ANGIOLOGY—THE LYMPHATIC SYSTEM 615 
termed (venous) sinuses; their wall consists of endothelium only; examples of 
this are the sinuses of the dura mater of the brain. A vein which connects one of 
these sinuses with veins outside of the cranium is termed an emissarium. 
A corpus cavernosum is an erectile structure which consists essentially of 
intercommunicating blood-spaces enclosed by unstriped muscle and fibro-elastic 
tissue. These spaces (Cavern) are lined with endothelium and contain blood. 
Some are to be regarded as greatly enlarged capillaries, since minute arteries open 
into them and they are drained by veins; others are intercalated in the course of 
veins. Distention of the cavern with blood produces the enlargement and harden- 
ing of the corpus cavernosum which is termed erection. 
Structure of Arteries.—The wall consists of three coats. The external coat 
or adventitia (Tunica externa) consists chiefly of fibrous connective tissue. In the 
deeper part are some elastic fibers, and in some arteries there are also longitudinal 
unstriped muscle-fibers. The middle coat (Tunica media) is composed of unstriped 
muscle and elastic tissue in medium-sized arteries. In small vessels there is chiefly 
muscular tissue, and in the largest trunks almost exclusively elastic tissue. The 
internal coat or intima (Tunica intima) consists of a layer of endothelial cells, resting 
on an elastic membrane. The sheath of the vessel (Vagina vasis) is a condensation 
of the surrounding connective tissue, and is attached more or less closely to the 
external coat. 
Structure of Veins.—The walls of veins are similar in structure to those of 
the arteries, but are very much thinner, so that veins collapse more or less com- 
pletely when empty, while arteries do not. The middle coat is very thin and con- 
sists to a large extent of ordinary fibrous tissue. The internal coat is also less 
elastic than in the arteries. In many veins this coat forms semilunar valves, the 
free edges of which are directed toward the heart. They are most numerous in the 
veins of the skin and of the extremities (except the foot), while in most veins of the 
body cavities and viscera they are absent or occur only where the veins open into 
larger ones or where two veins join. 
The walls of the vessels are supplied with blood by numerous small arteries, 
called vasa vasorum. These arise from branches of the artery which they supply 
or from adjacent arteries, ramify in the external coat, and enter the middle coat 
also. The nerves of the vessels (Nervi vasorum) consist of both medullated and 
non-medullated fibers. They form plexuses around the vessels, from which fibers 
pass mainly to the muscular tissue of the middle coat. 
THE LYMPHATIC SYSTEM 
The lymphatic system (Systema lymphaticum) is subsidiary to the venous 
part of the circulatory system, from which it arises in the embryo. It consists of 
the lymph vessels and glands. 
The lymph vessels (Vasa lymphatica) contain a colorless fluid, the lymph, 
which contains numerous lymphocytes.!. They resemble the veins in structure 
but have thinner walls and are provided with more numerous valves. The vessels 
are sacculated opposite the segments of the valves and have a characteristic beaded 
appearance when distended. The collecting lymph vessels do not usually form rich 
plexuses, as veins often do, their branching is more limited and less tree-like than 
that of the blood-vessels, and their caliber therefore increases less from the periphery 
toward their termination. All of the lymph is ultimately carried into the venous 
system by two trunks, the thoracic duct and the right lymphatic duct. Almost all 
! The term chyle is often applied to the lymph carried by the efferent vessels of the intestine 
when it contains products of digestion, and these vessels may be designated accordingly as lacteals 
or chyle vessels. 
