ANATOMICAL AND PHYSICAL DATA 



81 



vena cava. The auricular canal is probably represented by the 

 auriculo-ventricular bundle (conveniently designated as the a.-v. bundle), 

 which will again be referred to in relation to the conduction of the heart- 

 beat from auricles to ventricles (p. 147). This bundle starts from a 

 clump of primitive tissue, the auriculo-ventricular node (a.-v. 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 interventricular 

 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 primitive cardiac tube is still represented 

 from base 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 function- 

 ally the respiratory or lesser 

 circulation, and the systemic or 

 greater circulation. Starting from 

 the left ventricle, the blood passes 

 along the systemic vessels ar- 

 teries, capillaries, veins and, on 

 returning to the heart, is poured 

 into the right auricle, and thence 

 into the right ventricle. From 

 the latter it is driven through the 

 pulmonary 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 



Fig. 25. Diagram of Primitive Vertebrate 

 Heart, combining Features found in the 

 Eel, Dogfish, and Frog (Flack, after 

 Keith), a, Sinus venosus; b, auricular 

 canal; c, auricle; d, ventricle; e, bulbus 

 cordis; /, aorta; i-i, sino-auricular junc- 

 tion and venous valves; 2-2, junction of 

 canal and auricle; 3-3, annular part of 

 auricle; 5, bulbo-ventricular junction. 



is also in a sense a slow and stag- 

 nant side-stream of the blood- 

 circulation; for substances are 

 constantly passing from the 

 bloodvessels 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. 80), an endowment of blood- 

 vessels 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 func- 

 tion of accessory hearts. 



