i8o 



HISTOLOGY 



In the heart of adult frogs, the system of intermuscular clefts or lacunar vessels 

 is the only blood supply of the ventricular musculature; the coronary vessels are 

 limited to the epicardium. In turtles the coronary vessels supply an outer layer of 

 the ventricular muscles, but the greater part is still nourished by the central lacunae 

 or sinusoids. This sinusoidal circulation, which is characteristic of the adult heart 

 in lower vertebrates, occurs also in mammalian embryos, but it becomes vestigial in 

 adult mammals. 



A structure which has recently received much attention because of its 

 functional importance is a small band of muscle fibers, associated with 

 nerves, which passes from the septum between the atria into the septum 

 between the ventricles. This atrio-ventricular bundle or "bundle of His" 



(discovered independently in 

 1893 by Kent and His, Jr.) 

 represents the only connec- 

 tion between the muscula- 

 ture of the atria and ventri- 

 cles; it passes through the 

 fibrous tissue where the 

 annuli fibrosi come together. 

 The position of the bundle 

 is shown in Fig. 173, after 

 Curran (Anat. Rec., 1909, 

 vol. 3, pp. 618-632). Curran 

 finds more extensive branches 

 in the atria than others have 

 shown. They come from 

 both sides of the heart into 



Rd 



VC.L 



FIG. 173. THE ATRIO-VENTRICULAR BUNDLE (F. a. v.), AND 



THE POSITION OF THE "SlNO-ATRIAL NODE" (x) IN A 



HUMAN HEART. (After Curran and Aschoff.) 

 Ao., Aorta; A. p., pulmonary artery; F. o., fossa ovalis; S. c., 

 coronary sinus; R. d., right branch of the atrio-ventricular 

 bundle; and R. s., its left branch; V. c. i. f vena cava in- 

 ferior; V. c. s., vena cava superior. 



the inter-atrial septum, and converge from the fossa ovalis, the roots 

 of the tricuspid valve and the orifice of the coronary sinus to form 

 the atrio-ventricular node. This is "a small mass of interwoven fibers 

 in the central fibrous body of the heart, " and the main bundle, 2-3 

 mm. wide, passes from it into the inter-ventricular septum. It passes 

 under the pars membranacea, and divides into two branches which are 

 distributed to the right and left ventricles, respectively. Their extensive 

 ramifications have been modelled by Miss DeWitt. She describes the 

 models, and briefly summarizes previous investigations of the bundle, in 

 the Anatomical Record (1909, vol. 3, pp. 475-497); the subject is more 

 fully considered by Aschoff (Verh. d. deutsch. path. Gesellsch., 1910, 



PP- 3-35)- 



The atrio-ventricular bundle is composed of muscle fibers which are 

 pale macroscopically. They are larger than those of ordinary cardiac 

 muscle, but contain fewer fibrils, peripherally placed and surrounded 

 by abundant scaroplasm (Fig. 172). In the ventricle they are specially 



