THE CIRCULATORY SYSTEM. 



101 



auriculo-ventricular valves is thicker than that of the ventricular 

 surface. The roots or attached portions of these valves possess 

 thickenings the annul! fibrosi composed of supplementary 

 masses of fibro-elastic tissue. The auricular muscle is continued into 

 the valves for about one-third of their width, following closely the 

 general contours of the fold. Within the larger chordae tendineae 

 the papillary muscles extend for some distance, in addition to which 

 isolated muscle-bundles are also sometimes present. The semilunar 

 valves possess a thin 

 elastic layer on the 

 arterial surface, aug- 

 mented by a thick 

 stratum of connective 

 tissue, the bundles ex- 

 tending parallel with 

 the margin of the 

 valve; increased 

 strength is secured by 

 a fibro-elastic nodule, 

 or corpus Arantii, 

 which occupies the 

 middle of each leaflet. 

 Beneath the ventric- 

 ular endocardium, in 

 many animals (deer, 

 sheep, calf, pig, horse, 

 goat, dog, certain 

 birds, etc.), but not in 

 man, peculiar bands 

 the fibres of Purkinje 

 occur ; these are 

 muscular fibres whose 

 transverse striations 

 are limited to the pe- 

 ripheral zone, while 

 their centre is occupied 

 by a large continuous 

 mass of nucleated pro- 



Section of the heart, including a leaflet of the semilunar valve 

 of the pulmonary artery of child : a, a, cardiac, 6, b, arterial, 

 surface ; c , recess behind the valve (f), constituting part of a 

 sinus of Valsalva : ti. free border of valve ; e, thickening near 

 edge of valve corresponding to a corpus Arantii ; g, endothe- 

 lium, h, intima, /, media, k, adventitia, of the pulmonary artery ; 

 the adventitia is continuous with the principal fibrous layer of 

 the endocardium ; /, cardiac muscle ; , areolar tissue. 



toplasm. The fibres 



of Purkinje represent an embryonal condition of the muscular tissue, 

 since the peripheral part of the fibre alone has undergone differen- 

 tiation, while the central portion has remained indifferent protoplasm. 

 Among some lower vertebrates, as fishes, a similar condition of the 

 muscle-fibres is constant. 



