66 



MUSCULAR FIBRES OF THE HEART. 



trunk. It consists of the vena portarum formed by the union of the 

 intestinal or mesenteric and splenic veins, and it passes into the 

 liver, where it divides into capillaries, from which the hepatic veins 

 arise. These last veins join the inferior vena cava. 



Strictly speaking, however, there is no special portal circulation. 

 Similar arrangements occur in other animals in different places 

 e.g., snakes have such a system in their supra-renal capsules, and the 

 frog in its kidneys. 



When an artery splits up into fine branches during its course, and 

 these branches do not form capillaries, but reunite into an arterial 

 trunk, a rete mirabile is formed, such as occurs in apes and the eden- 

 tata. Similar arrangements may exist on veins, giving rise to venous 

 retia miraUlia. 



43. The Heart. 



Muscular Fibres of the Heart. The musculature of the mammalian 

 heart consists of short (50 to 70 /*, man), very fine, transversely striated 

 muscular fibres, which are actual uni-cellular elements (Eberth), devoid 

 of a sarcolemma (15 to 25 p broad), and usually divided at their 

 blunt ends, by which means they anastomose and form a net- 

 work. (Fig. 17, A, B.) The individual muscle-cells contain in their 



Fig. 17. 



A, branched muscular fibres from the heart of a mammal ; B, transverse section of 

 the cardiac fibres ; &, connective tissue corpuscles ; c, capillaries ; C, muscular 

 fibres from the heart of a frog. 



centre an oval nucleus, and are held together by a cement which is 

 blackened by silver nitrate, and dissolved by a 33 per cent, solution of 

 caustic potash. This cement is also dissolved by a 40 per cent, solu- 

 tion of nitric acid. The transverse striae are not very distinct, and 

 not unfrequently there is an appearance of longitudinal striation, pro- 

 duced by a number of very small granules arranged in rows within 



