A party examined by an adverse party may be examined on his own behalf, in respect to any matter pertinent to the issue. But if he testify to any new matter, not responsive to the inquiries put to him by the adverse party, or necessary to qualify or explain his answer thereto, or to discharge, when his answer would charge himself, such adverse party may offer himself as a witness, and he shall be so received.

[Code 1881 § 1877; 1873 p 371 § 176; 1854 p 234 § 65; RRS § 1906.]