THE HEART 



3 J 



Heart Muscle. — The heart muscle in structure is considered to 

 come midway between skeletal and involuntary muscle. Until 

 recently its fibres were described as short, branching, anastomos- 

 ing, and possessing no sarcolemma. There is good reason to believe 

 that this account is no longer correct, and that the fibres, instead 

 of being short, form a continuous sheet, fibrils passing from fibre 

 to fibre, and so constituting an anastomosis in every direction. 

 There is evidence also that a sarcolemma is present, though it is 

 delicate in structure. The fibrils, or sarcostyles, of which the 

 fibres are composed, are 

 both longitudinally and 

 transversely striated. 



Fibres, described as 

 the fibres of Purkinje, are 

 found beneath the endo- 

 cardium of the horse, ox, 

 and sheep. They give a 

 greyish appearance to the 

 part, and on microscopi- 

 cal examination are found 

 to be large polyhedral, 

 clear cells, containing 

 granular substance and 

 one or more nuclei. These 

 fibres are connected with 

 a peculiar band of muscle 

 which forms a connection 

 between the auricle and 

 ventricle. This band or 

 bundle is known as the 

 Auriculo - ventricular or 

 A.V. Bundle, and is the 

 discovery of recent years. 

 Prior to that it was held that the muscular structure of the 

 auricle and ventricle was -distinct, the chambers being separated 

 by a fibrous ring ; but in the heart of the ox and sheep this bundle 

 of muscle connecting the auricle and ventricle can easily be 

 traced by its paleness. This is proved to hold good for all 

 mammalian hearts. 



In Fig. 12 may be seen the arrangement of the band in the 

 right auricle, where it begins as a swelling known as the A.V. 

 Node (i). This node microscopically is found to consist of 

 peculiar branched cells. 



From the node is given off the main bundle (2), and the latter 

 divides into right and left divisions, one running to^the right 

 ventricle and one to the left. The right heart bundle (3) 



Fig. 12. — Right Auricle and Ventricle 

 of Calf, to show Auriculo - Ven- 

 tricular Band (after Keith). 



1, Auriculo- ventricular (A.V.) node; 2, main 

 auriculo- ventricular bundle ; 3, right septal 

 division of the bundle; 4, moderator 

 band. 



