468 NEUROLOGY. 



are plentiful in the spinal nerves. Some nerve fibres have no 

 termination, but form a loop by joining with others in the same 

 bundle, and so return to the axis ; while others, as the auditory 

 and optic nerves, have end organs of great complexity. 



Nerves which convey impressions to the centres are termed 

 centHpetal or afferent ; those which transmit stimuli from the 

 centres to the various organs, centrifugal or efferent ; while 

 those which pass from one centre to another are called inter - 

 central. 



Centrifugal nerves are called motor when they stimulate mus- 

 cular contraction, and if this contraction be in the coats of vessels, 

 the nerves are then vaso-motor ; when they stimulate the secre- 

 tion of glands, they are called secretory, and when they have the 

 power of diminishing function, . inhibitory ; some nerves are 

 believed to control the process of niftrition in the parts they 

 supply, and have been named trophic. 



Centripetal nerves which convey ordinary feeling are termed 

 sensory ; when they convey impressions peculiar to an organ, as 

 the nerves of sight and hearing, they are known as nerves of 

 special sense. Nerves conveying impressions which lead to the 

 stimulation of motor nerves are termed excito-motor ; if to that 

 of secretory nerves, excito -secretory ; and if to that of inhibitory 

 nerves excito-inhibitory. This reflection of nervous impulse from 

 one nerve to another is known as reflex action. 



CEREBRO-SPINAL SYSTEM. 



The centre of this system is the cerebrospinal axis, consisting 

 of the brain and spinal cord, which like their coverings or 

 meninges, are continuous with each other. 



The cerebro-spinal axis is invested by three distinct membranes 

 — viz., the dura mater externally, the arachnoid in the middle, 

 and the pia mater internally. 



MENINGES OF THE SPINAL CORD. 



The dura mater of the cord is a strong, inelastic membrane, 

 composed of intersecting bands and laminae of white fibrous tissue. 

 It extends from the foramen magnum, to which it is attached, and 

 where it is continuous with the dura mater of the brai-D, to the 

 posterior extremity of the neural canal, where, as a slender cord. 



