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.
Fig. 68. Dentition of a Dog (Canis familiaris) × ½. (Camb. Mus.)
| i 2. second incisor. | pm 1, pm 4. first and fourth |
| c. canine. | 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.