Bṛhad-āraṇyaka upaniṣad 1:1:1

Bṛhad-āraṇyaka upaniṣad is the concluding section of the Śatapatha brāhmaṇa of the white (vājasaneyi) yajurveda. By tradition and on linguistic grounds, it is considered the oldest, and is the most important of the upaniṣad. Even the most conservative scholarly opinion places it at c. 800 B.C.E. 

The opening verse outlines the equivalence between the sacrificial horse and the cosmos. The anatomical detail and cosmic equivalence clearly marks a transition from the sacrificial tradition (karma-kāṇḍa) to the pursuit of knowledge (jñana kāṇḍa) in the Brāhmiṇical scriptures.

Vājasaneyi is the patronymic of the sage Yajñavalkya, who tradition has it, founded this school. He plays a central role in propounding some of the key ideas of the Bṛhad-āraṇyaka upaniṣad, which were to inform Uttara Mimāṃsā (Vedānta) thinking centuries later. The text has been preserved in two rescensions - the Mādhyandhina and the Kāṇva. This translation comes from the Kāṇva rescension. I have taken the support of S. Radhakrishnan's and Patrick Olivelle's translations. It's a tough text to comprehend, and absorb, so I'll only be doing one verse at a time. 

1.1.1 The head (शिरः) of the sacrificial horse (अश्वस्य मेधस्य) , clearly, is the dawn (उषा); its sight (चक्षु:) is the sun (सूर्य) ; its breath (प्राणः) is the wind (वातः) ; its wide open mouth (व्यात्तम्) is the fire common to all men (अग्निर्वैश्वानरः). The body (आत्मा)* of the sacrificial horse is the year (संवत्सर:); its back (पृष्ठम्)  is the sky (द्यौ) ;  its abdomen (उदरम्) is the intermediate region (अन्तरिक्षम्); its flanks (पाजस्य) are the quarters; its ribs (पार्श्वम्) are the intermediate quarters (अवान्तारादिशः) ; its limbs are the seasons; its joints (परवाणि) are the months and fortnights; its feet (प्रतिष्ठा) are the days and the nights; its bones (अस्थीनि) are the stars; its flesh (मांसानि)  is the clouds (नभ ); its stomach contents (ऊवध्यम् ) are the sand (सिकता:) ; its intestines (गुदा) are the rivers (सिन्धवः) ; its liver (यकृत्) and lungs (क्लोमानः)  are the hills; its body hairs (लोमानि) are the herbs (ओषधयः)  and the trees (वनस्पतयः) ; its fore-quarter (पूर्वार्धः)  is the rising sun (उद्यन्); and its hind-quarter (जघनार्धः) is the setting sun (निम्लोचन्). When it yawns (विजृम्भते) lightening flashes (विद्योतते); when it shakes itself (विधूनुते) it thunders (स्तनयति) ; and when it urinates (मेहती), it rains (वर्षति); its voice [neighing] indeed, is speech itself. (वागेवास्य वाक्)

* Both S. Radhakrishnan and Olivelle translate आत्मा as body.