An instrument is described which can produce a limited, controlled region of cooling or freezing of biologic tissue. It can be used to provide temporary physiologic blockage of nerve conduction in a limited area of neural tissue within the central or peripheral nervous system. It can also safely produce, deep within the brain (e.g., cryothalamectomy), a spherical, limited lesion of consistent size. This cannula has been used to produce deep intracerebral lesions in human subjects as well as in in vitro and animal experimentation.