Taittirīyopaniṣat 01.08

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

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.