Taittirīyopaniṣat 01.08

अथाध्यात्मम्। अधराहनुः पूर्वरूपम्। उत्तराहनुरुत्तररूपम्। वाक् सन्धिः। जिह्वा सन्धानम्। इत्यध्यात्मम्॥
इतीमा महासंहिताः। य एवमेता महासंहिता व्याख्याता वेद। सन्धीयते प्रजया पशुभिः। ब्रह्मवर्चसेनान्नाद्येन सुवर्ग्येण लोकेन ॥८॥

Now, concerning the 'adhyātma', i.e. spirituality: the lower jaw is the earlier form, the upper jaw is the latter form, speech is the junction, the tongue brings them together; thus is the teaching about the 'adhyātma', i.e. spirituality.
These are great collections. Whoever knows these great collections as explained is united with progeny, cattle, spiritual brilliance, food and drink, and the heavenly world.