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control. When the nerve is unduly irritated the bold-back or inhibitory 

 force is increased, and the heart slows up in the same measure. The 

 left cavities of the heart, the pulmonary veins, and the aorta or systemic 

 artery, contain red or Horid blood, fit to circulate through the body. 

 The right cavities of the heart, with the venae cavee or systemic veins 

 and pulmonary artery, contain dark blood, which must be transmitted 

 through the lungs for renovation. 



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

 monary artery, undergo division as in the branching of a tree. Their 

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

 form diameter in each case, but successively diminish after and incon- 

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

 capillary 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 system 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 returns from the capillary vessels to the venous trunks. 



In the smaller vessels a frequent running together or anastoniosls oc- 

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

 blood, and must tend to promote equability of distribution and of press- 

 ure, and to obviate the effects of local interruption. The arteries are 

 highly elastic, being extensile and retractile both in length and breadth. 

 During life they are also contractile, being provided with muscular tis- 

 sue. When cut across they present, although empty, an open orifice j 

 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 usually described as being 

 formed of three coats, named, from their relative j)ositions, external^ 

 middle, and internal. This applies to their structure so far as it is 

 discernable by the naked eye. The internal, serous or tunica intima 

 is the thinnest, and continuous with the lining membrane 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 muscular and elastic fibers, thickest in the 

 largest arteries and becoming thinner in the smaller. In the smallest 

 vessels it is almost entirely m uscular. The external coat, tunica ad- 

 ventitia, is composed mainly of fine and closely- woven bundles of white 

 connective tissue, which chiefly run diagonally or obliquely round the 

 vessel. In this coat the nutrient vessels, the vasa vasorum, form a cap- 

 illary net-work, from which a few penetrate as far as the muscular coat. 



