CIRCULATORY CHANNELS 



155 



fibers which surround the vessel as a circular layer. They are smooth, 

 and the nucleus lies on one side in the main cell body. The largest 

 arteries show a longitudinal layer of muscle fibers which, near the heart, 

 become irregular in arrangement as they also are in that organ. 



In the Cephalopod mollusks is found a series of blood vessels that 

 are much the same in structure as those of Unio or of any other mollusk. 

 A point of importance here is the great development of muscle in all of 

 the larger channels, veins and arteries alike. These vessels are active 

 as pumping agents, operating without the 

 aid of many valves. Their powerful circu- 

 lar muscle layer sends waves of contrac- 

 tion along the vessel. These waves are so 

 strong that they close the vessel entirely and 

 drive the blood before them. 



Figure 138 shows a large mantle artery 

 from a common Florida octopus. The 

 inner layer consists of a very thin endothe- 

 lium lying on a thick, well-developed mem- 

 brane. This membrane, from its position 

 under a layer of surface cells, is a base- 

 ment membrane. But also, its compara- 

 tively great thickness reminds one of the 

 elastic membrane to be described later in 

 a human artery. It probably is greatly 

 strengthened and added to in substance by 

 the connective tissue on which it lies. It 

 lies in longitudinal folds in all but the 

 fullest expanded arteries, and it is possibly, but not necessarily, non- 

 elastic. 



The connective tissue lying outside of the membrane is very sparse 

 in our subject, the octopus, and can be seen to better advantage in the 

 contracted mantle artery of a squid. Its structure has no great signifi- 

 cance other than its function as a loose and movable connecting medium 

 between the layers. 



The muscle cells form a thick, powerful layer of circular fibers em- 

 bedded in a connective tissue which holds them together and at the same 

 time keeps them some distance apart. The individual muscle fibers are 

 comparatively short, bluntly spindle-shaped, and reach, in the average 

 example, about one tenth of the distance around the circumference of the 

 artery. 



The fine, sharp strands or fibrils of connective substance which bind 

 these fibers together are loosely arranged between them except on their 

 blunt ends. Here, these fibrils reach radially from the end of the 



mus.fi.b. \ 



FIG. 137. Part of a transaction 

 of a foot artery of Anodonta, 

 int., intima of endothelial cells 

 on a basal membrane of weak 

 development; mus.fi.b., muscle 

 fibril bundles (fibers); conn. I.e., 

 connective-tissue cell. (After 

 SCHNEIDER.) 



