500 HANDBOOK OF PHYSIOLOGY. 



they are bilateral. As has already been pointed out, these centres may 

 be directly or reflexly stimulated, as well as by impressions conveyed 

 downward from the cerebrum to the medulla. The condition of the 

 blood circulating in them is the direct stimulus. Its influence is no 

 doubt a tonic or else a rhythmic one. It is also supposed that there is 

 iu the medulla a special vaso-dilator centre not acting tonically, stimu- 

 lation of which produces vascular dilatation. The diabetic centre is 

 probably a part of the vaso-motor centre, at any rate stimulation of it 

 causes dilatation of the vessels of the liver. 



(5.) Bilateral chief centres for the secretion of Sweat exist in the 

 medulla. The centres on either side control the subsidiary spinal sweat 

 centres. They may be excited unequally so as to produce unilateral 

 sweating. They are probably automatic and reflex. 



(6.) Bilateral Spasm centres are said to be present in the medulla, 

 011 the stimulation of which, as by suddenly produced excessive venosity 

 of the blood, general spasms of the muscles of the body are produced. 



(c.) Control centres. These are centres whose influence may be 

 directed to controlling the action of subsidiary centres. They are 



(1.) The Respiratory centres, which probably control the action of 

 other subordinate centres in the spinal cord. 



(2.) The Car dio- Inhibitory centres, which act upon a local ganglionic 

 mechanism in the heart. 



(3.) The Accelerator centres, if they exist, probably act through a 

 local mechanism in the heart. 



(4.) The Vaso-motor centres control spinal as well as local tonic 

 centres. 



(5.) The medullary Sweat centres control the spinal sweat centres. 



(d.) Tonic centres. Of the centres whose action is tonic or con- 

 tinuous up to a certain degree, may be cited the vaso-motor and the car- 

 dio-inhibitory. 



It should not be forgotten that in the medulla are the centres for 

 the special senses, Hearing and Taste, and that other special centres are 

 supposed to be localized there, of which may be mentioned one, the 

 hypothetical Inhibitory heat centre, which controls the production of 

 heat by the tissues, independently of the vaso-motor centre. 



The Cranial Nerves. 



The cranial nerves consist of twelve pairs ; they appear to arise (su- 

 perficial origin) from the base of the brain in a double series, which 

 extends from the under surface of the anterior part of the cerebrum to 

 the lower end of the medulla oblongata. Traced into the substance of 

 the brain and medulla, the roots of the nerves are found to take origin 

 from various masses of gray matter. 



