PROPERTIES AND FUNCTIONS OF NERVES. 87 



measure about the T ^ of an inch in length on the average, and consist of 

 concentric layers of connective tissue; the nerve fibre penetrates the cor- 

 puscle and terminates in a rounded knob in the central bulb. Their 

 function is Unknown. 



The end bulbs of Krause are formed of a capsule of connective tissue in 

 which the nerve fibre terminates in a coiled mass or bulbous extremity ; 

 they exist in the conjunctiva, tongue, glans penis, clitoris, etc. 



Many sensitive nerves terminate in the papillae at the base of the hair 

 follicle ; but in the skin, mucous membranes, and organs of special sense 

 their mode of termination is not well understood. 



PROPERTIES AND FUNCTIONS OF NERVES. 



Classification. Nerves may be divided into two groups, viz. : 



(1) Afferent or centripetal, as when they convey to the nerve centres the 

 impressions which are made upon their peripheral extremities or parts of 

 their course. They may be sensitive, when they transmit impressions which 

 give rise to sensations ; reflective or excitant ', when the impression carried 

 to the nerve centre is reflected outward by an efferent nerve and produces 

 motion or some other effect in the part to which the nerve is distributed. 



(2) Efferent or centrifugal, as when the impulses generated in the 

 centres are transmitted outward to the muscles and various organs. They 

 may be motor, as when they convey impulses to the voluntary and invol- 

 untary muscles ; vasomotor, when they regulate the calibre of the small 

 blood vessels, increasing or diminishing the amount of blood to a part ; 

 secretory, when they influence secretion; trophic, when they influence 

 nutrition ; inhibitory, when they conduct impulses which produce a re- 

 straining or inhibiting action. 



Stimuli of Nerves. Nerves respond to stimulation according to their 

 habitual function ; thus, stimulation of a sensory nerve, if sufficiently strong, 

 results in the sensation of pain; of the optic nerve, in the sensation of light; 

 of a motor nerve, in contraction of the muscle to which it is distributed; of 

 a secretory nerve, in the activity of the related gland, etc. It is, therefore, 

 evident that peculiarity of nervous function depends neither upon any special 

 construction or activity of the nerve itself, nor upon the nature of the 

 stimulus, but entirely upon the peculiarities of its central and peripheral end 

 organs. Nerves react to the following kinds of stimuli : 



1. Mechanical : as from a blow, pressure, tension, puncture, etc. 



2. Thermal: heating a nerve at first increases and then decreases its excita- 



bility. 



