Anatomical study on 2 cases of the congenital partial defect of pectoralis major and minor muscles.

2 cases of the defect of pectoralis major muscle observed in male cadavers allocated for routine dissection were investigated anatomically, especially on the nerve supply for the defected muscles. In both cases, one on the left and the other on the right side, pectoralis major muscle was defected, the clavicular portion and a small part of sternocostal portion only persisting. Meanwhile, the ipsilateral pectoralis minor muscle was defected in the first case to be a string-like muscle band and was missing in the second case substituted by a membrane which seemed not to be the degenerated same muscle. Both the lateral and medial pectoral nerves supplied the pectoral muscles in the first case. One of rami of the medial pectoral nerve was distributed unusually to the most lateral part of the persisted sternocostal portion. Only the lateral pectoral nerve existed in the second case. In both cases nerves derived from 2 caudal segments, C8 and Th1, were lacking or poor. However, it was suggested that the defect of the muscles was not due to the nonparticipation of these nerves but due to the failure of caudal growth of the pectoral premuscle mass in a five-week embryo (Lewis 1901).