DIRECTIONS FOB LABORATORY WORK 785 



tunica externa. Sketch h.p. (147) a segment of the wall, noting the 

 fundamental tissues in each tunic. Note the longitudinal muscle bun- 

 dles in the tunica externa. Compare with portal vein; with hepatic 

 vein. 



(b) The Superior Vena Cava. Sketch h.p. (148) a segment of a 

 section stained with a specific elastic tissue stain noting the distribution 

 of elastic fibers. Compare with pulmonary vein. 



(3) VALVES. Study a longitudinal stained section of a large peri- 

 pheral vein of the lower extremities (e.g., long saphenous vein). Iden- 

 tify the valves, and note their distribution, position with respect to 

 tributaries, and their gross and minute structure. Sketch (149) a seg- 

 ment of the wall, including a valve. How many cusps are generally 

 included in a valve? Why are valves less numerous in the veins of the 

 upper than lower extremities, and why are they entirely lacking in the 

 abdominal and thoracic veins, and in veins of smaller calibre than 2 

 mm.? What specific structural differences obtain among the following 

 veins : venae cavae, mesenteric, external jugular, pulmonary, umbilical 

 and cranial? What is the functional significance of these differences? 



(C) SMALL VASCULAR CO MITES. 



(1) SMALL ARTERIES AND VEINS. Many sections will contain pairs 

 of smaller vessels. Study a larger pair in a stained section, and make 

 detailed comparison with respect of relative gross size and form, con- 

 dition of lumen, thickness of wall relative to diameter of bore, and rela- 

 tive thickness of the several tunics. What can you determine regarding 

 the presence of elastic membranes? Make (a) l.p. sketch of pair (150) ; 

 (b) h.p. sketch of segment of wall of each (151). 



(2) ARTERIOLES AND VENULES. (a) Find a pair of vascular comites 

 of this size in almost any stained section. Make h.p. sketch (152), not- 

 ing especially relative thickness of the several tunics in the vessels of 

 the pair. 



(b) In a stained toto mount of the mesentery or the pia mater find 

 a typical pair of comites and trace from arteriole and venules through 

 precapillary twigs into the capillaries. Sketch (153) such a system, 

 noting the differential marks between arteriole and venule, precapillary 

 artery and vein; and between the foregoing and capillaries. What are 

 sinusoids? Eetia mirabilia? How do they differ from capillaries? 



(c) In stained sections identify precapillary arteries and veins and 

 capillaries. Sketch (154). What is the chief differential feature be- 

 tween a precapillary artery and vein, and between these and a capillary? 



