Another important point is the situation of the ear drum. It is situated in the hardest bone found in the whole skeleton, called the petrosal. The nerve that gives the function called hearing comes from the brain down to the petrosal bone and enters by a small foramen or hole into the drum of the ear to give hearing. This nerve is called the auditory nerve. 2. CERVICAL VERTEBRAE OR NECK BONES.--These bones are seven in number. The first and second bones proceeding from the head receive special names. The first one is called the atlas, from which the head is suspended and to which it is attached; it somewhat resembles the body of a bird with wings extended. The second bone receives the name of dentata. This is the bone which allows the head to turn in any direction, hence it is sometimes called the axis or pivot of the neck. Between these two bones, on the upper surface, is the only place where the spinal cord is not covered with bone, a spot about three-fourths of an inch in diameter. The next four bones receive no name, and are about the same in size and length. The last, or the seventh bone, is only about one half the length of the preceding ones and receives no special name. 3. DORSAL VERTEBRAE OR BACK-BONE.--Dorsal bones are eighteen in number. The chief point of interest about them are the height of the spines on the upper surface of the bones. These large spines form the withers of the horse, as will be noticed in the skeleton. On either side of these bones the ribs are attached, 18 pairs corresponding with the number of bones in this region. 4. LUMBAR VERTEBRAE (or the bones which form the skeleton of the loins). These bones are six in number, and they are situated immediately above the kidneys. 5. SACRAL VERTEBRAE OR RUMP BONES.--There are five of these bones in the young horse, but in the adult they unite as a single bone, somewhat triangular-shape. These are situated between the upper hip bones and help to form the rump. Beneath these bones the bladder is situated. 6. COCCYGEAL VERTEBRAE OR TAIL BONES.--These are 18 to 20 in number. There is no complete canal, like in the previous vertebraes, for enclosure of the spinal cord. 7. PELVIC OR HIP BONES.--The pelvic bones are flat, but somewhat irregularly-shaped, and they form the sides, floor, and part of the roof of the pelvic cavity. Above they are connected with the sacrum, and below united to each other, in the adult, by ossification (union of bone). In the young animal, as above stated, and especially in the foetus, each side consists of three parts, which retain their names of ilium, ischium, and pubis, even after union by ossification. They all three meet in the acetabulum, or articular cavity for the femur or hip bone.