CHAPTER II 

 THE CIRCULATION OF THE BLOOD AND LYMPH 



THE blood can only fulfil its functions by continual movement. 

 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 rhyth- 

 mically 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 sacculatecl 

 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. In the primitive 

 vertebrate heart five parts can be distinguished as we proceed from 

 the venous to the arterial end : (i) The sinus venosus, into which the 

 great veins open ; (2) the auricular canal, from the dorsal wall of 

 which is developed(3) the auricle ; (4) the ventricle ; (5) the bulbus 

 arteriosus, from which the chief artery starts (Fig. 19, p. 70) . Amphi- 

 oxus, 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, ventricle, and bulbus arteriosus. 

 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 a venous sinus, 

 two auricles, a single ventricle, and an arterial bulb ; reptiles, two 

 auricles and two incompletely-separated ventricles. In birds and 

 mammals the respiratory and systemic hearts are completely 

 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. 



The sinus venosus seems to be represented in the mammalian 

 heart by certain small portions of tissue, especially the so-called sino- 

 auricular node, a little knot of primitive fibres at the mouth of the 



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