THE NERVOUS SYSTEM 



111 



strated, but there seems to be no vasodilator center. 

 Changes in the size of the blood vessels are caused by 

 the constriction of the muscular coat to reduce the size 

 and relaxation to increase size. Constriction dimin- 

 ishes and dilatation increases the quantity of blood in 

 the area of distribution of the vessels involved. The im- 

 pulses are carried by the sympathetic autonomic sys- 



Fig. 26. Diagram of section of spinal cord, showing tracts. (After K61- 

 liker) ; g, posterior median, and b,) postero-lateral columns; p.c., crossed 

 pyramidal, and p.d., direct pyramidal tracts; f, cerebellar tract. (After 

 Ho well.) 



tern and are not under the control of the conscious will, 

 though the higher centers of the brain can excite them 

 as may be seen in blushing a vasodilatation of the ves- 

 sels of the face. Ordinarily the center is excited re- 

 flexly, as when exposure to cold causes a constriction of 

 the vessels in the part affected. Fright, grief, the recep- 

 tion of bad news may cause the temporary unconscious- 



