782 DR PETTIGREW ON THE RELATIONS, STRUCTURE, AND FUNCTION, 



and left ventricle, occurs as a spiral slit in the septum (Plate XXIX. fig. 44 r), and 

 is guarded by two projecting muscular surfaces, which are rounded off for this 

 purpose. The opening into the left ventricle also occurs as a muscular slit (s) ; 

 and the orifices of many of the venous sinuses are closed by purely muscular adap- 

 tations ; the fibres in such instances running parallel to the slit-like opening (Plate 

 XXVIII. fig. 10), and being continuous with two or more bundles of fibres {b c), 

 which supply the place of musculi papillares. From the great variety in the 

 shape and structure of the auriculo-ventricular valves, and from the existence in 

 almost all of tendinous chords, which connect them with actively contracting 

 textures, there can, I think, be little doubt, that they possess an adaptive power 

 peculiar in a great measure to themselves ; this power being traceable to the 

 contractile properties residing in muscle. 



As it would greatly exceed the limits of the present paper, to give a detailed 

 account of the structure of the numerous auriculo-ventricular valves, to which 

 allusion has been made, I have selected for description the auriculo-ventricular 

 valves of the mammal, and those of man more particularly. Before, however, 

 entering upon this the most difficult part of the present investigation, a brief 

 account of the an-angement of the muscular fibres in the ventricles seems indis- 

 pensable ; these, as has been explained, modifying the action of the valves to a 

 very considerable extent. 



Arrangement of the Muscular Fibres in the Ventricles of the Mammal — Shape of 



THE Ventricular Cavities, &c. 



The fibres of the ventricles in the mammal, as I have ascertained from 

 numerous dissections,* are arranged in seven laj^ers ; three external, a fourth or 

 central, and three internal. The fibres constituting these layers in the left ven- 

 tricle, to which these remarks more particularly apply, pursue a spiral direction ; 

 the external fibres becoming more and more oblique, in a direction from left to 

 right downwards, as the central layer is approached — the internal fibres becoming 

 more and more vertical, in a direction from right to left upwards, as it is receded 

 from. The fibres, therefore, of corresponding external and internal layers, cross 

 each other. The fibres of the several layers are further arranged in two sets ; the 

 two sets, forming each of the external layers, being continuous at the apex and 

 at the base, with two similar sets belonging to a corresponding internal layer. 

 This arrangement of the fibres renders the ventricles bilaterally symmetrical, and 

 in part accounts for the great precision with which the heart acts, and for its roll- 

 ing movements. Its bearing on the action of the organ is obvious, for as muscular 

 fibres contract in the direction of their length, the more vertical external and 

 internal fibres, diminish the ventricular cavities from above downwards, and from 



* Of these upwards of an hundred are preserved in the University of Edinbui'gh Anatomical 

 Museum, where they may be examined. For a detailed description of the specimens, and for 

 accurate representations thereof, see Phil. Trans, vol. 154, pp. 445-500, Plates 12 to 16. 



