Shoulder

Brachium

Antebrachium

Facial and Masticatory Muscles

Thorax and Heart

Brain

Body Wall

Hip and Thigh

Shank and Foot

Digestive System

Urinary and Reproductive Systems

Hip and Tail

Facial and Masticatory Muscles of the Rat (Rattus norvegicus)

Photo and text by Yu Jin Kim and Ana Panduro

Part I: Overview

The facial muscles of Rattus norvegicus are adapted to their eating habits.  This species of rat is descended from a specialized herbivore, even though the living rat is an omnivore.  One important characteristic of the rodents is that their jaw moves in forward and backward motion while it chews whereas other mammals such as a pig or a cat move their jaws up and down.  Grinding power stroke is the forward motion and then they retrieve the jaw back into its original position.  This specialized movement is called “propalinal” and it is facilitated by the rat’s four-part masseter muscle. 

The masseter muscles attach more anteriorly than those of the other mammals in our class except for the beaver.  The anterior attachment point is similar in the beaver, another rodent.

There are two ear muscles in a rat.  The interscutularis is a thin, poorly developed muscle that connects the bases of both pinnae together.  Another muscle of the ear is the cervioauricularis, which originates from the median raphe of the neck.  It encircles the base of the pinna and inserts rostral to the ear.  This muscle elevates and rotates the external ear.  

Dissection:

In order to reveal facial muscles we first made a cut from the top of the head to the tip of the nose.  We had to be very careful not to cut too deeply or we might have cut through the muscles.  It was hard to peel off the skin because it was so thin and is became even harder when we got to the eye and mouth area because orbicularis oculi and orbicularis oris muscles are thin and are tightly attached to the skin.  Peeling skin off around the mouth was especially difficult because of the thick pad of mystacial muscles, which moves the whiskers.  The platysma in our rat is a well-developed muscular sheet but it is very thick and difficult to separate from subcutaneous fascia.  Although the dissection process was difficult, it was easy to identify the facial muscles once we had removed the skin.

Part II: Facial muscles:

Muscles below are all innervated by the trigeminal nerve