CH. XV.] CLASSIFICATION OF NERVES. 165 



up the nerves to the spinal cord, and then to the brain. The 

 mental process set up in the brain is called a sensation ; the 

 sensation, however, is referred to the end of the nerve where 

 the impulse started, and the sensation of pain does not appear 

 to occur in the brain, but in the finger. This is an instance 

 of a sensory nerve ; and the terms afferent and sensory may 

 often be used synonymously. The nerves of sensation may be 

 grouped as follows : 



a. The nerves of special sense ; that is, of sight, hearing, taste, 

 smell and touch. 



b. The nerves of general sensibility ; that is, of a vague kind of 

 sensation not referable to any of the five special senses just 

 enumerated ; as instances, we may take the vague feelings 

 of comfort or discomfort in the interior of the body. 



c. Nerves of pain. These do not appear to be anatomically 

 distinct from the others, but any excessive stimulation of a 

 sensory nerve whether of the special or general kind will 

 cause pain. 



The words " sensory " and " afferent," however, are not quite 

 synonymous. Just as we may have efferent impulses leaving 

 the brain for the heart or blood-vessels of which we have no 

 conscious knowledge, so also afferent impulses may travel to the 

 central nervous system which excite no conscious feelings. The 

 afferent nerve tracts to the cerebellum form a very good 

 instance of these. 



Then, too, the excitation of many afferent nerves will excite 

 what are called reflex actions. We are very often conscious of 

 the sensations that form the cause of a reflex action, but we do not 

 necessarily have such sensations. Many reflex actions, for instance, 

 occur during sleep ; many may be executed by the spinal cord 

 even after it has been severed from the brain, and so the brain 

 cannot be aware of what is occurring. 



A reflex action is an action which is the result of an afferent 

 impulse. Thus a speck of dust falls into the eye, and causes 

 movements of the eyelids to get rid of the offending object. The 

 dust excites the sensory nerve-endings in the conjunctiva, an 

 impulse travels to the centre of this nerve in the brain, and from 

 the brain a reflected impulse travels to the muscles of the 

 eyelid. As an instance of a reflex action in which secretion 

 is concerned, take the watering of the mouth which occurs 

 when food is seen or smelt. The nerves of sight or smell 

 convey an afferent impulse to the brain, which reflects, down the 

 secretory nerves, an impulse which excites the salivary glands to 

 activity. 



