424 



A MANUAL OF VETERINARY PHYSIOLOGY 



direction — viz., either to the centre or to the periphery. When 

 a nerve is removed from the body there is no difficulty in trans- 

 mitting an electrical impulse in either direction, but within the 

 body the law is as stated. We shall see presently that the dorsal 



roots of the spinal cord are sen- 

 sory or afferent, but Bayliss has 

 shown that if these roots from 

 the fifth lumbar to the first sacral 

 be divided, and the peripheral 

 end stimulated, vascular dilata- 

 tion of the vessels of the hind- 

 limbs occurs. The assumption 

 is, therefore, that efferent fibres 

 are passing out of the cord 

 through the dorsal roots, and 

 conveying motor impulses to the 

 bloodvessels in a direction oppo- 

 site to that in which the ordinary 

 sensory impulses pass. Bayliss 

 has termed these impulses anti- 

 dromic, as indicating they are 

 occurring in the opposite direc- 

 tion to the natural stream. 



Structure of Nerves. — Nerves 

 were originally grouped under 

 two heads, according to their 

 colour, as white and grey. The 

 microscope revealed the fact that 

 a difference in structure existed 

 between them, the white fibres 

 possessing a thick coating of 

 fatty substance which gave them 

 their colour, while the grey fibres 

 were without this covering. The 

 white substance is described as 

 the medullary sheath ; nerves so 

 covered are termed medullated, 

 while the ' grey fibres ' — a term 

 now rarely used — are spoken of 

 as the non-medullated. If a 

 medullated nerve be examined microscopically (Fig. 124, I), 

 it is found to consist of a central core, or axis cylinder, con- 

 sisting of fibrils, surrounded by a white substance known as 

 the medullary sheath, and outside this is another sheath, 

 or neurilemma. The medullary sheath does not extend con- 

 tinuously along the nerve ; it is broken at intervals, termed 



Fig. 124. — Normal and Degener- 

 ated Nerve Fibres (Barker, 



AFTER THOMA). 



I. Normal fibre : S, neurilemma ; m, 

 medullary sheath ; A , axis cylinder ; 

 L, Lantermann's line or cleft ; 

 R, node of Ranvier. II. degenerating 

 fibres : mt, drops of myelin ; a, re- 

 mains of axis cylinder. III. further 

 stage of degeneration : mt, drops of 

 myelin ; w, proliferating cells of 

 neurilemma. 



