THE CIRCULATION OF THE BLOOD AND LYMPH 



73 



superior vena cava. The auricular canal is probably represented by 

 the auriculo-ventricular bundle, which will again be referred to in 

 relation to the conduction of the heart-beat from auricles to ventricles 

 (p. 135). This bundle starts from a clump of primitive tissue, the 

 auriculo-ventricular node at the base of the interauricular septum 

 on the right side, below and to the right of the coronary sinus, and 

 runs down the inter ventricular septum. The sino-auricular and the 

 auriculo-ventricular nodes are connected by fibres which run in the 

 interauricular septum, so that it may be considered that the primi- 

 tive cardiac tubs is still represented from bass to apex of the adult 

 mammalian heart, although only by very slender threads of tissue, 

 amidst the massive secondary 

 developments of auricular and 

 ventricular muscle (Keith and 

 Flack). 



General View of the Circulation 

 in Man. The whole circuit of the 

 blood is divided into two portions, 

 very distinct from each other, both 

 anatomically and functionally 

 the respiratory or lesser circula- 

 tion, and the systemic or greater 

 circulation. Starting from the left 

 ventricle, the blood passes along 

 the systemic vessels arteries, 

 capillaries, veins and, on return- 

 ing to the heart, is poured into the 

 right auricle, and thence into the 

 right ventricle. From the latter 

 it is driven through the pul- 

 monary artery to the lungs, 

 passes through the capillaries of 

 these organs, and returns through 

 the pulmonary veins to the left 

 auricle and ventricle. The portal 

 system, which gathers up the 

 blood from the intestines, forms 

 a kind of loop on the systemic 

 circulation. The lymph-current 

 is also in a sense a slow and stag- 

 nant side-stream of the blood- 

 circulation ; for substances are 

 constantly passing from the blood- 

 vessels into the lymph-spaces, and returning, although after a com- 

 paratively long interval, into the blood by the great lymphatic trunks. 



Physiological Anatomy of the Vascular System. The heart is to be 

 looked upon as a portion of a bloodvessel which has been modified to 

 act as a pump for driving the blood in a definite direction. Morpho- 

 logically it is a bloodvessel ; and the physiological property of auto- 

 matic rhythmical contraction which belongs to the heart in so 

 eminent a degree is, as has been mentioned (p. 72), an endowment 

 of bloodvessels in many animals that possess no localized heart. 

 Even in some mammals contractile bloodvessels occur ; the veins 

 of the bat's wing, for example, beat with a regular rhythm, and 

 perform the function of accessory hearts. 



The whole vascular system is lined with a single layer of endo- 



FIG. 20. DIAGRAM OF THE GENERAL 

 COURSE OF THE ^CIRCULATION. 



RA, LA, right and left auricles ; RV 

 LV, right and left ventricles. 



