THE SINO- VENTRICULAR SYSTEM 163 



case of nerve trunks the fibres are grouped into bundles, the 

 funicuH, surrounded by a definite condensed connective time, the 

 perineurium. The several funiculi, together with their sheaths, 

 are still farther enveloped in a general fibro-elastic envelope, 

 the epineurium, which extends among and is directly united 

 with the perineural sheaths. The epineurium carries the blood 

 and lymph of the nerves. The perineural sheaths are closely 

 applied to the funiculi but are only attached by delicate tra- 

 beculae running across and terminating in the endoneurium. 

 The spaces under the perineurial sheaths which have been in- 

 jected by Key and Retzius (76) with Richardson's solution, 

 present pictures similar to the internal surface of the similarly 

 injected perifascicular sheaths of the S.V.S. A great many more 

 elastic than white fibres are present in the sheath thus permitting 

 considerable stretching under a strong pressure. In testing the 

 strength of the sheath it was found to stretch to such an extent 

 that ridges were found on the endocardium before the sheath 

 finally broke, permitting extravasation. 



Noting the above similarities between nerve sheaths and the 

 sheath of the S.V.S. , it seemed probable that these sheaths might 

 be further similar in having the same kind of lining cells. The 

 cells lining the perineurial sheaths of nerves are flat plate-like 

 endothelioid cells and resemble true endothelial cells in shape 

 and reaction towards silver nitrate. 



Numerous injections of silver nitrate solution into the peri- 

 fascicular sheaths were made to determine the presence or absence 

 of lining cells but all attempts to demonstrate lining cells were 

 fruitless, nothing but a brown mass appearing under the micro- 

 scope. However, after making further trials on real fresh hearts 

 some unexpected results were obtained. Outlines of lining cells 

 still failed to show, but in some of the preparations a reduction 

 had taken place in the intercellular substance between the 

 Purkinje cells. The outlines of these cells appeared as wavy 

 lines forming a network, which is similar in appearance to the 

 cell boundaries of endothelium and was at first mistaken for cells 

 of the sheath. Not only was there found to be a reduction in 

 the intercellular substance but the silver was also reduced by the 



