218 THE VASCULAR SYSTEM. 



were drawn, transverse bands (cut longitudinally), 0-33 mm. in 

 length, were numerous ; their central ends terminated in bands, the 

 fibres of which were cut transversely. The auricular septum (Fig. 

 141 a M] also possesses muscular fibres which cross one another in its 

 substance. 



The fibres of the truncus arteriosus resemble those of an ordi- 

 nary artery; those of the sinus venosus are arranged in rings, 

 mostly transverse, but others oblique, and a few longitudinal.] 



(2) The structure of the cardiac muscle-fibres (Figs. 138 and 

 141 a). 



[The cardiac muscle of the frog consists of spindle-shaped cells, 

 which as Pohl-Pincus pointed out, are of two kinds ; the main differ- 

 ence between the two being in the size of the nucleus, which is 

 much broader in one than in the other. 



Fig. 138. 



TAf 



Portion of a transverse section through the middle of the ventricle of R. temporaria. From 



same heart as the sections in Fig. 136. Oc. I, Syst. 7, Leitz. G. H. 



A Rod-shaped nuclei of muscle fibre. 



B Oval nuclei of muscle fibre. 



CM Circular layer of muscle. 



CT Connective-tissue. 



E Endothelium of the epicardium. 



TM Transverse bands of muscle. 



a. Cells with a broad nucleus (Fig. 138 B] : the nucleus is lenti- 

 cular or egg-shaped, 4 to 7 ju, broad, 8 to 14 or 1 8 /u, long; the 

 ratio of length to breadth being I : 2. 



/3. Cells with a narrow nucleus (Fig. 138 A) : the nucleus is 



