x AFFERENT AND EFFERENT NERVES 175 



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 in- 

 creased secretion of gastric juice. The secretion of other 

 glands is regulated in the same way. In some cases, how- 

 ever, 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 impulses 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. 



PRACTICAL DIRECTIONS. 



I- The Central NervOUS System (Fig. 6). Lay bare the brain 

 and spinal cord as directed on p. 33, noting the dura mater and pia 

 mater : the latter is densely pigmented over parts of the brain. The 

 specimen in 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 

 \fmgdorsal and ventral roots of the latter (compare Fig. 52) which pass 

 backwards for some distance before making their exit from the neural 

 canal ; and also the ganglia on the dorsal roots, lying just outside the 

 canal and each hidden in a whitish calcareous body in this region 

 (Fig. 51, c). (The ganglia, however, can be more easily made out at a 

 later stage.) Then sever the nerves very carefully from the brain and 



