The skull of a toy-terrier has much restricted areas of insertion for the temporal muscles; has weak zygomatic arches; and has extremely small attachments for the masseter muscles.
"Essays: Scientific, Political, & Speculative, Vol. I"
Herbert Spencer
They who seek for metaphysical causes of laughter are not mirthful, while they who are aware that laughter draws the zygomatic muscle backwards towards the ears, are doubtless very learned.
"A Philosophical Dictionary, Volume 7 (of 10) From "The Works of Voltaire - A Contemporary Version""
François-Marie Arouet (AKA Voltaire) Commentator: John Morley Tobias Smollett H.G. Leigh
The zygomatic arches are less strong than they are in an average white male; and the mastoids are small, even smaller than in an average adult white female.
"A Bilateral Division of the Parietal Bone in a Chimpanzee; with a Special Reference to the Oblique Sutures in the Parietal Bulletin of the AMNH, Vol. XIII, Article XXI, pp. 281-295, Dec. 31, 1900"
Ales Hrdlicka