The words pūrva 'front', apara 'near', adhara 'lower', uttara 'upper', are compounded when in construction with a word signifying a thing that has parts, provided that the thing having parts is distinguished numerically by unity; and the compound is tat-puruṣa,
The word ekadhikarana (the unity of substance) is the attribute of or qualifies the word ekadesin. This debars the Genitive Tat-purusha compound ordained by sutra 9 ; which would have placed the words purva &c., last in the compound, whereas being here exhibited in the nominative case 1.2.43 and 2.2.30, they take the precedence.
पर्व+कायस्य = पर्वकायः 'the front of the body'; अपरकायः 'the back of the body'; अधरकायः 'the lower part of the body '; and उत्तरकायः 'the upper part of the body.'
Why do we say 'what signifies a thing that has parts'? Observe पूर्व नाभेः कायस्य. But not so in पूर्वं छात्राणामामन्त्रय 'invite the fore-most of the pupils': because here the substratum (adhikarana) is not unity (eka). But how do we get the compounds like मध्याह्नः 'noon'; सायाह्नः 'evening'? The word ahna is compounded with every word signifying its parts, because we learn this by inference from sutra 6.3.110.,