DISSECTION OF THE BACK AND THORAX, 131 



exposed which are slightly inclined, but in a direction opposite to the 

 superficial fibres; and the obliquity of these increases until the inner 

 surface of the wall is reached, where the fibres have a degree of obliquity 

 equal to the most external fibres. So much the student will probably 

 be able to make out without much difficulty, but according to Pettigrew 



View of a Pabtial Dissection of the Fibres of the left Wall of the Ventricles Tin a 

 Sheep's Heart, designed to show the different Degrees of Obliquity of_.the 

 Fibres (Allen Thomson). 

 At the base and apex the superficial layer of fibres is displayed : in the intervening space, more 

 and more of the fibres have been removed from above downwards, reaching to a greater depth on 

 the left than on the right side. a', a'. The superficial layer of the right ventricle ; 6'. 6'. The same 

 of the left ventricle ; at 2 this superficial layer has been removed so as to expose the fibres under- 

 neath, which are seen to have the same direction as the supei-ficial ones over the left ventricle, but 

 diflerent over the right ; at 3 some of these have been removed, but the direction is only slightly 

 different ; 4. Transverse or annular fibres occupying the middle of the thickness of the ventricular 

 walls ; 5, 6, 7. Internal fibres passing downwards towards the apex to emerge at the whorl ; between 



c. c. the left ventricular groove, over which the fibres of the superficial layer are seen crossing ; in 

 the remaining part of the groove, some of the deeper fibres turn backwards towards the septum ; 



d. The pulmonary artery ; e. The aorta. 



the fibres are arranged in seven determinate layers — three external 

 which are oblique, three internal, also oblique but in the opposite 

 direction, and a central which is horizontal. Further, he describes the 

 fibres of the most external layer as turning in at the auriculo-ventricular 

 orifices and at the apex of the heart to become continuous with the 

 layer beneath the endocardium. In like manner, he supposes that the 

 second layer is continuous with the sixth, and the third with the fifth, 

 while the fourth or central layer has a zone-like arrangement. In truth, 

 however, the fibres of the same stratum anastomose not only with one 

 another, but also with the fibres of adjacent strata, as is shown by the 



