One of the dimensions of the process of semantic extension of the noun is the change of the grammatical properties from count to mass and mass to count that can be observed between the primary and extended sense. Although in English such changes are nothing unusual – numerous nouns are both count and mass nouns (e.g. Quirk et al. 1985, 247–248; Huddleston and Pullum 2002, 334–335), the phenomenon is still far from being explored and explained in detail. The paper focuses on two of its dimensions: the formation of such senses and the regularities found among the extended senses of nouns designating body parts. As it turns out, although these nouns are classified in dictionaries as either solely or predominantly count nouns, they possess a whole network of mass senses that designate untypical dimensions of the body. The analysis allows us to specify which dimensions these are, and which of these dimensions are shared by nouns referring to different parts of the body. The linguistic data, 180 utterances produced by native speakers of English, come from the Internet. The analysis is based on the assumptions of Cognitive Grammar (Langacker 2000a, 2000b, 2008, etc.) and such notions as conventionalization, extension, schematization, and encyclopaedic knowledge.