156 



MANUAL OF HISTOLOGY. 



color of the blood is seen through their semitranslucent walls. 

 Finally, they branch more frequently than the arteries. Three 

 main coats or tunics enter into the composition of most veins. 



These resemble the corre- 

 sponding arterial struc- 

 tures, and have likewise 

 received the names of in- 

 tima, for the internal endo- 

 thelial lining ; media, for 

 the middle muscular ; and 

 adventitia, for the external 

 connective-tissue coat. 



Veins, however, differ 

 from arteries in the feebler 

 development of their mus- 

 cular coat, in the compara- 

 tive paucity of elastic ele- 

 ments, a greater laxity 

 of their intima, and the 

 presence in some of valves. 

 We may distinguish veins of smaller calibre, or venules, 

 from the vessels of medium and large size. The venules, like 

 the arterioles, in certain respects resemble the capillaries. As 



FIG. 70. Portion of innominate vein of dog, after in- 

 jection of a solution of silver nitrate. The endothelial 

 cells and their nuclei are visible. The media shines 

 through this layer. 



B S 



FIG. 71. Arteriole and vennle from child's mesentery, treatment by acetic acid and glycerine: A, ar- 

 tery ; a, nucleus of muscle-cell of media ; ft, same in transverse section (optical). B, vein ; c. nucleus of 

 connective tissue constituting media, which in these minute veins contains no muscle-cells ; tf, nucleated 

 connective tissue. 



it may become important to differentiate the minuter forms 

 of vessels, we will here briefly indicate the main points of dis- 

 tinction between full-sized capillaries, small veins, and arte- 



