Proceedings of the Association of American Anatomists XIII 
not of reversional significance, since they cannot properly be homo- 
logized with divisions of the Pectoralis normally encountered in the 
mammalian series, notwithstanding the wide range of complexity which 
the Pectorales present in the mammalian orders below the primates. 
The group is again subdivided, according to the position occupied by 
the variant in reference to the plane of the Pectoralis major, into: 
A. Supernumerary muscles placed superficially to the Pectoralis 
major. 
1. Sternalis. 
2. Infraclavicularis. 
B. Deep supernumerary muscles in the interval between the Pectoralis 
major and minor. 
1. Chondro-coracoideus ventralis (Pectoralis minimus). 
2. Tensor semivagine articulationis humero-scapularis (Gruber). 
3. Some forms of the Preclavicularis. 
In attempting to define the derivation and significance of these varia- 
tions the following eticlogical factors deserve consideration: 
1. The relation between the variant muscles and coexisting defi- 
ciencies in the plane of the Pectoralis major, especially conditions indi- 
cating that the variant represents a portion of the normal muscle 
atypically displaced. 
2. Innervation of the variant. 
3. The peculiar type of development followed by the pectoral group, 
as determined by Mall and Lewis, together with the development and 
fusion of the sternal bar in the formation of the ventral thoracic wall. 
4, Atypical cleavage of the Ecto- and Entopectoralis, resulting in the 
production of the intermediate supernumerary pectoral muscles. 
5. The possible significance of some superficial pectoral slips as rem- 
nants of the thoraco-humeral panniculus. 
Conclusions.—1. The components of both the superficial and deep 
group of variants are frequently associated with defects in the muscular 
plane of the Pectoralis major. 
2. The innervation of all the muscles here considered is probably 
uniformly by branches derived from the anterior thoracic nerves. 
3. The superficial group develops in consequence of ontogenetic dis- 
turbances in the normal migration of the pectoral mass, and may be 
further influenced by faulty development in the closure of the ventral 
thoracic wall. The atypical widening of some of the intercostal spaces, 
adduced by Eisler as the main etiological factor in the production of the 
Sternalis, does not obtain uniformly. 
