Excerpted from De proprietatibus rerum 5.47

The navel is the midmost place of the body, so called as the shield-boss of the belly (umbo ilium), just as that part of shields is called the boss. The infant is fed as it hangs from the umbilical cord. According to Constantinus [Africanus ], the umbilical cord is made up of nerves, veins and arteries, and by its mediation the fetus attracts and sucks up refined blood, and receives its spirit through those same arteries. The umbilical cord is broken off near the womb when the fetus emerges, and it comes out with the fetus, and the midwives tie it off to a length of four fingers, and from this tying come the ends and roundness of the navel. Thus far Constantinus [Africanus].

original latin

Super Ezech. autem 16[.4] dicit Hieronymus [Comm. in Ezech. book 4] sic: naturale est infantibus ortis primo umbilicum praescindi. Deinde ad sanguinem diluendum aqua lavari, tertio humorem parvulorum sole siccari, quarto tenera corpora pannis constringi ne membra diffluant, et sic depraventur.

Ibidem etiam dicit glossa Gregorii, quod umbilicus est quo foetus alitur in utero, sicut arbores et virgulae per radices humore occulto terrae nutriuntur. In umbilico autem sita sunt genitalia mulierum, sicut in lumbis genitalia virorum, et ideo umbilici nomine luxuria designatur in Iob 40 [:11]. "Virtus eius in lumbis eius, et fortitudo eius in umbilico ventris eius."

De umbilico autem dicit Aristoteles in libro 13 [De part. anim. 693b23-24], quod omne animal generans et ovans habet umbilicum tempore partus et cum crescit partus avis, latet umbilicus, et nihil omnino apparet, quoniam continuatur cum intestino per aliquam partium venarum.

Item, idem libro 16 [De gen. anim. 745b34ff?]. Creatio umbilici non est nisi cortex, continens venas, et cum umbilico continuatur, et currit sanguis a matrice per venas umbilici, quasi per canalem* ad pueri nutrimentum. Crescit ergo conceptum per esse umbilici, ut dicitur in eodem.

* Original reading was per canale; this has been edited to make it easier to understand.