ON THE STRUCTURE AND FUNCTION OF THE MAMMALIAN HEART. 227 



The second part deals with the work of the Committee for the year 

 1914--L5. 



Pakt T. 



The work of the Committee since the date of the last Report ' has 

 been concerned (a) with investigations undertaken in order to confirm 

 the existence of conducting paths between the auricle and ventricle other 

 than the auriculo-ventricular bundle, and (b) with an examination of the 

 functions of such paths. 



With regard to the former, viz. the existence of conducting paths 

 between the auricle and the ventricle other than the aui'iculo-ventricular 

 bundle, it has been established that there exists on the right side of the 

 heart a bundle of muscular tissue which forms a path of conduction 

 between the auricle and ventricle, and which is capable of conducting 

 impulses between these chambers in the absence of the auriculo- 

 ventricular bundle. The evidence upon which this assertion is based 

 is partly histological and partly experimental. By histological examina- 

 tion it has been possible to establish the following facts. Serial sections 

 cut through the auriculo-ventricular junction at a certain point show 

 ventricular muscle fibres approaching the junction from three direc- 

 tions. These fibres are connected with fibres of auricular origin through 

 a mass of modified tissue presenting characters similar to those seen 

 in the sino-auricular and auriculo-ventricular nodes. 



The mass of modified tissue is well defined. It is usually of olive 

 shape, and consists of a central mass in which the modification is well 

 marked, with a peripheral portion which more nearly approaches normal 

 muscle in structure. With the peripheral portion on one side the ventri- 

 cular muscle is connected, whilst auricular muscle is connected with 

 the peripheral portion on the opposite side. 



On account of the character of the tissue and its resemblance to 

 nodal tissue in other parts of the heart, it is called a ' node. ' From 

 its position it is called for purposes of identification the ' right lateral 

 node. ' This structure is so placed that impulses passing down the 

 auricular wall would traverse the node before reaching the ventricle. 

 If, as suggested many years ago," impulses are conducted at a slower 

 rate over the modified tissue of the heart than over ordinary cardiac 

 muscle, there may be here, as in the case of the auriculo-ventricular 

 bundle, an explanation of the pause which occurs between the auricular 

 and the ventricular beat. It may be pointed out too that the sectional 

 area of the path is at one point extremely small, and there is evidence 

 in the work of Bomanes, Gaskell, and others to prove that in such 

 circumstances the rate of transmission is slow. The pause may, there- 

 fore, be regarded as being due partly to transmission through modified 

 tissue, and partly to transmission along a muscular path of narrow 

 cross -section. 



The experimental evidence of the existence of a path of conduction 

 between the auricle and ventricle in this situation is of the following 

 kind. It has been stated by many that auricular contractions fail to 



' Annuol Tteport, 1913, p. 258. 

 ' Journal of Physiolotjih xiv. No. 1. and xiv. Nos. 4 and 5. 



Q 2 



