248 



muscle-bundle. Those near the extremities terminate at onii end in 

 a tendon, at the other between other muscle-fibres. The fibres com- 

 posing the central portion of the bundle terminate by an intrafasci- 

 cular ending at each extremity (Fig. 4 b). In the dog the length of the 

 fibres varies from two to six or seven centimeters. 



The relations of nerves to the muscle -fibres may be determined 

 as follows: A thin flat band of muscle -fibres is dissected from the 

 muscle and treated like the thin muscle of a rodent. Fig. 5 shows 

 an external oblique muscle of a dog and the main nerve trunks dis- 

 tributing fibres to it; a-h, c-d represent the regions from which thin 

 bands of fibres have been removed, . treated and replaced. 



By the use of acetic acid, osmic acid and glycerine, the areas of 

 innervation of a muscle band are revealed. Fig. 4b and Fig. 5 show 

 instances of this kind. It is to be noted that areas of innervation 

 are scattered along the muscle-band. By treatment with osmic acid 

 or gold -chloride, followed by glycerine and then by the nitric- acid- 

 glycerine mixture, the individual fibres composing the band of muscle- 

 fibres may readily be isolated (Fig. 4 b). The nerve -ending may be 

 near one extremity or near the center of the fibre. As a rule, groups 

 of fibres are innervated in given areas and these groups of fibres inter- 

 digitate with other similar groups. This makes the internal archi- 

 tecture of the muscle very complex. 



Thus we find that in the rodents, in which the external oblique 

 muscle is comparatively slightly developed, a primary metameric seg- 

 mentation is well marked, that for the greater part the muscle-fibres 

 run from one transverse tendon to the next, and that the area of inner- 

 vation is across the central area of each metameric segment. At the 

 margins of the muscle conditions are more complex and resemble those 

 found in mammals with a more highly developed abdominal musculature. 



In the cat, dog, pig, and man, and probably in most mammals 

 with a highly developed abdominal musculature, the primary segmen- 

 tation is absent, the fibres interdigitate and the nerve supply is from 

 a complex nerve plexus to which the lateral branches of the abdominal 

 nerves contribute. 



Transverse tendons dividing M. obliquus abdominis externus meta- 

 merisally have been described as characteristic of many mammals by 

 Seydel 1 ). They were first noted by Leche in Tupaja and after - 



1) Ueber die Zwischensehnen und den metameren Aufbau des M. 

 obliquus thoraco-abdominalis der Säugetiere. Morphologisches Jahrbuch, 

 Bd. 18, p. 544, 1892. 



