THE HEART. 89 



inserted into a venous pathway. It is composed of the portal vein, 

 which represents the union of the veins of the abdominal viscera the 

 superior gastric, the superior and inferior mesenteric, and the splenic 

 veins and which breaks up in the liver into capillaries that again unite 

 to form the hepatic veins, which empty into the inferior vena cava. 



Strictly speaking, this differentiation of the portal system into a separate 

 circulation is not justifiable. In many animals similar conditions are found in 

 still other organs, as, for example, the suprarenal of the snake and the kidney 

 of the frog. When an artery breaks up into numerous small branches that shortly 

 reunite, without the intervention of capillaries, to again form an artery, the cluster 

 of branches thus formed is called a "wonderful network," rete mirabile, such as 

 is seen in apes and edentates. Microscopical networks of this character are found 

 in the mesentery of man. The glomerulus of Bowman's capsule in the kidney also 

 is an example of this peculiar arterial division. Analogous formations in the veins 

 are called venous "wonderful networks." 



THE HEART. 



The mammalian heart-muscle (Fig. 184, 8) is composed of short, closely and 

 finely striated, unicellular elements which are devoid of sarcolemma and, in man, 

 from 50 to 70 // long and from 15 to 23 /u wide. The ends are rather blunt and 

 generally split, and by these split ends the fibers are joined together anastomotically 

 to form a network. The individual muscle-cells are united by a cement-substance, 

 which is soluble in 33 per cent, potassium-hydrate solution and is stained black 

 by silver nitrate. Each cell at its center contains a nucleus, rarely two smaller 

 nuclei, 14 u- long by 7 // wide, in its central axis. The transversely striated sub- 

 stance frequently contains molecular granules arranged in rows. The fibrils are 

 placed side by side and are divided by the perimysium into bun dies, which, after 

 solution of the connective tissue by boiling, may be isolated. The shape of the 

 bundles on transverse section is rather circular in the auricles, while in the ventricles 

 it is rather flat and laminated; here also several of the smaller bundles may unite 

 to form a thicker band. The interstices between the bundles serve to carry the 

 lymph-vessels. 



ARRANGEMENT OF THE MUSCLE-FIBERS OF THE HEART AND 

 THEIR PHYSIOLOGICAL SIGNIFICANCE. 



Musculature of the Auricle. The study of the embryonal heart furnishes the 

 key to the understanding of the complicated arrangement of the muscle-fibers. 

 The simple heart-tube of the embryo exhibits an outer circular and an inner 

 longitudinal layer of muscle-fibers. The septum is formed later, so that it is 

 obvious that both in the ventricles as well as in the auricles the fibers belong, 

 in part at least, to both halves, as they originally enclose only a single cavity. 

 On the other hand, the fibers of the auricles are generally separated from those 

 of the ventricles by the fibrous ring; nevertheless certain of the muscle-bundles 

 pass from the auricles to the ventricles. In the auricles the embryonal arrange- 

 ment of the fibers remains fundamentally unchanged. In the ventricles the 

 arrangement is obliterated because during the process of development the fibers 

 here undergo a peculiar bending and looping, as in the stomach, together with 

 a spiral rotation. 



The musculature of the auricles is in general arranged in two layers: an outer 

 transverse, which is continuous over the two auricles, and an inner longitudinal. 

 The outer fibers can be traced from the entering veins upon the anterior and 

 posterior walls. The inner fibers are especially prominent where they are attached 

 vertically to the fibrous rings, but in certain parts of the anterior wall in particular 

 they are not arranged continuously. On the septum of the auricles the ring-like 

 muscular layer surrounding the oval fossa, the opening of the oval foramen in 

 the embryo, is especially prominent. Around the openings of the veins emptying 

 into the auricles are found circular muscle-bundles; these are least well marked 

 around the inferior cava, while- around the superior cava they are well developed 

 and extend upward around the vessels for 2.5 cm. (Fig. 22, II). At the entrance 

 of the four pulmonary veins in man and in some mammals, transversely striated 

 muscle-fibers, arranged in an inner circular and an outer longitudinal layer, ex- 



