AND BLOOD PRESSURE 131 



the vascular systems, the writer cites the following suggestive 

 passages from the admirable Principles of Animal Histology of 

 Dahlgren and Kepner, in which he underlines certain sentences 

 which bear in particular on the arguments that follow. 



" The main blood channel system itself has many differentiated 

 regions. The region of thin-walled capillaries and lacunae, the 

 strong-walled conducting vessels, the blood-forming organs, and 

 the muscular pumping stations or hearts . . . most specific of these 

 portions are the capillaries and lacunce, for it is here that the real 

 work of the blood is accomplished, the exchange of material with 

 the tissues. Here the walls of the vessels are thinnest or even 

 apparently wanting. In this case the connective tissue cells that 

 surround the channel, while not differentiated into definite channel 

 walls, act in that capacity, so that we cannot say that retaining walls 

 are altogether absent. The vessels of the periphery have in all cases 

 a larger total cross section than any other total cross section in 

 the circuit. This results in the surface of contact between blood 

 and tissue being large enough to effect necessary exchanges of 

 materials as well as making the circuit slower to give requisite 

 time for such exchanges. The smaller but more numerous branches 

 of the periphery unite to form large channels that serve to conduct 

 the blood to other portions of the periphery, or to and from the 

 central pumping stations, or to the blood glands. These vessels 

 and the veins, together with the vessels carrying blood back to 

 the periphery, the arteries, act as the long-distance carriers of 

 the circulatory system, and their walls are usually very strongly 

 constructed. 



" The pumping region comprises one or more parts of the larger 

 channel or channels that have acquired the power of rhythmic 

 contraction. Sometimes this region occupies a considerable 

 extent of the larger vessels. At other times it is found in a more 

 specialised form, occupying only a short section of the tube, but 

 very intensely developed. Such an organ is known as a heart. 

 Both of the preceding conditions may be found together, as 

 they are in the squid and other cephalopod molluscs where there 

 are three or five separate hearts, and in addition the larger purts 

 of the arteries are also constantly engaged in driving the blood on its 

 course by wave-like pulsations. Other regions of the blood-channel 

 system are found in which the walls are differentiated and in which 

 the blood moves but slowly and sometimes almost conies to rest. 



