The Jack Russell Terrier is perfectly designed to go to ground. This requires a chest of sufficient depth to give good heart and lung room, but without so much depth and width that the dog is encumbered underground. The well-sprung ribs extend well back, but must be capable of being spanned behind the shoulder by an average man's hand. The chest must be capable of being compressed so that the dog is unhindered when working underground. The back is of moderate length, and level, blending into a muscular, slightly arched loin with slight to moderate tuckup.
Skin is thick.
The hindquarters are strong and muscular. The bone, angulation, and musculature of the hindquarters are in balance with the forequarters. The stifles are well bent, and the hocks are well let down. When the dog is standing, the short, strong rear pasterns are perpendicular to the ground, and viewed from the rear, parallel to one another. The feet are fairly round, moderately small, well arched, and tight.