.menu a {
  display:block;
  border:none;
  background-position:right top;
  background-repeat:no-repeat;
  width:170px;
  height:40px;
}
.menu a:hover { background-position: 0 -107px; text-decoration: none; }
.menu .alt { display:none; }
.menu a#hoy { background-image: url(1.jpg) }
.menu a#solca { background-image: url(2.jpg) }
.menu a#asistencial { background-image: url(3.jpg) }
.menu a#cientifica { background-image: url(4.jpg) }
.menu a#social { background-image: url(9.jpg) }
.menu a#formacion { background-image: url(5.jpg) }
.menu a#publicaciones { background-image: url(6.jpg) }
.menu a#contactenos { background-image: url(7.jpg) }
.menu a#webmail { background-image: url(8.jpg) }