130 HUMAN PHYSIOLOGY. 



and the calamus scriptorius. The vasomotor fibres having their origin in 

 this centre descend through the interior of the cord, emerge through the 

 anterior roots of spinal nerves, enter the ganglia of the sympathetic, and 

 thence pass to the walls of the blood vessels, and maintain the arterial 

 tonus ; they may be divided into two classes, viz.; vaso-dilators, e.g., 

 chorda tympani, and vase-constrictors, e.g., sympathetic fibres. 



Division of the cord at the lower border of the medulla is followed by a 

 dilatation of the entire vascular system and a marked fall of the blood pres- 

 sure. Galvanic stimtdation of the divided surface of the cord is followed 

 by a contraction of the blood vessels and a rise in the blood pressure. 



The vasomotor centre is stimulated directly by the condition of the 

 blood in the medulla oblongata. When it is highly venous it becomes very 

 active and the blood vessels throughout the body are contracted and the 

 blood current becomes swifter; sudden anaemia of the medulla has a similar 

 effect. This centre may be increased in action with attendant rise of 

 blood pressure, by irritation of certain afferent nerve fibres. These are 

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

 by other afferent fibres with attendant fall of blood pressure. These are 

 known as depressor fibres. 



9. A Diabetic centre, irritation of which causes an increase in the amount 

 of urine secreted, and the appearance of a considerable quantity of sugar. 



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

 nerves, presides over the movements of respiration and its modifications, 

 laughing, sighing, sobbing, sneezing, etc. It may be excited reflexly by 

 the presence of carbonic acid in the lungs irritating the terminal pneumo- 

 gastric filaments ; or automatically, according to the character of the blood 

 circulating through it ; an excess of carbonic acid or a diminution of oxygen 

 increasing the number of respiratory movements ; a reverse condition dimin- 

 ishing the respiratory movements. 



11. A Spasm centre, stimulation of which gives rise to convulsive phe- 

 nomena, such as coughing, sneezing, etc. 



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

 eyelids and dilatation of the pupil. 



13. A Sweat centre is also localized in the medulla. 



NERVOUS CIRCLE OF RESPIRATION (ENTIRELY REFLEX), 

 p, . f Pulmonary branches of the pneumogastric. 



or I Superior laryngeal. 



n . \ Trifacial, or 5th pair. 



Nerves Nerves of &^ sensibilit y- 



[ Sympathetic nerve. 



