (the 

 drum), which is set in vibration by sound waves in the outer 

 atmosphere. The drum communicates its motions to a chain of 

 three small bones (the auditory ossicles), which again transmit 

 the vibrations to the liquid contained in a cavity in the bone of 

 the skull. This vibrating liquid then stimulates the nerve 

 terminations of the cochlear (auditory) nerve in a most complex 

 structure, called the organ of Corti, and the stimuli are trans- 

 mitted along the cochlear nerve to the auditory centres. The 

 organ of hearing thus excludes light, chemical and mechanical 

 stimuli, but allows the periodic variations of atmospheric pressure 

 that we call " sound " to reach the nerve terminations. 



The nerve terminations in the mucous membranes of the 

 mouth and nasal cavities are " bare " that is, they are exposed 

 to all physical stimuli: variations of temperature, some light, 

 mechanical pressure, and chemical activity. But they are 

 highly sensitive to the stimuli of different kinds of chemical 

 substances, so that they can distinguish between the latter. 

 And they are very susceptible in this way, so that, for instance, 

 one easily tastes the difference between the flesh of, say, cod 

 and haddock, or plaice and sole, a distinction which cannot yet 

 be made by chemical analysis. The nerve terminations of the 



