Full Title: Atrophy of the Frontalis and Temporalis Muscle
Author: Karen Finlay
Date of Publication: July 26, 2017
Research Paper Text:
ATROPHY OF THE FRONTALIS AND TEMPORALIS MUSCLE
By: Karen Finlay
July 26, 2017
The facial muscles of mammals are very complex and variable. Even though a dog does not have the same range of facial expression as humans do, the muscles of the dog’s face are well developed. They consist of several layers and differ in size and insertion points from one breed to another, but the innervation of the facial muscles by the facial nerve is constant.
Distinguishing one facial muscle from another is difficult and results from a lack of distinct connective tissue sheaths (fascia) between the layers. Adjacent layers of muscle often join together in various ways, or they may become independent and create separate muscles, each with their own name. Dissimilarities of this sort make neat classifications and actions difficult to describe.
The frontalis muscle is a thin, superficial facial muscle that lays across the top of the head, roughly from ear to ear, spanning the midline. Miller’s Anatomy of the dog categorizes the frontalis muscle as one of the “rostral extrinsic” ear muscles. It attaches at the rostral edge of the scutiform cartilage (part of the external ear), extends to the forehead and attaches to the nasofrontal fascia and into the zygomatic process (the bony protuberance at the side of the cheek forming the base of the eye socket). The frontal muscle fixes and pulls the scutiform cartilage rostrally, helping to move the ear forward.
The frontalis muscle lays directly atop the temporalis muscle, the largest muscle in the head, which raises the mandible and contributes to the action of closing the mouth and chewing. Since it is difficult to isolate the frontalis from the temporalis muscle, it is not possible to palpate it independently from the underlying temporalis. The two muscles combined give the top of the head behind the eyes it’s shape and fullness. Although the frontal muscle is not one of the muscles directly involved in mastication, due to its location relative to the temporal muscle, it will be affected by movement of or damage to of the temporal muscle.
Atrophy of the muscles involved in jaw movement (including the temporalis) would be more notable than observing the healthy muscles. If these muscles atrophy, the result would be a very distinctly evident “hollow” on either side of the midline of the frontal bone and giving the eyes a sunken look. Although atrophy of these muscles can simply be symptomatic of aging (as with all muscles), an immune mediated disease called Masticatory Muscle Myositis (MMM) is often to blame. This disease causes inflammation of the muscles of the jaw and temples, causing pain and dysfunction. The disease normally starts with swollen muscles on the top of the head, quickly followed by progressive muscle atrophy making opening and closing its’ mouth difficult and painful, and eventually causing the dog to be unable to move its jaw.
Miller’s Anatomy of the Dog, 4th edition