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॥

athādhyātmam। adharāhanuḥ pūrvarūpam। uttarāhanuruttararūpam। vāk sandhiḥ। jihvā sandhānam। ityadhyātmam॥ itīmā mahāsaṃhitāḥ। ya evametā mahāsaṃhitā vyākhyātā veda। sandhīyate prajayā paśubhiḥ। brahmavarcasenānnādyena suvaryeṇa lokena॥8॥

[अथ (atha) - now; अध्यात्मम् (adhyātmam) - pertaining to the spritiual aspect; अधराहनुः (adharāhanuḥ) - lower jaw; पूर्वरूपम् (pūrvarūpam) - former form; उत्तराहनुः (uttarāhanuḥ) - upper jaw; उत्तररूपम् (uttararūpam) - latter form; वाक् (vāk) - speech; सन्धिः (sandhiḥ) - junction; जिह्वा (jihvā) - tongue; सन्धानम् (sandhānam) - joining / bringing together; इति (iti) - thus; अध्यात्मम् (adhyātmam) - pertaining to the spritiual aspect; इति (iti) - thus; इमा (imā) - these; महासंहिताः (mahāsaṃhitāḥ) - great collections; यः (yaḥ) - who; एवम् (evam) - thus; एता (etā) - these; महासंहिता (mahāsaṃhitā) - great collection; व्याख्याता (vyākhyātā) - explained; वेद (veda) - knows; सन्धीयते (sandhīyate) - is joined; प्रजया (prajayā) - by progeny; पशुभिः (paśubhiḥ) - by cattle; ब्रह्मवर्चसेन (brahmavarcasena) - by spiritual lustre; अन्नाद्येन (annādyena) - by food and drink; सुवर्ग्येण (suvargyeṇa) - by heavenly; लोकेन (lokena) - by world;]

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.