THE MEDULLA OBLONGATA. 177 



may however be excited or inhibited in its activity by nerve impulses 

 reflected from the periphery by stimulation of various afferent nerves. 

 The heart beat will in consequence be slowed or increased in its rate by 

 the action of the cardio-accelerator center. 



8. A vaso-motor center, which, by alternately contracting and dilating the 

 blood-vessels through nerves distributed in their walls, regulates the 

 quantity of blood distributed to an organ or tissue, and thus influences 

 nutrition, secretion, and calorification. The vaso-motor center is situated 

 in the medulla oblongata and pons Varolii, between the corpora quadri- 

 gemina and the calamus scriptorius. The vaso-motor fibers having their 

 origin in this center descend through the lateral column of the cord, emerge 

 through the ventral roots of the thoracic or upper lumbar nerves, enter the 

 ganglia of the sympathetic, and thence pass to the walls of the blood-vessels, 

 and maintain an arterial tonus; they may be divided into two classes viz., 

 vase-dilatators and vase-constrictors. 



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. 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 be- 

 comes 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 vasomotor center may be increased, with attend- 

 ant rise of blood pressure, 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. 



Q. 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 modifications, laughing, 

 singing, sobbing, sneezing, etc. It may be excited reflexly by stimulation 

 of the terminal branches of the vagus nerve during an act of expiration; 

 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 diminishing the 

 respiratory movements. 



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

 such as coughing, sneezing, etc. 



