CIRCULATORY CHANNELS 149 



LITERATURE 



SCHNEIDER, K. C. " Lehrbuch der Algem. Histologie," Jena, 1898. 

 FOSTER, M. "Text-Book of Physiology." 



TISSUES OF CIRCULATION: THE HISTOLOGY OF THE CHANNELS 



The main blood-channel system itself has many differentiated regions. 

 The region of thin-walled capillaries and lacunae, the strong-walled con- 

 ducting vessels, the blood-forming regions, and the muscular pumping 

 stations or hearts are the chief grouping of these organs which must be 

 treated of in more detail in the seminar part of this section. Most im- 

 portant, or rather most specific of these portions, are the capillaries and 

 lacuna, for it is here that the real work of the blood is accomplished, the 

 exchange of materials with the tissues. This region will be spoken of 

 as the periphery. Here the walls of the vessels are thinnest or even ap- 

 parently 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 current 

 slower to give requisite time for such exchanges. 



The smaller but more numerous branches of the periphery unite to 

 form larger 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, 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 con- 

 structed. 



The pumping region comprises one or more parts of the larger 

 channel or channels that have acquired the power of rhythmic contrac- 

 tion. Sometimes this region occupies a considerable extent of the larger 

 vessels. At other times it is found in a more specialized 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 mollusks, where 

 there are three or five separate hearts, and in addition the larger part 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 some- 



