1.  Field of the Invention
The present invention relates to a magnetic disk cartridge, and more particularly to an improved magnetic head access opening and shutter structure in a magnetic disk cartridge having a rotatable magnetic disk encased in a cassette shell. 
2.  Description of the Related Art
A magnetic disk cartridge comprises a magnetic disk for magnetically recording analog and/or digital signals rotatably encased in a thin cassette shell.  The cassette shell is formed in the shape of a flat prism (substantially a hexahedron) with a top surface slightly larger than the magnetic disk, a bottom surface having an opening through which a center core supporting the center of the magnetic disk is exposed and narrow side surfaces extending between the outer peripheral edges of the top and bottom surfaces.  The magnetic disk cartridge is provided with a magnetic head access opening for insertion/withdrawal of magnetic heads used for magnetic recording and reproducing so as to enable the magnetic heads to be brought into contact with, or close to, the opposite surfaces of the magnetic disk from the exterior.  The magnetic head access opening is equipped with a shutter for preventing invasion of dust and the like when the opening is not in use. 
The magnetic head access opening of the prior art magnetic disk cartridge is a slender opening of substantially rectangular shape formed in the top and bottom surfaces of the cassette shell to extend in the radial direction of the magnetic disk.  The shutter member for opening/closing the magnetic head access opening is a piece of thin sheet metal of .hoarfrost.-shaped section which extends along both the top and bottom surfaces of the cassette shell and also over the side surface between them.  The top and bottom surfaces of the cassette shell are formed with shallow recesses constituting stepped regions for accommodating and guiding the shutter member. 
However, it is not absolutely necessary for the magnetic head access opening to be formed in the top and bottom surfaces of the cassette shell to extend in the radial direction of the magnetic disk.  Specifically, it can be provided at any location so long as it enables the magnetic heads to be brought in contact with, or close to, the opposite surfaces of the magnetic disk over the full radius thereof from the exterior.  The opening is preferably as small as possible so as to minimize invasion of dust into the cartridge.  The ability to fabricate smaller magnetic heads has afforded greater freedom in the positioning and sizing of the magnetic head access opening and has even made it possible to realize the desired small magnetic head access opening by providing the opening only in a side surface rather than in the top and/or bottom surface.  This can be achieved by forming the narrow side surface of the cassette shell with a magnetic head access opening of a size enabling access to the opposite surfaces of the magnetic disk from the exterior and inserting/withdrawing thin, arm-shaped pickups having tiny magnetic heads attached at their tips through this hole to conduct recording and reproduction while moving the pickups in the radial direction of the magnetic disk. 
Although a magnetic disk cartridge having a magnetic head access opening of this type can be equipped with a shutter member similar to that of the prior-art magnetic disk cartridge, adoption of the same structure causes problems.  This is because the shallow recesses formed in the top and bottom surfaces of the cassette shell as stepped regions for guiding the shutter member reduce the thickness of the main walls of the cassette shell to the extent that the main wall thicknesses at the portion where the magnetic head access opening is provided in the side wall of the cassette shell become so extremely thin as to make the magnetic head access opening susceptible to deformation during fabrication of the cassette shell or during use of the magnetic disk cartridge.  The extreme thinness of the main walls at this portion also degrades the strength of the magnetic disk cartridge.  Deformation of the magnetic head access opening portion prevents the shutter member slidably mounted on the cassette shell from sliding smoothly.  This is liable to make opening and closing of the shutter member difficult or impossible.