1909.] The Nerves of the Atrio-ventricular Bundle. 155 



small bundles isolated by more or less definite connective tissue sheaths. 

 Through the moderator band it reaches the lateral wall of the ventricle and 

 the papillary muscles. The left branch appears under the endocardium of 

 the septal wall of the left ventricle about 1*5 cm. below the junction of the 

 posterior with the right semilunar valve. Immediately under the aortic 

 opening it is covered with cardiac muscle. On reaching the surface of the 

 heart it spreads out and sends branches downwards and outwards to the 

 septal and lateral walls of the ventricle. 



In mammals generally, the course of the atrio-ventricular bundle agrees in 

 the main with this, but some slight differences are observed. Thus in the 

 sheep the band is smaller and less distinct. The right septal branch is 

 covered by cardiac muscle from the atrio-ventricular septum to the moderate 

 band ; the left branch appears under the endocardium 1'2 cm. beneath the 

 junction of the right and posterior aortic valves. In the pig there is a close 

 resemblance to this. It is interesting here to note how in all mammals the 

 bundle is in close apposition to the insertion of the aorta, behind its posterior 

 valve. 



Microscopically, as Tawara pointed out, one must also distinguish an atrial 

 and ventricular part. The atrial part begins as a complicated network of 

 branching fibres smaller than the atrial muscle cells with a nucleus lying in 

 undifferentiated protoplasm and with fibrillar less well developed and more 

 irregular than in the ordinary cardiac cell. Within this network lie 

 connective tissue with fat, blood-vessels, and nerves. From it emerges a 

 series of more or less parallel muscle fibres similar to those of the network 

 surrounded by a well-marked connective tissue sheath. The appearance of 

 these cells and the amount of connective tissue between them readily 

 distinguish them from the ordinary cardiac muscle. 



The ventricular section begins immediately where the atrio-ventricular 

 bundle breaks through the fibrous septum of the atrio-ventricular groove. It 

 consists of an irregular network of muscle cells surrounded by connective 

 tissue. Histologically it has no similarity with the atrial portion of the band 

 nor with the ordinary cardiac muscle cell. One, two or more of the cells of 

 the atrial strand pass into a much larger cell of irregular size and shape which 

 possesses many features in common with a Purkinje cell. As the ventricular 

 strand passes down the septum, these initial cells gradually pass into typical 

 Purkinje fibres which constitute the muscle cell component of the two arms 

 and their outspreading branches. 



Technique. — As a first step, it is necessary to be able rapidly to cut out the 

 bundle either after staining or in the fresh condition. This can be done after 

 a series of preliminary dissections. For the purpose of this research I limited 



