No person other than those specified in Article 3 (commencing with Section 934), and in Sections 939.1, 939.11, and 939.21, and the officer having custody of a prisoner witness while the prisoner is testifying, is permitted to be present during the criminal sessions of the grand jury except the members and witnesses actually under examination. Members of the grand jury who have been excused pursuant to Section 939.5 shall not be present during any part of these proceedings. No persons other than grand jurors shall be permitted to be present during the expression of the opinions of the grand jurors, or the giving of their votes, on any criminal or civil matter before them.