410 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



ular layer is the most vascular. The arterioles have 

 thin walls, and in old age may become brittle and 

 may easily rupture. 



No lymphatic vessels with definite walls have been 

 discovered in the central nervous system. The 

 blood-vessels are, however, surrounded by peri- 

 vascular spaces which probably function as lymph 

 channels. 



