OF THE CIRCULATION IN GENERAL. 295 



of the animal at the moment of death, that it is almost impossible to make 

 any direct measurement. The average capacity of the cavities may, how- 

 ever, be estimated, in the full-sized Heart, at about lour to five ounces. The 

 regurgitatiou of the blood is effectually prevented by valves, the structure 

 of which has also been investigated by Dr. Pettigrew, 1 and may be under- 



Fu . 117. FIG. 118. 



FIG. 117. Segment of human semilunar valve from the pulmonary artery suspended from the fibrous 

 band a a', x. r, Lunulse, which when the valve is in action become accurately applied to corresponding 

 lunuhe in the two remaining segments, c, Corpus Arantii, or portion usually thickened in old, though 

 not in young people; n, thickened convex border attached to fibrous ring of pulmonary artery ; o, 

 thinner portion of segment, j s', Fibrous bands which break up in the mesial line of the segment to 

 support and strengthen it. 



FIG. 118. Anterior segment of the human mitral valve, showing the threefold distribution of the 

 chord* tend! uege from above downwards, and from the mesial line towards the margins of thesegments. 

 z, base of segment ; x, aprx ditto; r r' s s' , chords tendinese from anterior portions (a c) of right and left 

 musculi papillares, bifurcating and losing themselves in margins of segments; b, d, posterior portions 

 of right and left musculi papillares which send chordae tendiueie to posterior segment of mitral valve. 



stood by an examination of the adjoining figures. In the case of the valves 

 of the pulmonary artery they appear to be not quite equal in size, nor are 

 they exactly upon the same plane ; when made to act by pouring water into 

 them, and watching them from below, it may be seen, according to Dr. Pet- 

 tigrew, that the smallest segment, which is situated highest, descends with a 

 spiral movement, and first falls into position ; the middle-sized segment, 

 which is placed a little lower, descends in like manner, fixing the first seg- 

 ment by one of its lunulee or crescentic surfaces ; the third and largest seg- 

 ment, which occupies a lower position than either of the others, descends 

 spirally upon the cresceutic margins of the other two, wedging and screwing 

 them more and more tightly into each other. The spiral movement is occa- 

 sioned by the direction of the sinuses of Valsalva, which curve towards each 

 other and direct the blood in spiral waves upon the mesial line of each seg- 

 ment. The muscular fibres of the heart are peculiar in presenting an inter- 

 mediate form between the striped and unstriped varieties of muscular tissue; 

 the fibres are striated, but destitute of a sarcolemma, and according to Eberth 2 

 are composed of broad cells that are often forked at their extremities or give 

 off lateral prolongations which, being firmly adherent to each other, produce 

 a deceptive appearance of anastomosis of the fibres. In a chemical point of 



1 Transact, of the ~Roy. Soc. of Eclinb., vol. xxii, part iii, 1864, p. 701. 



2 Virchow's Archiv, Bd. xxxvii, Heft 1, p. 100. See also Schweigger-Seidel, 

 Strieker's Histology, New Syd. Soc. Trans., vol. i, p. 248. 



