THE MUSCULAR FIBRES OF HEART. 



475 



Until such a specimen is obtained, the student may omit the 

 following description. 



The muscular fibres of the heart, although involuntary, are striped : but 

 they differ in their character from those of the voluntary muscles. The 

 fibres of the auricles are distinct from those of the ventricles. 



In the wall of the auricles the fibres are mostly transverse (fig. 172. a, J), 

 and are best marked near the ventricles, though they form there but a thin 

 layer ; and some of the fibres dip into the septum between the auricular 

 cavities. Besides this set, there are annular fibres surrounding the appen- 

 dages of the auricles and the endings of the different veins ; and lastly, 

 a few looped fibres (c, d) pass 

 obliquely over the auricle from 

 front to back. 



Dissection. The auricles having 

 been learnt, separate them from 

 the ventricles by dividing the 

 fibrous auriculo- ventricular rings. 

 Kext clean the fleshy fibres of the 

 ventricles by removing all the fat 

 from the base of the heart around 

 the two arteries (aorta and pul- 

 monary), and from the anterior 

 and posterior surfaces. 



Before cutting into the heart, 

 let the student note that the 

 anterior surface is to be recog- 

 nised by the fibres turning in at 

 the septum, with the exception of 

 a small band at the base and 

 another at the apex ; while on the 

 posterior aspect the fibres are 

 continued from the left to the 

 right ventricle across the septum. 



To show the direction of the 

 muscular bundles in the left ven- 

 tricle, divide the superficial fibres 

 in front longitudinally near the 

 septum, and transversely about 

 half an inch below the left auri- 

 culo-ventricular opening ; and re- 

 flect a thin layer of the fibres 

 carefully towards the left side. 

 In the same way a second layer 

 is to be reflected ; then a third, 

 and so on, each layer that is 

 raised being about as thick as 



the thin end of the scalpel. It will not be difficult to demonstrate thus six or 

 seven layers in the wall ; and as each is raised, the fleshy fibres will be seen 

 to change their direction (fig. 173). 



On the right side a similar dissection may be made, and a like number of 

 layers may be displayed, but greater care will be necessary owing to the 

 thinness of the wall. Make a vertical cut along the anterior aspect from the 

 root of the pulmonary artery to the apex of the ventricle ; and reflect the 

 several layers inwards and outwards. As the superficial ones are raised, 

 their fibres may be followed into the septum in front, and across the middle 

 line into the wall of the left ventricle at the back. 



Thickness of the ventricular walls. The wall of the left ventricle is in 

 general nearly three times as thick as that of the right. Its thickest part is about 

 one-fourth of its length from the base ; and at the apex it is very thin. The 

 free wall of the right ventricle is of more uniform thickness than the left. 

 The septum is about as thick as the wall of the left ventricle, except at the 



Muscular 

 substance of 

 heart. 



Fibres of 

 the auricles 



are trans- 

 verse, 



annular, 

 and looped. 



FlG - 



172. MUSCULAR FIBRES 

 AURICLES. 



a. Transverse fibres of the right, and 

 b, of the left auricle. 



c. Looped fibres of the left, and d, of 

 the right auricle. 



e. Superior cava. 



/. Inferior cava. 



g. Right, and h, left pulmonary veins. 

 Annular fibres surround the veins. 



and of 

 right. 



Thickness 

 of left ven- 

 tricle, 



of right, 



and of 



septum : 



