Got a simple question maybe someone an assist me in figuring it out. I'm using the below code to better under my understanding of javascript objects.

What reason is there for me getting this error message "this.SomeTxt is not a function"

Code:
var MyNewObj = new Test('random string');
MyNewObj.prntTxt();

function Test(Txt) {

		this.Txt = 'No string specified';

		this.prntTxt= function() {
				document.onmousedown = this.Func1;
			}

		this.Func1= function(e) {
                                if(e==null) {
						e = window.event;
					}
				if(e.target!=null) {
						var MyElem = e.target;
					} else {
						var MyElem = e.srcElement;
					}
                                if(MyElem.style.className = 'PrntNow')
				       this.SomeTxt('test');
                        }

                this.SomeTxt = function(MyTxt) {
		 		document.write(MyTxt);
			}
      }