496 MANUAL OF PHYSIOLOGY. 



In some nerve fibres there is but one very thin, transparent cov- 

 ering, termed the primitive sheath, while in others there is a thick 

 layer of doubly-refracting fluid inside the primitive sheath, in 

 immediate contact with the fibrils of the axis cylinder. This is 

 called the medullary sheath, or white substance of Schwann, be- 

 cause its peculiar refractive properties make it look white when 

 viewed in a direct light. According as the nerves have or have 

 not this medullary sheath, they have been termed "white" or 

 "gray." The former are by far the most plentiful, since they 

 make up the greater part of the ordinary nerves, while the gray 

 fibres only predominate in the sympathetic nerve and its ramifi- 

 cations and parts of the special sense organs. 



An ordinary nerve, then, is made up of a largejiumber of fibres 

 held together by connective tissue, and each of the fibres contains 

 a vast number of fibrils within its sheath. 



FUNCTIONAL CLASSIFICATION. 



Nerve fibres may be classified according to their function, in 

 the following way : 



I. Afferent nerves, those which bear impulses from the surface 

 to the nervous centres. These may be further divided into : 



(a.) Sensory nerves, when the impulse they convey gives rise to 

 a " perception." The perceptions may be the special sensations 

 which are transmitted from the organs of special sense, or those 

 of general sensation, giving rise to pleasure or pain. 



(b.) Reflex nerves, which communicate the impulse to some 

 other nerve elements, and thus give rise to some new forces, with- 

 out any perception of the stimulus. According to the result of 

 the excitation resulting from their reflected impulse they are 

 termed excito-motor, excito-secretory, and excito-inhibitory, etc. 



(c.) Those nerves which act both aa sensory and reflex nerves ; 

 these are the most numerous, the sensory or reflex action depend- 

 ing upon the condition of the nerve centres. 



II. Efferent nerves, which carry impulses from the centres to 

 the various organs throughout the body. According to the effect 

 their excitation produces they are termed ; (a) motor, going to 

 muscles and causing them to contract ; (ft) secretory, the stimula- 



