218 THE VASCULAR SYSTEM. 



were drawn, transverse bands (cut long-itudinally), 0*33 nun. in 

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

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

 141 a 31) 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. T38. 



rj/ 





B 



..c& 



Portion of a transvei-se section through tlie middle of the ventricle of /?. femporaria. From 

 same heart aa 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. 

 B Endothelium of the epicardium. 

 TM Transverse bands.of muscle. 



a. Cells with a broad nucleus (Fig. 138 ^) : the nucleus is lenti- 

 cular or egg-shaped, 4 to 7 jix broad, 8 to 14 or 18 m lo^gi the 

 ratio of length to breadth being i : 2. 



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



