There is great variability in the arterial supply of the pectoralis minor, with contributions received from three main sources: the lateral thoracic artery, the thoracoacromial artery, or directly from a branch of the axillary artery. Infrequently, the latter arterial source is present when contributions to the muscle from the other two arterial trunks are minimal. I call this direct artery the “lateral thoracic artery” because, in the few clinical encounters in which a direct artery from the axillary to the pectoralis minor occurred, there was no visible contribution to the muscle from other arterial sources more inferiorly. In the original ten dissections, a separate branch from the axillary artery was never encountered.

By far the predominant vascular pattern is from branches of the lateral thoracic or thoracoacromial arteries. The former passes around the lateral margin of the muscle after it has supplied it with a branch, while the latter passes around the medial margin of the muscle after it has issued an arterial branch, usually to the superior part of the muscle. One or the other branch is dominant, but in a clinical series, the dominant branch came more frequently from the lateral thoracic artery. When the two branches share the blood supply to the pectoralis minor, harvesting the muscle on the lateral thoracic contribution has been the choice, but in these patients, the superior portion of the muscle was not as well perfused and appeared duskier.

There is also extensive variability in the venous drainage of this muscle. More often than not, there is a separate vein that will drain the muscle adequately; however, in many cases, one of the venae comitantes must be used because the direct vein is absent. If there is some doubt about dominant venous drainage, it is best to harvest all the available veins and to observe under magnification which one best drains the muscle unit after microvascular transfer.

The nerve supply to the muscle has minimal variations. The majority of the pectoralis minor (four fifths) receives innervation from the medial pectoral nerve, while the most superior part of the muscle receives a tiny branch from the lateral pectoral nerve. Another substantial branch of the lateral pectoral nerve destined to innervate the pectoralis major uses the pectoralis minor only as a passway; thus harvesting the pectoralis minor necessitates taking of that nerve. However, these axons need not be lost, and if time is taken to redirect these fibers back to the pectoralis minor, additional innervation can be obtained through a process of direct neurotization.

The nerve supply to the pectoralis minor is multisegmental, receiving contributions from all five spinal nerves contributing to the brachial plexus, namely, C5, C6, C7, C8, and T1. Motor fibers from these spinal nerves reach the pectoralis minor through branches of two nerves, the lateral and medial pectoral nerves. The dominant innervation to the pectoralis minor is through the medial pectoral nerve, with contributions from C8 and T1. A major branch from this nerve is responsible in all cases for three fourths or two thirds of the muscle, depending on whether the muscle has three or four slips. Also constant is the innervation of the superior part of the pectoralis minor, which, in all cases, comes through a tiny branch from the lateral pectoral nerve. A larger lateral pectoral branch also penetrates the muscle, but it usually does not supply the muscle; instead, it courses through it on its way to the pectoralis major.

Contrary to reported variability in the nerve supply of the pectoralis major, the innervation of the pectoralis minor is relatively constant. Variations are limited to the presence or absence of the communicating loop, with fiber exchange between the medial and lateral pectoral nerves. It is worth mentioning that in two clinical cases of severe brachial plexopathy, with documented neuroradiologic and electrical findings of C8 and T1 avulsion, the inferior segment of the pectoralis minor responded to intraoperative electrical stimulation, raising the possibility that some motor fibers from T2 may contribute to the innervation of this muscle.

The dual innervation of the pectoralis minor makes it a unique muscle for facial reanimation procedures because the upper leaves can be motored separately from the lower portion of the muscle. This characteristic opens possibilities for independent eye and mouth movements, one of the greatest advantages of using this muscle for facial-muscle substitution (Figs. 157.1, 157.2, 157.3, 157.4).