324 DISSECTION OF THE THORAX. 



The fibres of the fourth layer are continued into the septum ventri- 

 culorum. 



Dissection. To display the layers and fibres of the right ventricle, 

 great care will I e needed because of the thinness of the wall ; but the 

 same number of layers exists in this, as on the other side of the heart. 



Make a vertical cut along the anterior aspect from the root of the pul- 

 monary artery to the aj ex of the ventricle ; and reflect the several layers 

 forwards and backwards from that incision. As the three outer are raised 

 let them be traced on the one hand into the part of the septum detached 

 from the left ventricle ; and on the other into the left ventricle through 

 the continuity of the common fibres behind. 



The RIGHT VENTRICLE possesses seven layers in its wall, like the left, 

 though they are much thinner. They are arranged as in the other ven- 

 tricle into three external, three internal, and a fourth or intermediate. In 

 like manner the wall decreases from the centre towards the base and apex, 

 but at the tip it is thicker than the apex of the left half of the heart 

 (Pettigrew). 



Directions of the fibres. In this as in the other ventricle, the fibres of 

 the three outer layers run down from the base to the fore part of the sep- 

 tum and the apex of the heart: the outer being most vertical. In the 

 fourth stratum the fibres have a transverse direction, as in the correspond- 

 ing layer of the left ventricle. And in the three inner layers they are 

 directed upwards from the apex to the base of the ventricle across the 

 fibres of the three outer strata, the deepest being the most vertical. 



Continuity of the fibres. The fibres are not distinct from those of the 

 left ventricle, but are derived in the three outer layers from the " common 

 fibres" and the septum, and from the auriculo-ventricular opening. They 

 are then continued forwards to the front of the septum ventriculorum, 

 where they leave the surface, and bending back construct the right part of 

 the septum : at the back of that partition they blend with the " common 

 fibres" of the left ventricle. The fourth layer fibres are continuous for 

 the most part with the " common fibres" crossing the posterior groove. 

 At the apex the three cuter layers do not enter in a whorl as in the left 

 ventricle ; but at the base they are continuous with the three inner as on 

 the other side of the heart. 



Many of the fibres of the external layer are attached to the ring of the 

 pulmonary artery ; and the narrow slip from the right to the left ventricle, 

 near the base in front, receives its fibres from the two outer strata. 



In this ventricle the fibres are arranged as if there had been originally 

 one common cavity in the heart the left ventricle, from which the right 

 had been detached during the growth by a pushing inwards of a partition 

 from the fore part. 



Septum ventriculorum. This partition between the two cavities has 

 been divided anteriorly into right and lei't parts by the previous dissection ; 

 and the layers of the ventricles may be traced into them. 



It is rather thicker than the wall of the left ventricle, and is formed by 

 the fibres of both ventricles. About one-third belongs to the right, and 

 two-thirds to the left ventricle. Where the two portions touch the fibres 

 mingle, and altogether behind lie the " common fibres" of the two ven- 

 tricles. 



Endocardium. Lining the interior of the cavities of the heart is a thin 

 membrane, which is named endocardium.- It is continuous on the one 

 hand with the lining of the veins, and on the other with that of the arteries. 



