THE VASCULAR SYSTEM. 213 



of the vessels, we distinguish, in both arteries and veins, large, 

 medium-sized, small, and precapillary vessels, and finally, the capil- 

 laries themselves. The latter connect the arterial and venous pre- 

 capillary vessels. In the lymphatic system we must further dis- 

 tinguish between the larger lymph-vessels, the sinuses, and the 

 capillaries. 



A. THE VASCULAR SYSTEM. 



J. THE HEART, 



In the heart there are recognized three main coats the endo- 

 cardium, the myocardium, and the pericardium or epicardium. 



The endocardium consists of plate-like endothelial cells, with 

 very irregular outlines. Beneath this endothelial layer is a thin 

 membrane composed of unstriped muscle-cells, together with a 

 small number of connective-tissue and elastic fibers. Below this is 

 a somewhat thicker and looser layer of elastic tissue connected ex- 

 ternally with the myocardium. Between the two layers are found, 

 here and there, traces of a layer of Purkinje 's fibers (compare p. 

 147). Purkinje's fibers are found in the heart of many mammalia, 

 although absent in the heart of the human adult. 



The auriculoventricular valves of the heart represent, in general, 

 a duplication of the endocardium. The layer of smooth muscle- 

 fibers found in the latter is better developed on the auricular surface. 

 At the points of insertion of the chordae tendineae the connective- 

 tissue layer is strongly developed and assumes a tendon-like con- 

 sistency. The semilunar valves of the aorta and pulmonary artery 

 have a similar structure. In the nodules of these valves the elastic 

 fibers are especially dense in their arrangement. 



The myocardium is made up of the heart muscle-fibers already 

 described (yid. p. 145). Between the heart muscle-fibers and 

 bundles of such fibers are thin layers of fibrous connective tissue 

 containing a network of capillaries. The myocardium of the auricles 

 may be divided into two layers, of which the outer is common to both 

 auricles, the fibers of which have a nearly circular arrangement, while 

 the deeper layer is separate for each chamber. The arrange- 

 ment of the heart muscle-fibers of the ventricles is complicated. 

 With special methods of maceration J. B. MacCallum was able to 

 show that "the superficial fibers are found to have origin in the 

 auriculoventricular ring, to wind about the heart spirally, and to end 

 in tendons of the papillary muscle of the opposite ventricle. The 

 deep layers also begin in the tendon of one auriculoventricular ring, 

 pass around to the interventricular septum, cross over backward or 

 forward in this septum, and end in the papillary muscle of the other 

 ventricle. In the light of this, the heart consists of several bands 

 of muscles with tendons at each end, rolled up like a scroll or like 

 the letter S." The musculature of the auricles is almost completely 



