25 
M. pectoralis B (figs. 3, 6 and 7, p.b.)—the second sector of 
the pectoral mass—is separated from the more anterior, A, by an 
almost transverse cleavage line, but its fibres are otherwise simply 
in series with the fibres of A. It arises from the whole length 
of the mesosternum, and also by deep digitations from the adja- 
cent parts of the sternal segments of the second to the sixth 
costal arches, inclusive. Its fibres pass obliquely outwards and 
forwards, converging to a thin flattened tendon which becomes 
continuous at the humeral end with the more fleshy insertion of 
pectoralis, A, which it partly overlaps. 
M. pectoralis, C (figs. 3, 6 and 7, p.c.), is separated from the 
rest of the muscle superficially by a sulcus of radial cleavage, but 
its insertion is totally different from that of the two preceding 
sectors. It is the most posterior and lateral of the three pectoral 
muscles, and on the whole its origin is deeper, and is largely 
covered by the muscle last described. 
It arises by a series of deep fleshy slips from the sternal seg- 
ments of the second to the seventh costal arches (inclusive), 
laterad of, and overlapped by, the corresponding slips of pectoralis 
B, from which it is completely separate. 
Its outer or axillary margin is not covered by B until it enters 
the axilla; it is, however, covered posteriorly (caudad) by the 
mesio-ventral portion of the superficial muscular sheet of the 
panniculus. The latter is here very thin, and consists of fibres 
arising from the anterior abdominal aponeurosis near the mesial 
line, and extending forwards and outwards. These fibres do not 
form a distinct band, but they probably represent the pectoralis 
quartus, which is otherwise absent. 
Immediately external to the lateral border of pectoralis C the 
paunicular fibres form a denser layer, covering a portion of the 
rectus abdominus, which courses forwards beneath them and 
parallel with pectoralis C. The fibres of the humeral portion of 
the panniculus stratum cover the whole lateral thoracic wall in a 
continuous sheet, extending from the surface of pectoralis C 
laterally and dorsally as far as the ventral border of the latissi- 
mus. Its fibres are directed forwards, converging to enter the 
axilla, and are then inserted into the border of a tendinous arch, 
which crosses the axillary vessels and nerves. The ventral end 
of this tendinous arch is attached to, and inserted along with, 
the insertion of pectoralis C into the greater (radial) tuberosity 
of the humerus close to the joint-capsule, and just external to, 
and also bridging over, the bicipital groove, and thus indirectly 
into the lesser tuberosity of the humerus. 
It appears to me very evident that the sectors denominated A and 
B together represent the “ pectoralis major ;”’ and that sector C repre- 
sents the “ pectoralis minor.”’ Further, a pectoralis quartus is not 
