142 THE ESSENTIALS OF HISTOLOGY. 



like that of a serous membrane, and consists of connective tissue with 

 elastic fibres in its deeper part, between which there may, in some 

 parts, be found a few plain muscular fibres. Fat is sometimes met 

 with under the endocardium. 



In some animals, e.g. the sheep, and sometimes also in man, large 

 beaded fibres are found under the endocardium. These are formed of 

 large clear cells joined end to end, and generally containing in their 

 centre two nuclei, whilst the peripheral part of the cell is formed 

 of cross-striated muscular tissue ; they are known as the fibres of 

 Purkinje. 



The valves of the heart are formed of folds of the endocardium 

 strengthened by fibrous tissue (fig. 165). This tissue forms a thicken- 

 ing near the free edge of the valve (c'). At the base of the auriculo- 

 ventricular valves a little of the muscular tissue of the auricle may be 

 found passing a short distance into the valve. 



The nerves of the heart are seen in sections underneath the epi- 

 cardium of both auricles and ventricles ; in the former situation they 

 are connected at intervals with small ganglia (fig. 163, g). Their 

 branches pass to the muscular substance, and after dividing into fine 

 fibrils, these eventually end in enlarged extremities, which are applied 

 directly to the muscular fibres (Ranvier). 



