HISTOLOGY. 63 



The veins are different from the arteries in that they 

 have thinner coats and less muscle tissue; their walls 

 are thinner than the arteries and their lumen is larger. 

 They have more fibrous tissue than the arteries. Ar- 

 teries are composed of three coats. The tunica intiw*, 

 the tunica media, and the tunica adventitia. The in- 

 ternal coat is lined with endothelial cells resting upon 

 a basement membrane of white fibrous tissue. Just 

 under this coat is located a band of yellow elastic 

 tissue, called the fenestrated membrane of Henle. 

 The middle coat is composed of the smooth muscle 

 running around the artery, and having a few fibers of 

 fibrous tissue interspersed with the muscle cells, es- 

 pecially the large arteries. The outer coat is com- 

 posed of white and yellow elastic tissue in abundance. 

 The nerve supply of the artery is derived from the 

 vasomotor system, and the blood supply of a part is 

 regulated by the contraction or expansion of the mus- 

 cle fibers in the middle coat of the artery. The outer 

 coat is for strength ; the inner one for smoothness ; the 

 middle one for contraction and expansion. All the 

 blood vessels are of mesoblastic origin. The vessels 

 are closed tubes. 



Exercise No. 39. Make a section of a blood vessel^ 

 mount, stain and preserve. 



