x AFFERENT AND EFFERENT XERVES 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 

 be temporarily increased. For instance, the presence of 

 food in the stomach acts, through the central nervous 

 system, upon the coeliac branch of the splanchnic artery, 

 causing a dilatation of its capillaries and promoting an 

 increased 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 

 impulses to the central nervous system which have 

 nothing to do with sensation, and fibres carrying im- 

 pulses 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 

 to 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 

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

 mater and pia mater : the latter is densely pigmented over 

 parts of the brain. The specimen on which this operation 

 has already been performed will do, if the dissection has 

 been done carefully. 



Observe the origins of the cerebral and spinal nerves, 

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



