490 
DE. PETTIGREW ON THE ARRANGEMENT OE THE FIBRES 
Fig. 4. Left ventricle of the sheep’s heart, seen posteriorly. Shows the fourth or trans- 
verse layer, which occupies a central position in the ventricular wall, and 
divides the external from the internal layers. The fibres of this layer are in 
the act of doubling or turning upon themselves. See pages 458 & 459. 
Fig. 5. Left ventricle of the sheep’s heart, seen posteriorly. Shows the fifth layer. See 
page 459. 
Fig. 6. Left ventricle of the sheep’s heart, seen posteriorly. Shows the sixth layer. See 
pages 459 & 460. 
Figs. 7 & 8. Left ventricle of the sheep’s heart, seen posteriorly. Show the seventh or 
last internal layer, the fibres composing which extend from the extreme base 
to the extreme apex. See pages 460, 461, & 462. 
Fig. 9. Left ventricle of the sheep’s heart, as seen posteriorly from above. Shows the 
course pursued by the two sets of fibres constituting the superficial or first 
external layer. See pages 454, 455, & 456. 
f g. Posterior set of fibres of first layer, winding in a spiral direction from 
base to apex to enter the apex anteriorly. 
e d. Anterior set of fibres of the first layer, winding in a spiral direction from 
base to apex to enter the apex posteriorly. As the convexity of the 
posterior set of fibres fits accurately into the concavity of the anterior 
set, they are linked or twisted into each other so as completely to close 
the apex and render it bilaterally symmetrical. The internal continua- 
tions of the major portions of the two sets of fibres forming the first layer 
are seen at x and y of figs. 12, 13, 14, & 15, where they appear as the 
anterior and posterior musculi papillares. 
Fig. 10. Bird’s-eye view of the apex of the left ventricle of the sheep’s heart. Shows 
the two sets of fibres constituting the superficial or first external layer, sepa- 
rated from each other, and entering the left apex to become internal without 
breach of continuity. See pages 454, 455, & 456. 
e d. The anterior set of fibres, curving or twisting into or round the posterior 
set. 
f g. The posterior set of fibres, curving or twisting into or round the anterior 
set. 
Fig. 11. Bird’s-eye view of the apex of the left ventricle of the sheep’s heart. Shows the 
appearance presented by the left apex when the two sets of fibres composing 
the first layer have been removed. See pages 456 & 457. 
f g. Undissected portion of the anterior set of fibres of the first layer. 
1. Anterior set of fibres of the second layer, preparing to enter the left apex 
posteriorly. 
Jc. Posterior set of fibres of the second layer, preparing to enter the apex ante- 
riorly. 
Fig. 12. Transverse section of the left ventricle of the sheep’s heart, half an inch above 
