These are primarily letters, with some receipts for medical services rendered included. They relate to the general social and intellectural life of Philadelphia and to publications on medical topics. Many of the letters are introductions to Europeans when Pennock first visited there. Letters dated post-1867 are to relatives and colleagues (?).

This collection of Morton letters forms part of a bound volume of incoming correspondence. It is a companion volume to the original letters in the Morton collection at the American Philosophical Society, which were also originally bound. Most of the subjects of the two collections overlap, but additional subjects in the microfilmed collection include American Indians, archaeology, Egyptology, and phrenology.