The excitable foci of the cerebral cortex are well myelinated long before the unexcitable are so.
Larger and thicker in the rabbit, when excited it gives rise in that animal to movements of the eyes and of the fore-limbs and neck; but it is only in much higher types, such as the dog, that the cortex yields, under experimental excitation, definitely localized foci, whence can be evoked movements of the fore-limb, hind-limb, neck, eyes, ears and face.
F2' =objectand image-side foci of eyepiece.
F 1 ' =objectand image-side foci of objective.
Metrical relations between the axes, eccentricity, distance between the foci, and between these quantities and the co-ordinates of points on the curve (referred to the axes and the centre), and focal distances are readily obtained by the methods of geometrical conics or analytically.