CHAPTER XXI. THE SKELETON OF THE DOG[139] (_Canis familiaris_). I. EXOSKELETON. The exoskeleton of the dog includes three sets of structures: 1. hairs, 2. claws, 3. teeth. =Hairs= and =claws= are epidermal exoskeletal structures, while =teeth= are partly of dermal, and partly of epidermal origin. 1. =Hairs= are delicate epidermal structures which grow imbedded in little pits or follicles in the dermis. Specially large hairs forming the =vibrissae= or =whiskers= grow attached to the upper lip. 2. =Claws= are horny epidermal sheaths, one of which fits on to the pointed distal phalanx of each digit. They are sharply curved structures, and being in the dog non-retractile, their points are commonly much blunted by friction with the ground. The claws of the pollex, and of the hallux when it is present, however do not meet the ground, and therefore remain comparatively sharp. 3. =Teeth=[140]. Although as regards their mode of origin, teeth are purely exoskeletal or tegumentary structures, they become so intimately connected with the skull that they appear to belong to the endoskeleton. Each tooth, as has been already described, consists of three distinct tissues, dentine and cement of dermal origin, and enamel of epidermal origin. [Illustration FIG. 68. DENTITION OF A DOG (_Canis familiaris_) × 1/2. (Camb. Mus.) _i_ 2. second incisor. _c._ canine. _pm_ 1, _pm_ 4. first and fourth premolars. _m_ 1. first molar.] The teeth of the dog (fig. 68) form a regular series arranged along the margins of both upper and lower jaws, and imbedded in pits or =alveoli= of the maxillae, premaxillae, and mandibles. They are all fixed in the bone by tapering roots, and none of them grow from persistent pulps. They are divisible into four distinct groups, the =incisors=, =canines=, =premolars= and =molars=. There are three incisors, one canine and four premolars on each side of each jaw. But while there are three molars on each side of the lower jaw, the last is wanting in the upper jaw. The dentition of the dog may then be represented by the formula _i_ 3/3 _c_ 1/1 _pm_ 4/4 _m_ 2/3 × 2 = 42. In each jaw there is one large specially modified tooth called the =carnassial=, the teeth in front of this are more or less pointed and compressed, while those behind it are more or less flattened and tuberculated. =Teeth of the upper jaw.= The first and second =incisors= are small teeth with long conical roots and somewhat chisel-shaped crowns. Surrounding the base of the crown there is a rather prominent ridge, terminated laterally by a pair of small cusps. This ridge, the =cingulum=, serves to protect the edge of the gums from injury by the hard parts of food. The third incisor is a good deal like the others but larger, and has the cingulum well developed though not terminated by lateral cusps. All the incisors are borne by the premaxillae, the remaining teeth by the maxillae.