GENERAL OLKJAKOLOGT. Ill 



which, therefore, belong generally to the higher animals, and 

 function whether a l)ody cavity is present or not. Blood-vessels 

 are tubes with fluid contents, the blood, which ti-ansports the 

 oxygen received through the respiratory organs, as well as the food 

 absorbed from the digestive tract, and later gives these up to the 

 tissues. Since such an interchange of substances presupposes that 

 the blood circulates in the vessels, definite parts in the course of 

 the blood-vessels are contractile; they are covered \)y muscles which 

 by their contraction narrow the tube and \mA\ the fluid forwards. 

 In the lower forms wide areas in the course of the blood-vessels are 

 contractile; in higher animals a greater regularity of circulation 

 is reached; a definite specialized muscular part of the course, the 

 heart, alone propels the blood. 



The Higher Development of the Heart. — A free motion of the 

 heart is only possible when it is separated from the contiguous 

 tissues and enclosed in a special cavity (fig. 04). Hence wo see 

 that the heart always lies either free in the body cavity or enclosed 

 in a special pouch {]i), the pericardium (in all cases a portion of 

 the general body cavity, but not always of the ccelom, which has 

 Ijecome independent). The division of the heart into a part which 

 receives the blood, the atrium or auricle (Ii), and a part which 

 drives the blood onward, the ventricte (k), is of less functional 

 importaiice; hence this division is not carried out in all cases. 

 There are also special mechanisms within the heart, the valves 

 (H), which, by closing, prevent the blood from flowing back when 

 the walls relax at the end of the contraction. 



Blood-vessels. — In order that the blood system may properly 

 perform its function, in addition to circulation, it is necessary that 

 the nutritive substances be readily taken np and given out again 

 to the tissues. The part of the course of circulation concerned in 

 this must have easily permeable walls, must be widely distributed 

 in the body, and have a large superficial area. These demands are 

 met by the capillaries (c), extremely fine and thin-walled tubes, 

 which surround and permeate all organs. Through their walls, 

 usiuilly formed of a thin epithelial layer alone, the proteid substances 

 for nourishing the tissues can pass, and the oxygen can i.)e 

 cxchano-ed for carbon dioxide. Between the heart aiul the capil- 

 laries there exists, corresponding to their different functions, great 

 differences in structure; they must therefore be united Ijy special 

 transitional vessels — vessels which begin large and thick-walled at 

 the heart, and by branching, and thinning of their walls, pass 

 gradually into the cai)illaries; of such vessels there are two kinds, 



