Circulatory System 



67 



lining of the vessel and so placed that any tendency of the blood to flow 

 backward — that is, away from the heart — distends the pockets and oc- 

 cludes the passage (Fig. 68). The wall of a capillary is reduced to the 

 extreme of thinness. It is only one cell in thickness, 

 being the continuation of the delicate lining 

 (endothelium) of the adjoining small arteries and 

 veins, and quite devoid of muscle and connective 

 tissue. Lymphatics resemble veins but are nar- 

 rower than the corresponding veins and have even 

 thinner walls, the internal pressure of the circula- 

 tory current being at its minimum in lymphatics 

 and at its maximum in arteries. Certain localized 

 and more strongly contractile regions of larger 

 lymphatics, lymph-hearts, assist the flow of the 

 lymph. Valves similar to those in veins help prevent 

 backing of the lymph. In mammals, lymph-hearts 

 are rarely, if ever, present, but valves are espe- 

 cially numerous (Fig. 69). 



RELATIONS OF RESPIRATORY ORGANS TO 

 CIRCULATION 



Fig. 68. Valve 

 in vein. Arrows in- 

 dicate direction of 

 pressure. (Top) 

 Blood moving to- 

 ward heart; valve 

 open. (Bottom) 

 Pressure reversed; 

 pockets of valve di- 

 lated, occluding 

 passage. 



The respiratory organs are interpolated into the 

 main course of the circulation immediately anterior 

 to the heart so that the blood becomes oxygenated 

 before it starts on its way to the capillaries, where 

 its essential work is to be done. 



In gill-breathers without lungs, the heart consists of two cham- 

 bers: a posterior receiving chamber, the auricle or atrium; and an 

 anterior more strongly muscular ventricle (Fig. 65). The ventricle 

 pumps the blood cephalad into a short ventral aorta, whence several 

 pairs of afferent branchial arteries distribute the blood to the sev- 

 eral pairs of gills, in whose lamellas or filaments the arteries pass into 

 capillaries where the oxygenation occurs (Figs. 64, Squalus; 70). From 

 these branchial capillaries the now "pure" blood passes into several 

 pairs of dorsal efferent branchial arteries, which all converge into the 

 anterior end of the chief artery of the body, the median dorsal aorta, 

 which extends along the roof of the coelom. The pairs of arteries inter- 

 vening between ventral aorta and dorsal aorta encircle the pharyngeal 

 region of the digestive tube. They are called aortic arches. In adult 

 fishes there are usually four or five aortic arches. The aortic arches cor- 

 respond approximately in number to the branchial clefts and alternate 

 with the clefts in position (Figs. 64, 70). 



From certain of the efferent branchial arteries, a pair of carotid 



