tephrylometer

teph·ry·lom·e·ter

(tef'ri-lom'ĕ-tĕr),
An instrument for measuring the thickness of the cerebral cortex; it consists of a graduated tube of thin glass that is inserted into the brain substance, so the depth of the gray matter can be read off on the scale.
[G. tephros, ashen, + hylē, stuff, + metron, measure]

tephrylometer

An obsolete device used to measure the thickness of cerebral grey matter.