498 MANUAL OF PHYSIOLOGY. 



the primitive sheath, in immediate contact with the fibrils of the 

 axis cylinder. This is called the medullary sheath, or white sub- 

 stance of Schwann, because 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 ordi- 

 nary nerves, while the gray fibres only predominate in the sym- 

 pathetic nerve and its ramifications and parts of the special sense 

 organs. 



An ordinary nerve, then, is made up of a large number 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. 



(6.) 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 as 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, () motor, going to 

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

 tion of which calls forth the activity of a gland ; (y) inhibitory, 



