TIIK HEART. 131 



ventricle. Practically none of the strands of fibres begin and 

 end in the same ventricle. It will thus be seen that the heart 

 is made up of various layers of muscle, all of which have their 

 origin in the tendon of the auriculo ventricular ring of one side, 

 and end in the tendon of a papillary muscle of the other side. 

 Their fibres in passing over in the septum thus take a scroll- 

 shaped course. In the light of this, the heart consists of several 

 bands of muscle with tendons at each end, rolled up like a 

 scroll or like the letter S. At the same time it is to be 

 observed that the growing points in a very young heart are 

 just under the endocardium. Karyokinetic figures are found 

 there, and the cells in that region are -younger than at the 

 periphery of the heart. If the heart be then unrolled, these 

 growing points would appear at each end of the bands of 

 muscle that make up the heart. For a more detailed descrip- 

 tion of this dissection of the heart, the reader is referred to the 

 original article. 



In the muscle of the heart wall there is a rich network of 

 blood capillaries, which run parallel to the fibres and send 

 branches which surround them. 



Elastic tissue is found abundantly in both the auricles and 

 ventricles. 



The annuli fibrosi, which consist of firm connective tissue 

 containing elastic fibres, separate the muscle of the auricles 

 from that of the ventricles, and form a place of attachment for 

 those muscles. 



(3) JEpicardium is a connective-tissue membrane rich in 

 elastic fibres. Under it there is usually a quantity of fat, 

 which is gathered in masses in certain places. The upper sur- 

 face of the epicardium is covered by flat endothelial cells. 



The heart valves are connective-tissue structures formed by 

 a reduplicature of the endocardium, and contain connective 

 tissue and smooth muscle. Their surface is covered by endo- 

 thelial cells. No blood-vessels are present in the heart valves. 



The pericardium is a connective-tissue membrane containing 

 many elastic fibres, and on its free inner surface is covered 

 by a layer of endothelial cells. 



