196 HUMAN PHYSIOLOGY. 



arterial tonus ; they may be divided into two classes viz., vaso- 

 dilators (e. g., chorda tympani) and vaso-constrictors (e. g., 

 sympathetic fibers). 



Division of the cord at the lower border of the medulla is fol- 

 lowed by a dilatation of the entire vascular system and a marked 

 fall of the blood pressure. Electric stimulation of the distal surface 

 of the cord is followed by a contraction of the blood-vessels and a 

 rise in the blood pressure. 



The vaso-motor center is stimulated directly by the condition of 

 the blood in the medulla oblongata. When the blood is highly venous 

 this center becomes very active, the blood-vessels throughout the 

 body are contracted, and the blood current becomes swifter ; sudden 

 anemia of the medulla has a similar effect. The action of the vaso- 

 motor center may be accelerated, with attendant rise of blood pres- 

 sure, by irritation of certain afferent nerve-fibers. These are known 

 as pressor fibers. On the other hand, its action may be depressed 

 by other fibers, with attendant fall of blood pressure. These are 

 known as depressor fibers. 



9. A diabetic center, irritation of which causes an increase in the 

 amount of urine secreted and the appearance of a considerable 

 quantity of sugar in the urine. 



10. Respiratory center, situated near the origin of the pneumogastric 

 nerves, presides over the movements of respiration and its modifi- 

 cations, laughing, singing, sobbing, sneezing, etc. It may be excited 

 renexly by the presence of carbonic acid in the lungs irritating the 

 terminal pneumogastric filaments ; or automatically, according to the 

 character of the blood circulating through it ; an excess of car- 

 bonic acid or a diminution of oxygen increasing the number of 

 respiratory movements ; a reverse condition diminishing the respi- 

 ratory movements. 



11. A spasm center, stimulation of which gives rise to convulsive 

 phenomena, such as coughing, sneezing, etc. 



12. A center for certain ocular functions, governing the closure of 

 the eyelids and dilatation of the pupil. 



13. A sweat center is also localized in the medulla. 



