247 



in the distal portion of the muscle, well defined macroscopic units, 

 each about half-a-centimeter wide and a millimeter thick, which extend 

 from the costal to the distal margin of the muscle and terminate at 

 each end in a well marked tendon (Fig. 4a). Nerves of innervation 

 enter bundles of this kind at several places, as shown at the right in 

 the figure. 



After maceration in the nitric-acid-glycerine mixture above men- 

 tioned it is easy to isolate small fibre-bundles, less than half-a-milli- 

 raeter in diameter, which run completely across the muscle. In a series 

 of cross-sections taken through a secondary segment, it is not, how- 

 ever, possible to follow any given group of fibres from one extremity 

 of the segment to the other. The disposition of the connective-tissue 

 is far too irregular to admit of this. The small fibre-bundles isolated 

 after maceration are therefore to be looked upon as artificial secondary 

 units, the muscle-fibres themselves being the primary units. 



By carefully teasing one of these small bundles of fibres the in- 

 dividual fibres composing it may be isolated. When this is done it 

 is found that none of the individual fibres run the entire length of the 



Fig. 4. a Macro- 

 scopic "unit" isolated from 

 the distal portion of a 

 human external oblique 

 muscle, b In the center 

 is shown a flat band of 

 fibres dissected from the 

 external oblique muscle 

 of a dog. The main nerve 

 trunks crossing near this 

 band are represented by 

 the broad dark lines. The 

 areas of distribution of mo- 

 tor nerves to the muscle- 

 fibres composing the band 

 are shown by groups of 

 dots. At each side a few 

 isolated muscle-fibres are 

 shown. For the sake of 

 clearness they are repres- 

 ented disproportionately 

 wide. One-half natural 

 size. 



Fig. 5. The ex- 

 ternal oblique muscle of 

 a dog. The main nerve 

 trunks only are shown. 

 The areas of innervation 

 of two flat bands of fibres 

 a-b, c-d are represented 

 by groups of clots. Two- 

 fifths natural size. 



TT 



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Y 



Fie. 4. 



Fig. 5. 



