35 



one hundred and fifty sections from four full-grown mice and a half- 

 grown white reat; but as other duties have for the present interfered 

 with our work, we now give the results so far as they have extended. 



In all of our specimens, especially those from the ventricle of the 

 mouse, we find the fine inter-lamellar network depicted by Retzius. 

 This network is spread through the whole thickness of the heart muscle, 

 and is perfectly apparent both on cross and longitudinal section, and 

 extends from the auriculo-ventricular groove to the extreme apex of 

 the ventricle. The fibres composing it are of medium size as well as 

 extremely fine ones. The main fibres course generally in the direction 

 of the long axis of the muscular bundles, and here and there give 

 off not very numerous branches, which in their turn divide and sub- 

 divide between the muscular fibres, spreading out as they proceed, 

 and again give off branches dichotomously. These latter branches may 

 either join other fibrillae, or send out terminal filaments. All the 

 fibres have frequent knotty varicosities upon them, rarely sufficiently 

 closely set to present a pearl-string appearance, and at other times 

 longitudinal enlargements of the fibre may be found extending over 

 considerable distances, all the varicosities however being very small. 

 The final terminal branches are usually short, and terminate in a 

 simple rounded end-knob, either between the muscular fibres or upon 

 the convexity of the sarcous bundle. That they do not end within 

 the muscular substance is perfectly apparent on cross sections, where 

 the bulbs may be very clearly and distinctly seen adjusted between 

 the muscular fibres, touching but not entering the sarcous substance 

 of either. 



Figure 1 shows clearly the relation of the nerve plexus to the 

 muscular fibres, and in fig. 2 nos. 1 and 3 are shown more highly 

 enlarged terminal endings from the varicose plexus. Figure 3 has 

 also in its upper portion, a single fibre from the network, crossing 

 another variety of nerve filament of larger calibre. 



Besides the widely extended varicose meshwork, we meet with 

 here and there, fibres of coarser calibre and without varicosities that 

 apparently have no connection with the network, and end in a totally 

 different manner from the ultimate fibrillae of the nerve networks 

 described above. These non-varicose fibres are found only in limited 

 numbers, scattered here and there in the muscular tissue, commonly 

 at some distance from the larger blood-vessels. They follow an undu- 

 lating course between the muscular spindles, and whereever their end- 

 terminations appear, they are always complex, and form figures of 

 considerable size and intricacy. The most usual form observed is 



3* 



