Loss Of Button Function Within Onload Script?

I cannot figure out why navigation panel buttons are losing CSS formatting and their function as HTML buttons.The page markup (in index.html) is here:[URL]There is one external CSS and one external JS file that contains all the I have traced the problem to a single Javascript line during the initialization (first loading) of the document.

Code:

61: contentPanel.replaceChild(content, contentPanel.firstChild);

This is line 61 in the function jumpToPage() in the ncluded external Javascript file ReGenaVetLabs.js Up to this script statement, the buttons are active and the mouse cursor shows as a pointer. After it executes, the cursor is no longer a pointer, and the buttons not clickable.It does not make sense to me why the left-side navigation buttons should be affected.

The page layout has four parts:

1. top panel (constant)
2. left-sided middle panel for navigation with buttons (constant)
3. right-sided middle panel for content
4. bottom panel (constant)

"(constant)" means it does not change from page to page.Although there is only one physical page---which is index.html in this setup---the content is loaded using scripts: the page is full of text and images that are contained inside DIV elements whose display is turned off (style="display:none;") and the DIV content is loaded using Javascript, simply by using the DOM .replaceChild() method for the DIV element that makes contains panel labeled 3 above...the content panel.Later on, I will make each page a separate HTML page, to deal with browsers whose script use is deliberately disabled. But this development is easier for me, and it avoids the new page loading appearance: browser window clears, browser loads wanted page.