How command "manpath" works?

[Zhao You Bing <zhaoyb@cad.zju.edu.cn>]

I downloaded j2sdk from SUN and extracted it to /usr/local
and I found the /usr/local/j2sdk/man is in the list of command "manpath"

But I haven't set manpath manully and it's not listed in /etc/manpath.config

I'm wondering how manpath works, who can give me a hint or tell me in 
which package I can found the source code of "manpath" so I can check it 
myself?

Thank u very much!!
--
Zhao YouBing, Ph.D student
State Key Lab of CAD&CG,Zhejiang University,
Hangzhou, 310027, P.R.China
Tel  : 0571-87951045(O), 87933444(H)
Email: zhaoyb@cad.zju.edu.cn
MSN  : zhaoyoubing@hotmail.com