VEINS OF MAMMALIA. 



549 



424 



Epithelium from vena cava of Sheep, com". 



is followed by fasting, by long intermission of the supply of 

 chyme ; consequently the provision for the arterial supply is 

 simplified, and at the same time adapted to the more rapid assimi- 

 lation which the hungry or famished frame requires. 



Modifications of the arterial supply of the mammary glands, 

 exemplified by large epigastric and subcutaneous abdominal 

 branches, anastomosing with the internal and thoracic-mammary 

 arteries, accompany the position and extension of the mammary 

 glands from the thoracic to 

 the inguinal regions in many 

 quadrupeds. 



The ultimate capillaries of 

 the arterial ramifications 

 either open directly into 

 venous capillaries, or into 

 sinuses, as in erectile and 

 uterine structures, whence 

 the venous capillaries begin. 

 In exceptional cases, as in 

 the Bat's wing, arteries of 

 the second and third order of 

 branches have been observed to pass into veins of corresponding 

 size, without the intermedium of capillaries. 1 



349. Veins of Mamma- 425 



lia,- -The delicate structure- 

 less coat of the capillaries ^^^^^^^p 

 present, as the venules en- &S$. 

 large, an epithelial lining of 

 flat, usually rhomboid, nu- 

 cleate scales, fig. 424. The 

 middle tissue of the vein 

 includes sparing delicate un- 

 striped fibres in an abundant 

 bed of connective or areolar 

 tissue, in which maybe distin- 

 guished an internal stratum 

 of wavy longitudinal fibres, 

 fig. 425 , a middle stratum 

 of intermixed circular and 



lOngltUClinal fibres OI elastic Longitudinal vertical section of wall of subclavian vein 



tissue, imbedded in a nidus of an Ox : magn " 20 diara ' CCI11 "' 



of white or contractile fibres, b, and these degenerating into an 



1 cxcvn". p. 968. 



