Taittiriya Upanishad 1.3.4

1.3.4

athādhyātmam /
adharā hanuḥ pūrvarūpam /
uttarā hanuruttara rūpam /
vāksandhiḥ /
jihvāsandhānam /
ityadhyātmam /
itīmā mahāsam̐hitāḥ /
ya evametā mahāsam̐hitā vyākhyātā veda /
sandhīyate prajayā paśubhiḥ /
brahmavarcasenānnādyena suvargyeṇa lokena // 4//
iti tṛtīyo 'nuvākaḥ //

Vāyu is the link. This is the meditation with regard to the worlds. Then follows the meditation with regard to the shining things. Fire is the first letter. The sun is the last letter. Water is the rallying point. Lightning is the link. This is the meditation with regard to the shining things. Then follows the meditation with regard to knowledge. The teacher is the first letter. The student is the last letter. Knowledge is the meeting-place. Instruction is the link. This is the meditation with regard to knowledge. Then follows the meditation with regard to progeny. The mother is the first letter. The father is the last letter. The progeny is the focal point. Generation is the link. This is the meditation with regard to progeny. Then follows the meditation with regard to the (individual) body. The lower jaw is the first letter. The upper jaw is the last letter. Speech is the meeting-place. The tongue is the link. This is the meditation with regard to the (individual) body. These are the great juxtapositions. Anyone who meditates on these great juxtapositions, as they are explained, becomes conjoined with progeny, animals, the splendour of holiness, edible food, and the heavenly world.