XIII.] STRUCTURE AND ACTION OF THE HEART. 145 



a. The two bundles of nerve fibres running 

 down the septum and having clusters of 

 nerve cells on them (this is best seen with a 

 low power) ; a few of the nerve fibres are 

 medullated. 



b. From the nerve bundles, nerve fibres extend, 

 forming a plexus over the septum ; in this 

 plexus nerve cells occur either singly or in 

 small groups. 



c. Not infrequently a cell may be seen to give 

 off a process, sometimes a spiral process 

 may also be made out. 



d. The septum is chiefly composed of a thin 

 plexus of fibres made up of faintly-striated 

 muscle cells, the cells and their nuclei are 

 elongated and more nearly resemble in form 

 the cells of unstriated muscle than the cells 

 of mammalian heart muscle (cp. Lesson vili. 

 12). The outlines of the individual cells 

 are not distinct. 



e. No capillaries are present. 



Numerous nerve fibres and cells may also be 

 seen in the sinus and at the junction of the 

 septum with the ventricle (Bidder's ganglia) ; a 

 few occur in the auricles and the basal portion 

 of the ventricle. A piece of the auricles should 

 be mounted in glycerine to observe the inter- 

 lacing bundles of fibres, the further development 

 of which gives the ventricle the spongy appear- 

 ance shown in a section ; as in the septum (and 

 rest of the frog's heart) no capillaries will be seen. 

 L. 10 



