150 HUMAN PHYSIOLOGY. 



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. Galvanization ot 

 the upper end of the divided nerve causes all of the preceding 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 fol- 

 lowed by a dilatation of the vessels, an increased rapidity of the circulation, 

 and an elevation of temperature in the anterior and posterior limbs ; gal- 

 vanization of the peripheral ends of these nerves causes all of these phe- 

 nomena 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- obi ongata, 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 

 it ceases to dominate the medulla oblongata and the spinal cord. These 

 centres then act more vigorously, and diminish the calibre of the cerebral 

 blood vessels through the action of the vasomoter nerves, producing a con- 

 dition of physiological ancemia and sleep; during this state waste products 

 are removed, force is stored up, nutrition is restored, and waking finally 

 occurs. 



