AFFERENT AND EFFERENT NERVES 175 



of slight contraction. By various circumstances these 

 stimuli may be diminished for any given artery and at 

 the same time stimuli pass down another kind of 

 vaso-motor fibres : the artery will then dilate and the 

 blood-supply of the organ to which it is distributed will 



>e temporarily increased. For instance, the presence of 



:ood in the stomach acts, through the central nervous 

 system, upon the cceliac branch of the splanchnic artery, 

 causing a dilatation of its capillaries and promoting an 



ncreased secretion of gastric juice. The secretion of 

 other glands is regulated in a similar way. In some 

 cases, however, it has been proved that the nerves of a 

 gland do not act simply by producing dilatation of the 

 capillaries, but have a direct effect upon the gland-cells, 

 causing an increased secretion. 

 You will thus note that there are nerve-fibres carrying 



mpulses to the central nervous system which have 

 nothing to do with sensation, and fibres carrying im- 



>ulses from the central nervous system which have 

 nothing to do with motion, but result in increased 

 secretion or in stoppage of motion. It is therefore best 



o use the term afferent (which includes sensory) for a 

 nerve carrying an impulse to the brain and spinal cord, 

 and efferent (including motor) for one carrying an impulse 

 in the other direction (p. 166). 



PRACTICAL DIRECTIONS 



I. The Central Nervous System (Fig. 6). Lay bare the 

 )rain and spinal cord as directed on p. 34, noting the dura 

 nater and pia mater : the latter is densely pigmented over 

 )arts of the brain. The specimen on which this operation 

 las already been performed will do, if the dissection has 

 Deen done carefully. 



Observe the origins of the cerebral and spinal nerves, 

 toting the long dorsal and ventral roots of the latter (compare 



