316 THE CIRCULATION OF THE BLOOD. [CHAP. XXVIII. 



to the opinion which prevailed to the time of Herophilus that it 

 contained vital spirits or air (spiritus irvev^a] during life, and 

 the arteries were called irvevfiariKa ayyeia* 



Arteries are cylindrical tabes, whose walls are formed mainly by 

 a highly elastic material, whereby the cylindrical form is preserved 

 and the collapse of the tube is prevented. For the same reason 

 when an artery is cut across, its mouth is patulous, and remains so. 



The walls of arteries consist of three different textures : First, 

 the external tunic, composed of areolar tissue, and commonly called 

 the cellular coat: secondly, the middle coat, or fibrous tunic; 

 and thirdly, the epithelial tunic. 



The external tunic is that through the medium of which the artery 

 is connected with neighbouring structures, and it also forms a nidus 

 for the support of the nutrient blood-vessels of the arterial wall. 

 These minute vessels, named vasa vasorum, are derived from neigh- 

 bouring arteries; they ramify freely in the external tunic, and 

 send minute branches to a certain depth in the wall of the artery. 

 In a well-injected subject, they may be seen filled with injection 

 on all the larger arteries, and when great vascular congestion has 

 accompanied or preceded death, these vessels participate in the 

 general plethora, and may be seen distended with blood on the 

 aorta and its larger branches. 



In some of the larger arteries, a few pellets of fat may be found 

 in -the outer layers of the external tunic, which consist of very 

 loose areolar tissue : the inner layers of this tunic are, however, 

 very condensed, and adapt themselves closely to the middle coat of 

 the artery to which they adhere intimately, probably by reason of 

 the continuity of some of their fibres with those of the middle tunic. 

 The same elements are found in the external coat of arteries, as in 

 areolar tissue elsewhere, namely, the white and yellow fibrous tissue, 

 but the former predominates in quantity so much that in some 

 situations it seems to be the sole constituent of the tunic. 



The extensibility, toughness, and power of resistance which this 

 tunic enjoys, by reason of the large quantity of white fibrous tissue 

 which it contains, adapt it admirably as the external investment of 

 the arterial tube. It serves to give mechanical support to the 

 other tunics, and being the medium in which the nutrient blood- 

 vessels are distributed, it contributes to a certain extent to their 

 nutrition. Hence there is no other tunic, the loss of which, an 



* This idea respecting the office of the arteries is thus expressed by Cicero. 

 " Spiritus ex pulmone in cor recipitur et per arterias distribuitur, sanguis per 

 venas." De Nat. Deor., L. ii. 



