Can anyone please help with this function?

I have a large spreadsheet file with numerous worksheets.
Each sheet represents one month of the year and is named Jan05, Feb05, Mar05
etc.

In the "Summary" sheet I want to automatically reference a range in the
other sheets.

For example:

In the Summary sheet column A, I have the sheet names:
Jan05
Feb05
Mar05
etc.

In column B, (let's use B5 as an example) I want a formula that reads
something like:
=B4 + Apr05!C7

This seems simple enough, but how can I write a formula that 'automatically'
enters the sheet name (i.e. "Apr05") in the formula?
The row containing the data for Apr05 is only created when that month comes
along, in order to keep the file size as small as possible at any time.

Thanks,

Coober