How did Johannes Hevelius' long telescope work? Why all the round holes?
In general,  the less the support structure weighs, the easier it is to move the whole thing around.  Home-user telescopes usually have a solid body because it's by far the cheapest and simplest way to get a small rigid structure.

Large observatory 'scopes often have a minimal framework supporting the secondary (the far element in a reflector);  here the primary refractor is supported with a similar structure.

Most certainly the support blocks do not contain lenses.  For one thing, just imagine trying to align and focus a 12 or 15-element system!  For another, producing the lenses for telescopes of that size was incredibly tedious back then (and now, really).  

There are a couple great chapters in "The Age Of Wonder" by Richard Holmes about a couple of the early astronomers &amp; scope builders in Great Britain for those who are interested.