The ornamental design for a part of medicalinstrument, as shown and described.
1. Part of medical instrument
1.1 : Right
1.2 : Front
1.3 : Bottom
1.4 : Left
1.5 : Top left
1.6 : Top
1.7 : Bottom left
1.8 : Back
The broken line showing in FIGS. 1.1-1.8 is for the purpose of illustrating portions of the part of medical instrument that form no part of the claimed design.