THE VASOMOTOR REFLEXES. 193 



a fall in the general blood pressure. Like phenomena are often 

 seen following fright, pain, grief, and excitement. The patient 

 becomes suddenly pale, dizzy, and may faint, losing conscious- 

 ness entirely. This is due to a fall in the arterial blood pressure 

 produced by a temporary inhibition of the vasoconstrictor nerves 

 and perhaps also by a slowing of the heart, due to vagus stimula- 

 tion. If the person be standing, the blood naturally flows to the 

 vessels of the abdominal viscera and dependent portions of the 

 body, and the brain is thereby rendered bloodless. The treat- 

 ment of these cases is to elevate the feet and abdomen and to 

 lower the head. 



In case the depression of the blood pressure slowly develops 

 because of the gradual onset of fatigue in the vasomotor and 

 other nervous centers, a condition known as surgical shock super- 

 venes. The treatment of this condition demands plenty of air, 

 stimulants, saline or blood transfusion, and measures to main- 

 tain the body temperature. 



The Pressure Effects of Gravity on the Blood Flow vary 

 according to the posture of the body. In the upright position 

 the blood vessels of the feet support a column of blood of rela- 

 tively great height, but when the individual is lying down this 

 ceases to be the case. In spite of this, by means of the delicate 

 adjustments which the nervous system can bring about in the 

 heart and the blood vessels, there is little difference in the pres- 

 sure of the blood in the arteries in any position which the person 

 may assume. The blood vessels and nerves soon lose this power 

 if it is not continuously exercised. This is illustrated in patients 

 who have been confined to their beds for a time. If they try to 

 walk or to stand up suddenly, they become very dizzy and may 

 faint, which means that the blood has left the vessels of the 

 brain and is gathered by the force of gravity in the vessels of 

 the dependent parts of the body. With a normal vasomotor 

 mechanism, the vessels of the feet and viscera would quickly 

 constrict to such an extent that the blood pressure would remain 

 at its normal height in the vessels of the brain. 



The fact that stimulation of sensory nerves by the gross meth- 

 ods of the laboratory results in very profound changes in the 



