AUTOMATIC REGULATION OF THE HEART. 55 



auriculo-ventrieular valves is an immediate prolongation of the endocardium of the 

 auricles, and is directed towards the auricles. The semi-lunar valves have a thin 

 elastic layer directed towards the arteries, which is thickest at their base. The 

 connective-tissue layer directed towards the ventricle is about half the thickness 

 of the valve itself. 



The auriculo-ventrieular valves also contain striped muscular fibres. Radiating 

 fibres proceed from the auricles and pass into the valves, which, when the atria 

 contract, retract the valves towards their base, and thus make a larger opening for 

 the passage of the blood into the ventricles ; according to Paladino, they raise the 

 valves after they have been pressed down by the blood-current. This observer also 

 described some longitudinal fibres which proceed from the ventricles to enter these 

 valves. There is also a concentric layer of fibres arranged near their point of 

 attachment, and directed more towards their ventricular surface. These fibres 

 seem to contract sphincter-like when the ventricle contracts, and thus approximate 

 the base of the valves, and so prevent too great tension being put upon them. The 

 larger chordae tendinese also contain striped muscle, while a delicate muscular 

 network exists in the valvula Thebesii and valvula Eustachii. 



Purkinje's Fibres consist of an anastomosing system of greyish fibres which exist in the sub- 

 endocardial tissue of the ventricles, especially in the heart of the sheep and ox. The fibres are 

 made up of polyhedral clear cells, containing some granular protoplasm, and usually two nuclei 

 (fig. 31). The margins of the cells are striated. Transition-forms are found between these cells 

 and the ordinary cardiac fibres ; in __^^_^ 



fact, these cells become continuous 

 with the true fully developed cardiac 

 fibres. They represent cells which 

 have been arrested in their develop- 

 ment. They are absent in man and 

 the lower vertebrates, but in birds and 

 some mammals they are well marked 

 (Schweigger-Seidel, Ranvier). 



Blood- Vessels occur in the auriculo- 

 ventrieular valves only where muscular ^^ : ^P~ / " :'.' . Jf^/ P 

 fibres are present, while the semi-lunar ^\v- I ' f 

 valves are usually devoid of vessels ^- v^ 

 except at their base. The best figures 

 of the blood-vessels of the valves are ^^~~^^V " 



given by Langer. The network of 

 lymphatics in the endocardium reaches Yis. 31. 



towards the middle of the valves. . . . , , . , , , .", j., , , , 1 -,> r. 



Weight of the Heart. -According P^kinje s fibres isolated with dilute alcohol c, cell ; / ? 

 to W. Miiller the proportion betweeS _ stnated subs t an ^ 5 ^ nucleus, x 300. 



the weight of the body and the heart in the child, and until the body reaches 40 kilos., is 5 

 grms. of heart-substance to 1 kilo, of body-weight ; when the body-weight is from 50 to 90 

 kilos, the ratio is 1 kilo, to 4 grms. of heart-substance ; at 100 kilos. 3 '5 grms. As age advances, 

 the auricles become stronger. The right ventricle is half the weight of the left. The weight 

 of the heart of an adult man is about 309 grms. ; female, 274 grms. [According to Laennec the 

 heart is about the size of the closed fist of the individual.] Blosfeld and Dieberg give 346 grms. 

 for the male, and 310 to 340 grms. for the female heart. The specific gravity of the heart- 

 muscle is 1*069. The thickness of the left ventricle in the middle in man is 11'4 mm., and in 

 woman 11 15; that of the right is 4*1 and 3 "6 mm. respectively. 



47. AUTOMATIC REGULATION OF THE HEART. Anatomical Investigations. The two 

 coronary arteries arise from the first part of the aorta in tfie region of the sinus of Valsalva. The 

 position of origin varies (1) either the orifices lie within the sinus, or (2) their openings are 

 only partially reached by the margins of the semi-lunar valves (which is usually the case in the 

 left coronary artery of man and the ox), or (3) their orifices lie clear above the margins of the 

 valves. Post-mortem observations seem to show that during contraction of the ventricle it is 

 very improbable that the semi-lunar valves constantly cover the origin of the coronary arteries. 



Automatic Regulation of the Heart. Brucke attempted to show that during 

 the systole, or contraction of the ventricle, the semi-lunar valves covered the 

 openings of the coronary arteries, so that these vessels could be filled with blood 



