Aunt
Polly was a person deeply interested in "quack" medicines. These ranged from water treatments to hot baths, shower baths and sponges. Aunt Polly was always on the lookout for suitable subjects to experiment upon, but (un-fortunately) Tom was the only one available. In fact, Tom's recent depression had given Aunt Polly the opening that she wanted. Poor Tom had no other alternative but to meekly submit to yet another of her home remedies, the effects of which could neither be foreseen nor calculated.
Besides Peter the cat, the main characters in the
novel include Tom, Aunt Polly, Becky Thatcher, her brother Jeff, and Sid. But only Aunt Polly and Tom make an appearance in this chapter - aided and abetted by the cat. Obviously, Tom is the best-known character, and the entire chapter revolves around him, his Aunt Polly, and the cat.
The main reasons why Tom gave the cat a taste of the Pain-Killer were that the cat was ready, willing and available. Perhaps, he just wanted to have a good time, in a childish sort of way. Or, perhaps, being the victim of (frequent) experimentation by his Aunt Polly, he had decided that it was time to do some experimentation on his own. The Pain-Killer had an immediate effect upon Peter the cat...the cat behaved as it had been injected with stimulant. However, Aunt Polly - the same Aunt Polly who has no hesitation in experimenting upon Tom - objects to Tom trying the same with the cat. In fact, his Aunt Polly later softens a little, and tells Tom that he no longer needs to be force-fed the medicine he was taking.
There are many essential details (background / plot / characters) that are required in a short story, but may be left out in a single chapter of the novel. Two differences are readily apparent:
i) Characters - In a single chapter of a thick novel, not all the main characters will make an appearance at the same time. They will appear as and when required by the denouement of the novel. If the plot demands it, and not otherwise.
ii) Ending - Perhaps the most importance difference between a short story and a single chapter (of a novel) lies in the ending. The short story has a definite - and identifiable ending. This may or may not be the case with a single chapter; in a chapter ending, many matters are left un-resolved, to be sorted in later chapters.
iii) Subplots - usually, there is space enough for a single plot in the short story, but there is no restriction upon the number of plota and subplots in a novel.
iv) Background - the background material is usually covered in the first few chapters of the novel; and the reader will feel lost unless the chapter is more-or-less self contained.