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 emissaritun. 



A corpus cavemosum is an erectile structure which consists essentially of 

 intercommunicating blood-spaces enclosed by unstriped muscle and fibro-elastic 

 tissue. These spaces (Cavernae) 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 cavernte 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, 

 caUed 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 l3miphatic 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. ... i ■':' t 



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 nupierous 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 cahber 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 



1 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 accordmgly as lacteals 

 or chyle vessels. 



