The palpable landmarks
The zygomatic arch, the cheekbone, is formed by the zygomatic process of the temporal bone and the temporal process of the zygomatic bone; it borders the temporal fossa laterally. The external occipital protuberance, at the nape, is in the dog the highest point of the skull; the nuchal crest is its transverse extension. The external sagittal crest, present in carnivores and variable between breeds, runs along the midline of the skull between the two temporal fossae and gives attachment to the temporalis muscle.
The paired nasal bones form the roof of the nasal cavity and articulate laterally with the maxillae; the incisive bone, the most rostral, carries the upper incisors. The mandible has a body, a ramus, a coronoid process (where the temporalis attaches), a condylar process (the jaw joint) and an angular process. The temporomandibular joint joins the mandibular fossa of the temporal bone with the condylar process; the temporalis muscle protects it from forward luxation.
Skull types and the cephalic index
Dolichocephalic breeds have a more developed longitudinal skull axis (saluki, borzoi, collie); brachycephalic ones have a short, wide skull with a short muzzle, wide zygomatic arches and a rounded neurocranium (bulldog, pug, Boston terrier); mesaticephalic breeds are in between. The measure is the cephalic index: skull width × 100 / skull length, the length being measured from prosthion to inion. Numerical thresholds for the categories do not appear in a peer-reviewed source; some authors treat skull shape as a continuum. The term “stop” (the angle between forehead and muzzle) comes from show-ring vocabulary, not from the anatomical nomenclature consulted.
The orbit and the eye
The bony rim of the orbit is incomplete laterally in the dog; the circle is closed by the orbital ligament, between the zygomatic process of the frontal bone and the zygomatic arch. Seven muscles attach to the eyeball in domestic mammals: four recti, two obliques and the retractor bulbi, which pushes the third eyelid (nictitating membrane) over the cornea as protection; humans have no third eyelid. In the choroid, in the upper half of the fundus, lies the tapetum lucidum, a reflective layer 9-12 rows of cells thick at its thickest, which sends light back through the photoreceptors; the retina is rod-dominated, hence adapted to dim light. Color vision is dichromatic (two types of cones, ~429 and ~555 nm).
The ear
Three parts: the external ear (the pinna and the ear canal), the middle ear (the tympanum, an air-filled chamber with three ossicles and two small muscles, enclosed in the tympanic bulla, large in carnivores) and the inner ear (the labyrinth with the cochlea and the vestibular apparatus: hearing and balance). The pinna gets its shape from the auricular cartilage, thin at the tip and rolled like a tube at the base; it captures and localizes sound and is moved by groups of auricular muscles (rostral, lateral and caudal, partly attached to the scutiform cartilage), innervated by the facial nerve; each ear moves independently. The exact number of these muscles does not appear in a verifiable source.
The limits of hearing: Heffner's (1983) behavioral audiograms, reproduced by Mason et al. (2024), give at 60 dB SPL limits of about 60-95 Hz at the low end and 41-47 kHz at the high end, with no obvious differences between breeds, from the chihuahua to the Saint Bernard; the original text could not be read, and the low limits are approximations. Wording like “hears four times better” appears even in serious sources, but without a method, and is not used here.
