Several reasons for this trend are possible, one of which is that neural cells have a relative constant size. Some brain functions, like the brain pathway responsible for a basic task like drawing breath, are basically similar in a mouse and an elephant. Thus, the same amount of brain matter can govern breathing in a large or a small body. While not all control functions are independent of body size, some are, and hence large animals need comparatively less brain than small animals. This phenomenon can be described by an equation: "C" = "E" / "S2/3 , where E and S are brain and body weights respectively, and C is called the cephalization factor. To determine the value of this factor, the brain- and body-weights of various mammals were plotted against each other, and the curve of E" = "C" × "S"2/3 chosen as the best fit to that data.