If, after the date of a mortgage, any accession is made to the mortgaged property, the mortgagee, in the absence of a contract to the co ntrary, shall, for the purposes of the security, be entitled to such accession.

Illustration.

   (a)  A mortgages to B a certain field bordering on a river. The field is increased by alluvion. For the purposes of his security, B is entitled to the increase.

   (b)  A mortgages a certain plot of building land to B and afterwards erects a house on the plot. For the purposes of his security, B is entitled to the house as well as the plot.