164 



conclude that the fascia does not represent a rudimentary m. supra- 

 costalis. The cephalad prolongations of the m. rectus abdominis de- 

 scribed by LiviNi ('05), Kazzander ('04) and others as extending to 

 the third or even to the second rib are stronger in the lower inter- 

 costal spaces than in the upper, and the fibers, unlike those of the 

 fascia under consideration, pass upward and lateralward in the upper 

 intercostal spaces. For these reasons it does not seem possible to 

 consider the fascia as a derivative of the m. rectus abdominis. The 

 same reasons would debar one from homologizing it with the m. sterno- 

 costalis externus of lower vertebrates. It is therefore impossible to 

 regard this fascia as a derivative of either the m. supracostalis, the 

 m. rectus abdominis, or the m. sterno-costalis externus, and in de- 

 termining its derivation, we are limited to the consideration of the m. 

 serratus anterior, a derivation from which is positively indicated by the 

 direction of the fibers, by their frequent continuity with those of the 

 muscle or its perimysium, and by the occasional prolongation forward 

 in the fascia of fibers of the m. serratus anterior. 



The fact that the fibers in the anterior portion of this fascia do 

 not correspond in direction with those of the serrate muscle, but pass 

 almost vertically downward, may be understood by reference to the 

 developmental history of the m. serratus anterior. This muscle is 

 developed in the cervical region, as is indicated by its innervation and 

 clearly shown by Lewis ('01). If the pre-muscle mass constituting the 

 serrate muscle of the embryo during the course of its migration caudal- 

 ward, acquired at its ventral border an attachment to the first rib, 

 the fibers so attached while originally passing transversely lateralward, 

 would, as the muscle moved caudalward, take on a direction at first 

 oblique and finally in the case of the more anterior fibers almost 

 vertical. This suggestion as to the possible origin of the fascia ex- 

 plains completely the direction of all of its fibers. A similar ex- 

 planation for the origin and disposition of the m. sternalis has been put 

 forward by Huntington ('04). We have not been able so far, to find 

 reported in the literature an actual attachment of the pre-muscle mass 

 to the first costal cartilage. But since, at the time when the serrate 

 pre-muscle mass is migrating caudalward, the ribs are only partially 

 developed, and exist as processes extending lateralward from the 

 vertebral column, the tip of the first rib lies in close relation to the 

 serrate pre-muscle mass, and it would seem quite possible for such an 

 attachment to be formed. 



I wish to thank Dr. B. C. H. Harvey very heartily for his guid- 

 ance and assistance in the preparation of this paper. 



