ORGANS OF THE BODY. 405 



cavity, but passes then, not into the opposite wall of the same, but into 

 the wall of the left ventricle, arriving eventually at the left fibrous ring, 

 where it terminates. 



Besides this peculiar arrangement of the fibres, which is, however, on 

 the whole a longitudinal one, there is also a circular set. This takes its 

 rise from the left annulus, and surrounds the wall of the left ventricle in 

 figures of eight, while other fleshy bundles arising in the same region 

 envelope the right chamber in simple loops. These different masses of 

 fibres lie between the longitudinal. From the right annulus also, though 

 in much smaller number, similar fibres take their rise, encircling the wall 

 of the left ventricle in the same kind of simple loops. Finally, we have 

 another set of circular fibres, which, springing from the right annulus, 

 return to be inserted into the same, encircling in their course the conus 

 arteriosus. 



The musculse papillares are formed both from, the longitudinal and trans- 

 verse fibres. 



In conclusion, we must devote a few lines to those peculiar structures, 

 discovered in the year 1845 in the hearts of horses, cows, sheep, and 

 pigs, which have been named, in honour of the discoverer, the fibres of 

 Purkinje. 



These present themselves as flat grey jelly-like threads, spread out in a 

 reticular manner, immediately under the endocardium, on the internal 

 surface of the ventricles. They penetrate further into the muscular 

 papillares, and stretch across various depressions in the walls of the heart. 



Purkirtje's fibres (which were subsequently found to exist in the hearts 

 of deer and goats) are structures whose significance is far from being 

 understood as yet. We may see that they consist of rows of round or 

 polygonal nucleated bodies, ranged side by side, or one over the other, 

 which have received the name of " the granules." Between these is 

 noticed a plexiform or reticulated arrangement of the so-called "interstitial 

 substance." The latter consists of thinner or thicker fibres of striped 

 muscle, which can be followed into the substance of the heart. Those 

 cell-like bodies which lie in the interstices also frequently present a 

 transverse and longitudinal striation, and may unite finally with the 

 surrounding striped network to form stronger muscular fibres. 



For our own part we look upon the whole as a strange complicated 

 interlacement of cardial or endocardial muscle fibres, which have remained 

 stationary at an embryonic stage of development. We refer the reader 

 to the genesis of the latter ( 172). 



221. 



All the cavities of the heart, with their inequalities and projections, are 

 clothed with an endocardium of varying thickness. This structure is 

 thinnest in the ventricles, where it is presented to us in the form of a 

 delicate membrane, and thickest in the atrium sinistrum, where it forms 

 a tough lining. 



It consists of several layers. As a substratum may be recognised an 

 elastic lamina with abundant elastic fibrous networks, and corresponding 

 poorness in connective-tissue. Internally appears a specially dense lamella 

 of an elastic network supporting a coating of simple endothelium (p. 139). 



The external layer contains, besides, in the ventricles, smooth and 

 transverse muscle fibres ; but in the auricles only a few scattered contractile 

 fibre-cells are to be found (Schweigger-Seidel). 



