462 NERVES. 



.*. 



Classification of Nerves. There are three kinds of nerves, 

 classified according to the direction in which they carry impulses : 

 1. Efferent; 2. Afferent; and 3. Intercentral. 



Efferent Nerves. Inasmuch as nerves of this kind carry im- 

 pulses away from nerve-centers, they are also called centrifugal 

 nerves. They were formerly spoken of as motor nerves. All 

 motor nerves are efferent, for they carry impulses outward, but all 

 efferent nerves are not motor. A nerve which carries an impulse 

 to a muscle, and thus brings about motion, is properly called a 

 " motor nerve " ; but one that conducts an impulse to a gland, the 

 results of which are the activity of its cells and the production of 

 a secretion, is improperly named a motor nerve, although it is un- 

 questionably an efferent nerve. Secretory is a much more appro- 

 priate name. Efferent nerves may be divided as follows : (1) 

 motor ; (2) vasomotor ; (3) accelerator ; (4) secretory ; (5) trophic ; 

 and (6) inhibitory. 



Motor nerves terminate in muscles, and convey to them im- 

 pulses which cause and regulate their contraction. 



Vasomotor nerves, although distributed to the muscular tissue 

 of blood-vessels, and thus act as motor nerves, regulate the 

 amount of blood supplied to a part, and it seems wise to separate 

 them from the purely motor nerves and put them in a class by 

 themselves. 



Accelerator nerves are nerves which carry impulses that increase 

 the rhythmic action of an organ, as the sympathetic nerves to 

 the heart. 



Secretory Nerves. The impulses which these nerves carry to 

 glands bring about their secretion. The chorda tympani is a 

 striking example. 



Trophic nerves are supposed by some to govern the nutrition of 

 the structures to which they are distributed, entirely independently 

 of the regulation of the blood-supply. It is still a mooted ques- 

 tion whether such nerves exist. 



Efferent inhibitory nerves carry impulses which restrain or 

 inhibit the action of the organs to which they are distributed. 

 The pneumogastric, so far as the heart is concerned, is such a 

 nerve. Without its restraining influence the heart would beat 

 much faster. 



Afferent Nerves. The fact that these nerves carry impulses to 

 the nerve-centers has led to their being called also centripetal 

 nerves. They were formerly called sensory nerves, but there is the 

 same impropriety in using these terms synonymously as in the case 

 of efferent and motor nerves. All sensory nerves are afferent, but 

 all afferent nerves are not sensory. Afferent nerves may be divided 

 as follows, although the distinction is by no means so well marked 

 as in the efferent nerves : (1) Sensory ; (2) nerves of special sense ; 

 (3) thermic nerves ; (4) excitoreflex ; and (5) inhibitory. 



