CHAPTER III 

 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 rhythmically contractile 

 muscular organs, or hearts, upon which the chief share of the work 

 of keeping up the circulation falls. 



SECTION I. PRELIMINARY ANATOMICAL AND PHYSICAL DATA. 



Comparative. In Echinus a contractile tube connects the two vascu- 

 lar 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. 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. 25, p. Si). Amphioxus, the lowest verte- 

 brate, 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 pul- 

 monary 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 near the mouth of the superior 



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