736 THE VASCULAR SYSTEM. ae 1 
narrow and pointed in the arteries, whilst in the veins they are somewhat shorter 
and broader. 
The most essential structural difference between capillaries and the arteries and 
veins which they unite together, is to be found in the presence, in both of the latter, 
of involuntary muscular fibres inter- 
posed between the endothelial lining 
and the outer connective tissue — 
sheath. In small vessels, e.g. capil- 
lary arterioles, the muscle cells are 
few in number and more or less 
scattered. In larger vessels the 
Al, Capillary—with simple endothelial walls. A®, Larger capillary walls become stronger anc thicker, 
Faieh connective tissue sheath, ‘‘adventitia capillaris.” B, pouscular fibres increase and torm 
Capillary arieHole —showing muscle cells of middle coat, few : , 
and scattered. C, Artery—muscular elements of the tunica & Continuous layer, whilst yellow 
Sere ade aed nara bead elastic and ordinary white connec- 
tive tissue are added in varying proportions. The walls of the vessels thus 
become more complex, and numerous strata may be distinguished ; these, however, 
are for convenience regarded as forming three layers, which are known as the inner, 
middle, and outer coats, superadded to which is the inv esting common sheath. 
Structure of Arteries.—The walls of arteries are stronger and thicker than those 
of veins of corresponding size, the inner and middle coats being respectively par- 
ticularly rich in elastic and muscular elements. 
Inner coat (tunica intima).—The simple endothelial layer of the arterioles is 
strengthened by the addition of yellow elastic tissue, which usually forms a fene- 
strated membrane, but sometimes consists of longi- 
tudinal fibrils. In arteries of medium size the 
elastic lamina is separated from the endothelium by 
a layer of connective tissue consisting of branched 
cells and numerous fibrils. In the larger arteries 
the sub-endothelial connective tissue is considerably 
increased, and delicate elastic fibres appear which 
connect it with the more externally situated fene- 
strated elastic layer. 
The middle coat (tunica media) in the capuil- 
lary arterioles consists solely of scattered unstriped 
muscle fibres; the individual fibres are circularly 
disposed, but ‘do not entirely surround the vessel. 
In small arteries the muscle cells are so far increased 
in amount that they form a continuous though thin 
layer. As the arteries increase in size additional 
layers of muscle cells are added, and the greater 
thickness of the arterial wall is mainly due to this 
increase of the muscular elements of the middle coat. 
In the larger vessels delicate laminee of elastic tissue 
alternate with the layers of muscular fibres, and in 
the aorta and the carotid arteries, as well as in some of 
the branches of the latter, the elastic elements largely pig, 543. -TRansveERSE SECTION 
ial 
N. externa 
T. media 
T. intima 
Fie. 542.—Srructure OF BLOOD VESSELS (Diagrammatic. ) 
preponderate. In the first part of the aorta, in the — THRovGH THE Watt or A LarcE 
pulmonary artery, and in the arteries of the retina, the ARTERY. 
muscular fibres are entirely replaced by elastic tissue. 4» Tmeyinpima. By tunes media 
The external coat (tunica externa) of an artery 
consists almost entirely of fibrillated connective tissue, with connective tissue 
corpuscles lying in corresponding spaces. In all but the smallest arteries numerous 
elastic fibres are also present. The elastic element is specially strong near the 
middle coat in small and medium-sized vessels, and is sometimes descri bed as the 
external elastic membrane of Henle. In some arteries longitudinally arranged 
unstriped muscular fibres are also found in the external coat. 
The sheath of an artery (vagina vasis).—In addition to the three coats 
above described, arteries are enclosed in a sheath of the surrounding connective 
