ARTEEIES 



183 



MUflCLC 9COMCMT 



- - AOVBNTITIA 



FIG. 202. SEMI-DIA- 

 GRAMMATIC ILLUS- 

 TRATION OF SMALL 

 BRANCH OF PULMO- 

 NARY ARTERY OF 

 Ox. 



(After Piana.) X70. 



more or less complete third layer of scattered bun- 

 dles of longitudinally .arranged smooth muscle cells. 

 The umbilical vein is very similar but contains more 

 elastic fibers, and a distinct internal elastic mem- 

 brane. 



The cerebral and meningeal arteries have very 

 thin walls and, exclusive of a relatively very well 

 developed internal elastic membrane, contain but 

 little elastic tissue. 



The iliac, splenic, renal, superior mesenteric and 

 dorsalis penis contain scattered longitudinal bundles 

 of muscle in the media next the intima. 



In the pulmonary arteries the media is excep- 

 tionally well developed. This is the case to an ex- 

 treme degree in the pulmonary arterioles of the cat. 

 The pulmonary arteries and veins are very similar 

 in structure. In the guinea pig and opossum the 

 media of the arterioles consists throughout of thick 

 oval segments of circularly disposed smooth muscle 



alternating with narrow intervals where the muscle layer is relatively thin. 

 In ox, sheep and pig such segmented condition of the media is modified 

 in that the segmentation is spirally disposed. 



The media of the roots of the aorta 

 and the pulmonary artery consists largely 

 of cardiac muscle. 



In the subclavian artery the longitu- 

 dinal surpasses the circular ftiuscle in the 

 media. In the arch of the aorta and in 

 the upper portion of the descending aorta 

 longitudinal muscle bundles are found in 

 the intima, media and adventitia (von 

 Bardeleben). The common carotid, com- 

 mon iliac and common femoral (cruralis) 

 contain both longitudinally and spirally 

 arranged muscle fibers in the media. In 

 general, where large arteries are subjected 

 to bendings the circular muscle fibers are 

 reinforced by oblique (spiral) and longi- 

 tudinal bundles in the media. This is 

 conspicuously the case in the common 

 iliacs, the popliteal, and the brachial ar- 

 teries (MacCordick, Anat. Anz., 44, 11, 

 1913). 



FIG. 203. SEMI-DIAGRAMMATIC 

 ILLUSTRATION OF DIVIDING 

 SMALL BRANCH OF PULMONARY 

 ARTERY OF GUINEA-PIG. 



Pulmonary arterioles of opos- 

 sum are almost identical. X 50. 



