Semasiology

Semantic motivation is based on the co-existence of direct and figurative meaning e.g. mouth (as a part of human face) and mouth (of the river), leg ( of a dog) and leg (of a table), foot (of a man) and foot (of a hill). We should remember that direct meaning is non-motivated, only figurative (transferred) meaning is semantically motivated.

MEANING AND POLYSEMY

It is very important to distinguish between the lexical meaning of the word in speech and its semantic structure in language. If a word has only one meaning in language it is called monosemantic. If a word has more than one meaning in the language it is called polysemantic.
Monosemantic words are few in number, they are mainly scientific terms, e.g. hydrogen, molecule.Most of English words possess more than one meaning, and therefore are polysemantic.
The semantic structure of a polysemantic word may be defined as a structured set of interrelated meanings. Polysemy exists only in language, not in speech. Polysemy does not interfere with the communicative function of the language because in speech the meaning is contextual. Only one of all meanings of the word can be used while we are speaking, and we understand it through the context.
Context is the minimal stretch of speech necessary and sufficient to determine which of the possible meanings of a polysemantic word is used. In polysemantic words, however, we are faced not with the problem of analysis of individual meanings, but primarily with the problem of interrelation and interdependence of the various meanings in the semantic stricture of one and the same word.
This problem may be approached from two different angles. If polysemy is viewed diachronically, it is understood as the growth and development or, in general, a change in the semantic structure of the word. Semantic changes result as a rule in new meanings being added to the ones already existing in the semantic structure of the word. Some of the old meanings may become obsolete or even disappear, but the bulk of English words tend to increase in number of meanings.
Synchronically we understand polysemy as the coexistence of various meanings of the same word at a certain historical period of the development of the English language. In that case the problem of inter- relation and interdependence of individual meanings making up the semantic structure or the word must be investigated along different lines
It should be noted, however, that the actual arrangement of meanings in the semantic structure of any word, in any historical period is the result of the semantic development of this word within the system of the given language.
Taking into account that the semantic structure is never static; the relationship between the diachronic and synchronic evaluation of individual meanings may be different in different periods of the historical development language. This is perhaps best illustrated by the semantic analysis of the word revolution. In l600 when this word was registered in dictionaries the meaning “revolving motion’ was both primary (diachronically) and central (synchronically). In Modern English the arrangement of meanings in the semantic structure of the word revolution has considerably changed, the meaning 'a complete overthrow of the established government or regime" has become synchronically central and its most frequent meaning.