286 Elementary Biology. 



arteriosus, by which pure blood leaves the heart to pass to 

 the tissues. It very soon branches into a number of large 

 arteries afterwards to be specified. Lastly, one vessel opens 

 directly into the left auricle, while the sinus venosus mani- 

 festly is connected with the right auricle. 



Before entering into further details with regard to the 

 circulatory system it will be necessary for us to understand 

 clearly the nature of the three important sets of vessels con- 

 cerned in the transport of the blood, namely, the arteries, 

 veins, and capillaries. 



Arteries and veins have fundamentally the same struc- 

 ture, but they differ in the relative amounts of the 



FIG. 149. TRANSVERSE SECTION OF THE WALL OF A TYPICAL ARTERY. 

 (Quain.) 



c 



& ?". 



"' -' : '" \^ 



a, epithelial layer ; b, elastic membrane ; c, tunica media ; d, tunica adventitia. 



separate elements entering into the composition of their 

 walls. The wall consists of an internal epithelial lining of 

 flat squames followed by a sub- epithelial membrane com- 

 posed of elastic fibres : this is known as the tunica intima. 

 The tunica media consists of a thick layer of circularly 

 arranged muscle-fibres of the non-striped variety, plentifully 

 mingled with delicate elastic fibres. The whole is strengthened 

 and attached to adjacent parts by a tunica adventitia com- 

 posed of connective tissue in which all three elements of 

 that tissue cellular, fibrous and elastic are well represented. 

 The veins differ from the arteries chiefly in two points ; 

 first, in that the various coats are thinner ; and secondly, 



