or THE VALVES OF THE VASCULAR SYSTEM IN VERTEBRATA. 783 



below upwards ; the downward movement preceding the upward by an almost 

 inappreciable interval of time. In that brief space, however, which elapses 

 between the downward and upward movements, the ventricles, owing to the 

 contraction of the more circular fibres, are visibly diminished from without in- 

 wards ; and it is important to note this circumstance, as the auriculo-ventricular 

 orifices are, at this instant, reduced in size, and the mitral and tricuspid valves, 

 consequently liable to a certain amount of displacement. The ventricular wall 

 of the left ventricle, as was known to Gerdy and other investigators, is thickest at 

 the upper part of its middle third (Plate XXVIII. fig. 35 j), and tapers towards the 

 apex {w) and base {v') respectively ; and it is interesting to observe that the thickest 

 part of the ventricular wall, corresponds with the widest portion of the ventricular 

 cavity, whence the blood is projected into the aorta; a fact of some significance, 

 since the contractions at this point are necessarily more intense than at any other. 

 As the two sets of fibres composing the first external layer are continuous at the 

 left apex with the two sets of fibres forming the carnese columnae and musculi 

 papillares, and these structures, especially the latter, bear an important relation 

 to the segments of the bicuspid valve, with which they are connected by the 

 chordae tendinese, a more minute description than that given of the other layers, 

 is requisite for clearness. On looking at the left auriculo-ventricular opening 

 (Plate XXVIII. fig. 30 h), the fibres of the first layer are seen to arise from 

 the fibrous ring surrounding the aorta (a), and from the auriculo-ventricular tendi- 

 nous ring (n) in two divisions; the one division {d) proceeding /;-owz the anterior 

 portions of the rings, and winding in a spiral nearly vertical direction, from before 

 backwards, to converge and enter the apex posteriorly; the other set (Plate XXVIII. 

 fig 30/) proceeding from the posterior portions of the rings, and winding in a 

 spiral direction from behind forwards, to converge and enter the apex anteriorly. 

 Having entered the apex, the two sets of external fibres are collected together, 

 and form the musculi papillares and carneae columnse ; the one set, viz., that 

 which proceeded from the auriculo-ventricular orifice anteriorly and entered the 

 apex posteriorly, curving round in a spiral direction from right to left upwards, and 

 forming the anterior musculus papillai-is (Plate XXIX. fig. 50 y), and the carnece 

 columnce next to it; the other set, which proceeded from the auriculo-ventricular 

 orifice posteriorly, and entered the apex anteriorly, curving round in a correspond- 

 ing spiral direction, and forming the posterior musctdus papillaris (Plate XXIX. 

 figs. 50 and 51 <«), and adjoining carneoe columnce. As the external fibres con- 

 verged on nearing the apex, so the internal continuations of these fibres radiate 

 towards the base ; and hence the conical shape of the musculi papillares. I am 

 particular in directing attention to the course and position of the musculi papil- 

 lares, as they have hitherto, though erroneously, been regarded as simply vertical 

 columns, instead of more or less vertical spiral columns.* The necessity for in- 



* Plate XXIX. fig. 49, a; y, gives the spiral track of the musculi papillares. 



