56 CIRCULATORY SYSTEM. 



Endothelial outlines in blood vessels. Intestine of a small 

 mammal, injected with AgN0 3 (27) and p. in methylated spirit. 

 Spread the opened intestine peritoneal surface downwards on the slide, 

 and scrape away the mucous coat with the back of a scalpel, clear 

 witli clove oil, mount in balsam. 



(L) A branching system of blood vessels in which the arteries 

 are smaller than the veins, their branches being united by capil- 

 laries. (//) Endothelial outlines of the arteries are more fusifom 

 than those of the veins. Trace the passage of the arteries 

 into the capillaries. Observe that the cell territories in the 

 latter are larger and somewhat jagged. Around the arteries 

 perivascular lymphatics are sometimes observed. Their endo- 

 thelial ceils have sinuous outlines. 



Aorta. T.S. (p. 3, s. 19, c. P., m. B.) (L) The middle coat is 

 the predominant structure, and consists largely of elastic tissue 

 in circularly distributed*networks interconnected in all directions. 

 In its meshes lie muscular fibres. On the inner aspect is 

 the thin intima, and externally is the adventitia, with the 

 same structure as before. The abundant elastic tissue in 

 the middle coat is the characteristic feature in the large artery. 



Sinus of valsalva. L.S. Man (p. 11, c. G., s. 19, m. F.). (L) 

 Recognise the aorta, trace it to its junction with the base of the 

 ventricle, a portion of which is included in the section, the 

 junction is effected by the intervening connective tissue of the 

 Tendo cordis, stained pink. On one side of this junction there is 

 loose connective tissue which leads into the substance of the semi- 

 lunar valve, which consists of open connective tissue, more com- 

 pact on its ventricular aspect, and covered on both sides with 

 endothelium. The recess between the aorta and the valve is the 

 sinus. Note that the elastic tissue of the wall of the aorta thins 

 down, and is replaced by pink-stained connective tissue where 

 it forms the wall of the sinus. (H) Having noted the thinning 



