568 



THE VASCULAR SYSTEMS 



usually reaches the anterior papillary muscle by passing along the moderator band when this 

 is present. The undivided portion of the auriculoventricular bundle consists of narrow, some- 

 what fusiform fibres, but its two divisions and their terminal strands are composed of Purkinje 

 fibres. The bundle is not always easily recognized in the human heart, but is readily demon- 

 strated in the heart of the sheep or calf. 



A constant bursa or lubricating mechanism is in relation with the main bundle, according 

 to Curran, 1 and a special artery, arising from the right coronary, enters the bundle at its begin- 

 ning and follows it in direction. 



The Purkinje fibres are very much larger in size than the cardiac cells, and differ from them 

 in several ways. In longitudinal section they are quadrilateral in shape, being about twice as 

 long as they are broad. The central portion of each fibre contains one or more nuclei and is 

 made up of granular protoplasm, with no indication of striations, while the peripheral portion is 

 clear and has distinct transverse striations. The fibres are intimately connected with each other, 

 possess no definite sarcolemma, and do not branch. 



FIG. 424. Schematic representation of the auriculoventricular bundle of His. The bundle, represented in 

 red, originates near the orifice of the coronary sinus, undergoes slight enlargement to form a node, passes for- 

 ward to the ventricular septum, and divides into two limbs. The ultimate distribution cannot be completely 

 shown in this diagram. 



The epicardium, or visceral layer of the pericardium, is a serous membrane analogous in 

 structure to the endocardium, but contains no smooth muscle tissue. It is thin, smooth, glisten- 

 ing, and transparent, and is reflected over the roots of the great vessels to the inner surface of the 

 pericardial sac, forming there the parietal layer of the pericardium. 



Applied Anatomy. Clinical and experimental evidence go to prove that the auriculoven- 

 tricular bundle conveys the impulse to systolic contraction from the auricular septum to the 

 ventricles, and much attention has recently been paid to it, because it appears to become 

 attacked by various disease processes and to lose much of its conducting power in many cases 



i The Anatomical Record, December, 1909, vol. iii, No. 12. 



