Why does Atticus ask Tom Robison to stand up? Also, why doess Lee begin chapter 19 with a description of Tom trying to take the oath in the trial??

Atticus asks Tom to stand so that the jury can see that his left arm is a foot shorter than his right, and his left hand is shriveled from an accident.

In Chapter 19, Lee begins with a description of Tom Robinson trying to take his oath. He attempts to use his good right hand, and then put his bad left hand on the Bible. His left hand continuously falls off....... this shows the reader in no uncertain terms that Tom could not have used this hand to hurt Mayella.