However these breed do not show a reduction of the whole skull but only of the upper jaw. The braincase stays mainly stable and does not show this tremendous variability. A different inheritance of upper and lower jaw is obvious when we take a look at all those breeds that show very different lengths of their upper and lower jaws. This is most common with breeds with shortened muzzles. Such a phenomena can also be detected with the St. Bernard Dog, as this breed too shows an upward bending of the muzzle.
Small dogs have a much larger relative brain size than large dogs.
This phenomena is known as Haller's law, which says that small individuals from one species have relative larger brains than their larger relatives. The answer to this law lies in the different relations of bodies and surfaces in different sizes. Furthermore main cerebral centers may not sink below a certain size as to assure their full functionality. The mentioned Sagittal crest is a bony protrusion whose growth is induced by the muscles of the jaw. Within certain limits the size of the crest may be modified by strain put upon it from the muscles.