86 



LECTURE III. 



tained in them in the form of circular or longitudinal 

 layers. A small vein or a small artery can contract only 

 in proportion to the quantity of muscle with which it is 

 provided, and they are only distinguished by the circum- 

 stance that either the longitudinal or the transverse mus- 

 cular layers are the more strongly developed. 



I have called your attention to this point because you 



can see from it, how a simple anatomical discovery may 

 supply the most important information with regard to 



Fig. 26. Small artery from the base of the cerebrum after the application of acetic 

 acid. A. Small trunk ; B and C, larger branches ; D and E, branches of the 

 smallest size (capillary arteries), a, a. External coat, with nuclei, which run in the 

 direction of the length of the vessel, and are seen first in a double and afterwards 

 in a single layer, with a striated basis-substance ; at D and E the coat is reduced 

 to a single layer, with longitudinal nuclei, which here and there have been replaced 

 by masses of fat-granules (fatty degeneration). b, b. Middle coat (circular fibrous, 

 or muscular, coat), with long, cylindrical nuclei, which run transversely around the 

 vessel, and at its borders (where they look as though they had been cut across) 

 present the appearance of round bodies ; at D and E transverse nuclei of the middle 

 coat becoming continually scarcer, c, c. Internal coat, at D and E with longitudi- 

 nal nuclei. 300 diameters. 



