196 ■ TEE CAT. [cuap. vii. 



The blood and lymph arc contained and conveyed on their course 

 by the various sets of vessels and other parts already enumerated, 

 about each of which there is much to be said. 



§ 4. The ARTERIES are strong and very clastic tubes, so that 

 when empty they remain open and do not collapse. They are 

 lined internally with an epithelial layer, external to which is a layer 

 of elastic tissue. External to this again is a stratum of organic 

 muscular fibres, arranged in bundles placed mainly at right angles 

 to the course of the artery and tending to surround it, though some 

 fibres are longitudinal and others oblique in direction. External 

 again to the muscular layer is a layer of clastic tissue (and elastic 

 fibres are also more or less mixed with the muscular fibres), and 

 finally the whole is enclosed by a layer of connective tissue. 



In the smallest arteries the clastic coat is absent, while the 

 muscular coat is relatively more developed than in the larger 

 arteries. Arteries, generally, run deeply in well protected situations. 

 As they advance they divide and subdivide into smaller and smaller 

 branches. Different branches of the same, or of different trunks, 

 may unite together, and such unions are termed anastomoses. 

 Arteries generally run in a rather straight manner, but they may 

 pursue a very tortuous course. Sometimes an artery may suddenly 

 break up into a number of small anastomosing branches, which 

 reunite to form a single vessel. Such a network is called a rcfe 

 mirahilc The presence of the muscular coat enables the arteries to 

 diminish their capacity by contracting their muscular fibres, or, by 

 relaxing them, to enlarge it, since these fibres are contracted to a 

 certain moderate amount in the normal state of the arteries.* The 

 walls of the arteries are themselves supplied and nourished by minute 

 vessels termed vasa vasonun. 



§ 6. The VEINS are weak and thin-walled tubes , much less elastic 

 than the arteries — collapsing when emptied. They are lined in- 

 ternally with an epithelial layer, external to which is elastic tissue 

 and a stratum of organic muscular fibres invested externally by 

 connective tissue. 



In some veins this muscular layer is absent, while it is excep- 

 tionally well developed in a large vessel going to the liver, the 

 portal vein, and in that coming from the spleen. The veins ramify 

 through the body, as do the arteries, but are more numerous and 

 have greater capacity. They are arranged in a superficial and 

 deep set — the deeper veins accompanying the corresponding arteries, 

 as what arc called vcnxB comites. Veins anastomose together more 

 frequently than do arteries ; their walls, like those of arteries, are 

 supplied with rasa rasoruni. 



The^ veins are generally furnished with certain structures not 

 found in the arteries, namely, vat res. These are crescentic /folds of 

 membrane, so arranged that in each the semilunar edge of the fold 



* These muscles arc under the control I the so-called vaso-motor and raso-dilator 

 of special parts of tlie nervous system — ' nerves. 



