CHAPTER II. 

 THE CIRCULATION OF THE BLOOD AND LYMPH. 



THE blood can only fulfil its functions by continual move- 

 ment. This movement implies a constant transformation 

 of energy ; and in the animal body the transformation of 

 energy into mechanical work is almost entirely allotted to 

 a special form of tissue, muscle. In most animals there 

 exist one or more rhythmically contractile muscular organs, 

 or hearts, upon which the chief share of the work of keeping 

 up the circulation falls. 



Comparative. In Echinus a contractile tube connects the two 

 vascular rings that surround the beginning and end of the alimentary 

 canal, and plays the part of a heart. In the lower Crustacea and 

 in insects the heart is simply the contractile and generally sacculated 

 dorsal bloodvessel ; in the higher Crustacea, such as the lobster, it is 

 a well-defined muscular sac situated dorsally. A closed vascular 

 system is the exception among invertebrates. In most of them the 

 blood passes from the arteries into irregular spaces or lacunae in the 

 tissues, and thence finds its way back to the heart. Amphioxus, 

 the lowest vertebrate, has a primitive lacunar vascular system ; a 

 contractile dorsal bloodvessel serves as arterial or systemic heart, a 

 contractile ventral vessel as venous or respiratory heart. From the 

 latter, vessels go to the gills. Fishes possess only a respiratory heart, 

 consisting of a venous sinus, auricle, and ventricle. This drives the 

 blood to the gills, from which it is gathered into the aorta ; it has 

 thence to find its way without further propulsion through the systemic 

 vessels. Amphibians have two auricles and a single ventricle; 

 reptiles, two auricles and two incompletely-separated ventricles. In 

 birds and mammals the respiratory and systemic hearts are com- 

 pletely separated. The former, consisting of the right auricle and 

 ventricle, propels the blood through the lungs ; the latter, consisting 

 of the left auricle and ventricle, receives it from the pulmonary veins, 

 and sends it through the systemic vessels. 



General View of the Circulation in Man. The whole circuit 



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