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nerve is cut the lieart-boats increase rapidly, and in fact tlie organ 

 works without control. When the nerve is unduly irritated the hold- 

 back or inhibitory I'orce is increased, and the heart slows nj) in the 

 same measure. The left cavities of the heart, the i)ulmonar3- veins, 

 and the aorta or systemic artery, contain red or florid blood, fit to 

 circulate through the body. The right cavities of the heart, with the 

 vena? cava» or systemic veins and pulmonary arterj-, contain dark 

 blood, which must be transmitted through the lungs for renovation. 



The arteries, commencing in two great trunks, the aorta and the 

 pulmonary artery, undergo division as in the l)ranching of a tree. 

 Their branches mostly come off at acute angles, and are commonly of 

 uniform diameter in each case, but successively diminish after and in 

 consequence of division, and in this manner gradually merge into the 

 capillar}- system of blood-vessels. As a general rule, the combined 

 area of the branches is greater than that of the vessels from which 

 they emanate, and hence the collective capacity of the arterial sj's- 

 tem is greatest at the capillary vessels. The same rule applies to the 

 veins. The'effect of the division of the arteries is to make the blood 

 move more slowly along their branches to the capillary vessels, and 

 the effect of the union of the branches of the veins is to accelerate the 

 speed of the blood as it i-et urns from the capillary vessels to the venous 

 trunks. 



In the smaller vessels a frequent riuining together or anastomosis 

 occurs. This admits of a free communication between the currents of 

 blood, and must tend to j^romote equability of distribution and of 

 Ijressure, and to obviate the effects of local interruption. The arte- 

 ries are highly elastic, being extensile and retractile both in length 

 and breadth. During life they are also contractile, being jn-ovided 

 with muscular tissue. When cut across they present, although empty, 

 an open orifice; the veins, on the other hand, collapse. 



In most parts of the body the arteries are inclosed in a sheath 

 formed of connective tissue, but are connected so loosely that when 

 the vessel is cut across its ends readily- retract some distance within 

 the sheath. Independently of this sheath arteries are usuallj^ de- 

 scribed as being formed of three coats, named, from their i-elative 

 positions, external, middle, and internal. This applies to their struc- 

 ture so far as it is discernible bj-tlie naked eye. The internal, serous 

 or tunica intima is the thinnest, and continuous with the lining mem- 

 brane of the heart. It is made up of two layers, an inner, consisting 

 of a layer of epithelial scales, and an outer, transparent, whitish, highly 

 elastic, and perforated. The middle coat, tunica media, is elastic, 

 dense, and of a yellow color, consisting of non-striated muscuhir and 

 elastic fibers, thickest in the largest arteries and becoming thinner in 

 the smaller. In tlie smallest vessels it is almost entirely muscular. 

 The external coat, tunica adventitia, is composed mainly of fine and 

 closely woven bundles of white connective tissue, which chiefly run 



