Because it is warm and moist under finger nails. and it is normally quite cold and dry under toilet seats. the conditions for life are far more favourable under the finger nails.
Nails (and finger nails) have volume.
Finger nails are made of a protein called keratin.
No one invented finger nails.
It can if he is too rough. Check his finger nails over before you allow this. Long nails can cut your tissue and dirty nails are not good either so to protect your self check his hands. Make him wash them and trim nails.
Actually, finger nails contain protein (called "keratin").
By system, you mean category.. I'm not sure if you mean finger nails or like tool nails, so I'll give you both. Finger nails: Body parts Tool nails: Hardware
Soil is different from dirt, as dirt is the stuff under your finger nails.
Grease under finger nails at supper time. Usually seen with a band-aid on at least one finger/knuckle.
Finger nails (and toe nails) are made of keratin.
Long nail are easily damaged, and can scratch or catch on material. Also, dirt can get under the nails (usually after doing a dirty job) which doesn't look nice.
Nails