132 HUMAN PHYSIOLOGY. 



less decided than in the cerebro- spinal system. Irritation of the ganglia 

 does not produce any evidence of pain until some time has elapsed. If 

 caustic soda be applied to the semilunar ganglia, or a galvanic current be 

 passed through the splanchnic nerves, no instantaneous effect is noticed, as 

 in the case of the cerebro-spinal nerves ; but in the course of a few seconds 

 a slow, progressive contraction of the muscular coat of the intestines is 

 established, which continues for some time after the irritation is removed. 

 Division of the sympathetic nerve in the neck is followed by a vascular 

 congestion of the parts above the section on the corresponding side, attended 

 by an increase in the temperature ; not only is there an increase in the 

 amount of blood, but the rapidity of the blood current is very much 

 hastened, and the blood in the veins becomes of a brighter color. Gal- 

 vanization of the upper end of the divided nerve causes all of the preced- 

 ing phenomena to disappear ; the congestion decreases, the temperature 

 falls, and the venous blood becomes dark again. 



The sympathetic exerts a similar influence upon the circulation of the 

 limbs and the glandular organs ; destruction of the first thoracic ganglion 

 and division of the nerves forming the lumbar and sacral plexuses is 

 followed by a dilatation of the vessels, an increased rapidity of the circu- 

 lation, and an elevation of temperature in the anterior and posterior 

 limbs ; galvanization of the peripheral ends of these nerves causes all of 

 these phenomena to disappear. Division of the splanchnic nerve causes a 

 dilatation of the blood vessels of the intestine. 



These phenomena of the sympathetic nerve system are dependent upon 

 the presence of vasomotor nerves which, under normal circumstances, 

 exert a tonic influence upon the blood vessels. These nerves, derived 

 from the cerebro-spinal system, the medulla oblongata, leave the spinal 

 cord by the rami communicantes, enter the sympathetic ganglia, and 

 finally terminate in the muscular wall of the blood vessels. 



Sleep is a periodical condition of the nervous system, in which there is 

 a partial or complete cessation of the activities of the higher nerve centres. 

 The cause of sleep is a diminution in the quantity of blood, occasioned by 

 a contraction of the smaller arteries under the influence of the vasomotor 

 nerves. 



During the waking state the brain undergoes a physiological waste, as a 

 result of the exercise of its functions ; after a certain length of time its 

 activities become enfeebled, and a period of repose ensues, during which 

 a regeneration of its substance takes place. 



When the brain becomes enfeebled there is a diminished molecular 

 activity and an accumulation of waste products ; under these circumstances 



