Three or more simple sentences may indeed form a compound sentence; but, as they cannot be joined in a _cluster_, they must have two or more connectives.3.

The last clause erroneously suggests, that any or every conjunction "_sometimes connects only words_;" but the conjunctions which may connect only words, are not more than five, whereas those which connect only sentences are four times as many.
29.

NINTH DEFINITION:--"Interjections are words _thrown in between the parts of a sentence_, to express the passions or emotions of the _speaker_; as, 'O Virtue! how amiable thou art!'"-- _Murray, and many others_.

This definition, which has been copied from grammar to grammar, and committed to memory millions of times, is obviously erroneous, and directly contradicted by the example.

Interjections, though often enough thrown in between the parts of a _discourse_, are very rarely "thrown in between the parts of a _sentence_." They more frequently occur at the beginning of a sentence than any where else; and, in such cases, they do not come under this narrow definition.