The Quran

وَإِنَّ لَكُم فِي الأَنعامِ لَعِبرَةً ۖ نُسقيكُم مِمّا في بُطونِهِ مِن بَينِ فَرثٍ وَدَمٍ لَبَنًا خالِصًا سائِغًا لِلشّارِبينَ

Muhammad Asad

And, behold, in the cattle [too] there is indeed a lesson for you: We give you to drink of that [fluid]which is [secreted from] within their bellies between that which is to be eliminated [from the animal's body] and [its] life-blood: milk pure and pleasant to those who drink it.¹

Milk - in itself a glandular secretion - is not necessary for the mother-animal's life (or, as it is here metonymically described, its "blood"); on the other hand, it is not just something that the body eliminates as being of no further use to its metabolism: hence it is referred to as a substance "between that which is to be eliminated [from the animal's body] and [its] life-blood".