36 THE CIRCULATORY SYSTEM. 
titia or outer coat. In the larger arterioles there is a middle coat 
formed by a single layer of smooth muscle cells. 
How many coats are there to the larger arteries ?—for example, 
the radial artery ? 
Three coats are usually described, viz: The intima, media and 
adventitia. And in most parts of the body the arteries are surrounded 
in addition by a connective-tissue sheath. The outer arterial coat is 
connected to this sheath by slender filaments of connective tissue. 
The connection is so loose that when the vessel is cut across it retracts 
a little way into the sheath. 
Describe the intima. 
It is composed of three parts: An inner portion formed by a single 
layer of delicate endothelial cells placed edge to edge, an intermediate 
or subepithelial layer composed of fine fibrillated connective tissue, in 
the cell-spaces of which are lying a number of branching cells, and 
finally, a layer of elastic tissue called the fenestrated membrane of 
Henle, which separates the intima from the media, and which is com- 
posed of a branching network of elastic fibres. 
Describe the media. 
It is composed entirely of smooth muscle cells arranged in a some- 
what circular fashion about the vessel. The thickness of an arterial 
wall depends in great part upon the amount of smooth muscle present. 
In the aorta this layer is replaced in part or entirely by elastic con- 
nective tissue. 
Describe the adventitia. 
It is made of bundles of white fibrous ‘connective tissue, with a vari- 
able amount of elastic fibres between. The whole mass of fibres is 
felted together, and in it ramify the fine bloodvessels or vasa vasorum 
which carry the blood to nourish the walls of the artery. This coat 
also contains nerve filaments, which are usually distributed to the 
middle coat. 
In what do the walls of veins resemble arterial walls ? 
They are made up of three coats—intima, media and adventitia. 
The component parts of these coats are made up of the same elements 
as in the arteries. 
How do they differ ? 
The muscular coat is thinner and is intermixed with white fibres, 
and is not clearly defined from the outer coat. The inner coat is thin- 
