if (document.images)
{            
img1on  = new Image();  img1on.src  = "_i/toc_ynhh_over.gif"; 
img2on  = new Image();  img2on.src  = "_i/toc_ynhch_over.gif";  
img3on  = new Image();  img3on.src  = "_i/toc_ynhph_over.gif";
img4on  = new Image();  img4on.src  = "_i/toc_ynhhs_over.gif";
img5on  = new Image();  img5on.src  = "_i/toc_yusm_over.gif";
img6on  = new Image();  img6on.src  = "_i/toc_mdlink_over.gif";

img1off  = new Image(); img1off.src  = "_i/toc_ynhh.gif"; 
img2off  = new Image(); img2off.src  = "_i/toc_ynhch.gif";  
img3off  = new Image(); img3off.src  = "_i/toc_ynhph.gif";
img4off  = new Image(); img4off.src  = "_i/toc_ynhhs.gif";
img5off  = new Image(); img5off.src  = "_i/toc_yusm.gif";
img6off  = new Image(); img6off.src  = "_i/toc_mdlink.gif";
}

function imgOn(imgName)
{
 if (document.images) {
 document[imgName].src = eval(imgName + "on.src");       
 }
}

function imgOff(imgName)
{
 if (document.images) {
 document[imgName].src = eval(imgName + "off.src");        
 }
}