406 MANUAL OF HISTOLOGY. 



The valves betAveen the auricles and ventricles (valvulce tricuspidales 

 and mitrales) are duplicatures of endocardium, Avith a strong middle layer 

 of fibrous tissue, derived principally from the fibres of the annulus fibro- 

 cartilagineus, and expansions of the tendons of the musculi papillares. 



On one aspect they are clothed with the strong endocardium of the 

 auricle, on the other by the thinner of the ventricle. 



Under the first of these endocardia muscular bands are prolonged into 

 the valves from the muscular substance of the auricle penetrating to 

 various depths (Gussenbaur). 



Finally, the whole is covered with simple endothelium. The semi- 

 lunar valves also of the arteries have a similar structure, except that the 

 middle layer is thinner. 



The blood-vessels of the heart present in its muscular substance the 

 most typical form of the elongated mesh-work (p. 370). Several capil- 

 laries pass immediately and together into one strong venous root. The 

 ready outflowing of the blood is thus better provided for than elsewhere. 

 The endocardium is only provided with vessels in its undermost connec- 

 tive-tissue layer. A few may also be seen in the auriculo-ventricular 

 valves, but none in the semilunar (Gerlach). 



The heart is supplied with lymphatic, vessels in considerable number, and 

 according to Ebertli, Belajeff, Wedl. The two leaves of the pericardium, 

 as well as the endocardium, contain dense networks of coarser or finer 

 trunks. In the interior of the auricles they appear more scanty than in 

 the ventricles. In the chordae tendinese, on the other hand, they are not to 

 be found, and in the semilunar and auriculo-ventricular valves are only 

 present in small number. The fleshy substance of the heart does not 

 appear to be so richly supplied with them as was formerly supposed by 

 Luschka. 



The nerves of the heart have their origin from the cardiac plexus, which 

 is itself made up of branches from the vagus and sympathetic. 



The course of the numerous nervous stems is alongside of the blood- 

 vessels until they spread out in the auricles and ventricles. The auricles 

 are poorer in nerves than the ventricles, of which the left is the most 

 richly supplied. The nerves of the heart appear more or less grey, and 

 consist of fine medullated tubes with an intermixture of Eemak's fibres. 

 They terminate for the greater part in the muscle, while some of them 

 may be traced into the endocardium. All efforts to elucidate the mode 

 of ultimate termination here have hitherto proved futile in man and the 

 mammalia generally. The occurrence of numerous microscopically small 

 ganglia is also peculiar. The latter appear on the nerves imbedded in 

 the substance of the heart, especially in the neighbourhood of the trans- 

 verse groove and septum ventriculorum. 



Physiology, as is well known, has brought to light the interesting fact 

 that these two kinds of fibre elements are entirely different in function. 

 Whilst the sympathetic, namely, preside over the contraction of the 

 muscle, having their chief centres of energy in the ganglia just referred 

 to, so that the heart continues to pulsate after removal ; the vagus fila- 

 ments exercises a completely opposite influence, causing, when stimulated, 

 an interruption to the motor power of the sympathetic elements, and to such 

 an extent also that the heart comes to a standstill in a condition of diastole 

 (E. Weber). It is possible that the fibres of the vagus may terminate in 

 the cardiac ganglia, i.e., in their cells. 



Regarding the composition of the muscle of the heart, vide chemistry of 



