902 CENTRAL NERVOUS SYSTEM 



muscular acts. For this purpose the diffuse activity of this system is 

 an obvious advantage. 



In this connection the relation of the adrenal medulla to the thoracico- 

 lumbar outflow is of interest. Its cells are derived in the embryo from 

 the same neuroblasts which give rise to the effector neurons of the sym- 

 pathetic ganglia. They are innervated directly by the connector neurons 

 of the thoracico-lumbar outflow so that they are strictly homologous 

 with the effector neurons of this system. It has been shown that without 

 exception the effects produced on any organ by epinephrine, the secre- 

 tion of these glands, is the same as that produced by the excitation of its 

 thoracico-lumbar innervation. These gland cells may be considered as 

 nerve cells which have changed their mode of acting on their effectors, but 

 still produce the same effect through their secretion. If these glands are 

 brought into action, as Cannon 21 has shown them to be during emotional 

 conditions, they will reinforce the action of the thoracico-lumbar nerves 

 and because their hormone is distributed through the blood, ensure the 

 general, diffuse response characteristic of the action .of the thoracico- 

 lumbar system. 



It must not be supposed that the reactions of the thoracico-lumbar 

 system always has this diffuse character. The regulation of vasomotor 

 reactions is largely carried out through its neurons, and it is possible 

 for vasoconstriction to occur in one part of the vascular bed without 

 affecting other parts. The shifting of the mass of the circulating blood 

 from one part of the body to another in response to the varying needs 

 occasioned by the varying conditions of each organ must be controlled by 

 these nerves. The local reflex response of the cutaneous blood vessels to 

 temperature changes is a case in point (page 744). 



The Effects of Impulses from the Viscera upon Central Nervous Ac- 

 tivity. Just as the visceral organs may be inhibited through the thora- 

 cico-lumbar outflow in adaptation to the activity of the skeletal muscles, 

 so visceral conditions may influence the state of the central nervous 

 system and the muscles which it controls. We have considered the effects 

 of afferent impulses from the viscera in producing pain (page 864). In 

 the presence of visceral disease afferent impulses tend to produce " de- 

 pression" in the central nervous system, so that muscular activity is 

 avoided and the sufferer seeks retirement in which to recover. The dis- 

 agreeable mental conditions produced by constipation are probably also 

 nervous in origin, as is indicated by their prompt relief following a satis- 

 factory movement of the bowels. The vigorous contractions of the stom- 

 ach musculature in hunger not only give rise to this sensation, but make 

 the subject restless and irritable. Certain reflex acts are reinforced by 

 impulses set up during hunger, as the knee jerk. The cough attending 

 diseases of the pulmonary region is believed by Pottenger 18 to be due to 



