Muscles of the Ear

The facial muscles of mammals are very complex and variable. Even though the dog does not have the same range of facial expression as that seen in humans, the muscles of the dog’s face are well developed. They consist of several layers and numerous slips that may differ in size or insertion from one individual to another. Their innervation, however, by the nervus facialis is constant.

Ernst Huber, assistant in the Anatomical Institute at the University of Zurich, set himself the task of investigating the facial musculature and its innervation in carnivores. His well-illustrated publications are the most detailed findings available on the muscles of the face and ear of carnivores.

The difficulty in distinguishing one facial muscle from another results from their lack of distinct connective tissue sheaths or fascia between the delaminated layers and slips. Thus adjacent layers and slips often join each other in various ways, or slips may become independent and constitute separate muscles deserving of a name. Variations of this sort, which make neat classifications difficult, often show the relations of one muscle to another and clarify the ontogenetic and phylogenetic history of the slip or muscle. (A similar situation was encountered by Evans [1959] with the digastric and hyoid muscles of the dog.)

Scutiform Cartilage

A small, boot-shaped cartilaginous plate, the scutiform cartilage (cartilago scutiformis) or scutellum, is located in the rostroauricular muscles medial to the ear. Several muscles that move the ear attach to the scutiform cartilage. In addition to some short auricular muscles, the broad m. interscutularis passes from its attachment on the medial dorsal border of one scutiform cartilage to the other and, in so doing, blends with the m. frontalis. The scutiform cartilage is thought to be a detached portion of the conchal spina helicis that separates at birth or shortly afterward, so it should be considered a part of the external ear. Deep to the scutiform cartilage there is a fatty cushion, the corpus adiposum auriculare, that extends over a portion of the temporal muscle and around the base of the ear, giving the overlying muscle mass more mobility. Huber (1922) found many individual variations in auricular and facial muscles of dogs caused by delaminations, divisions, and fusions. Frequently, shared muscle fibers indicated common origins and shared relationships. Many of these variations were shown by Huber (1922, 1923) and reproduced by Leahy (1949). A few examples are reproduced here.

Because the external ears of mammals vary so much in shape, it is difficult to homologize the parts in different species. Huber (1922, 1923), in his two-part, well-illustrated monograph on the facial muscles of the dog, calls attention to the work of Boas (1912), who devised a method for studying and describing the auricular cartilage. Boas introduced a uniform nomenclature that allowed comparisons between the ears of mammals regardless of the contour of the ear in life. Anatomic preparation consists of macerating the ear to remove the skin and rolling it open to lie flat. Leahy’s redrawing of Huber’s illustration of the flattened dog ear (after Boas [1912]) is reproduced here with modified labels as Figure. When the auricular cartilage is flattened, each margin (anterior = Nomina Anatomica Veterinaria [NAV] medial; posterior = NAV lateral) can be said to bear seven lobulations or projections. These are numbered A1 to A7 and P1 to P7. For the details of muscle attachment to these processes or how they relate to each other when the ear cartilage is rolled into its normal position, see Boas (1912), Huber (1922, 1923), or the translation from Leahy (1949).

The borders of the auricular cartilage were named anterior and posterior for a long time, and these were the terms used by Boas (1912) and Huber (1922, 1923) in naming the parts of the auricle. The anterior and posterior auricular muscles of these authors are now rostral and caudal auricular muscles (NAV, 2005), with added designations of dorsal and ventral auricular muscles. Miller (1948) designated the rostral muscles as preauricular and the caudal muscles as postauricular in early editions of his dissection guide. Because the external muscles of the ear develop by splitting and delamination of muscle sheets or bundles, there may be superficial and deep layers, each of which may be further delaminated and given a Roman numeral.

Because various directional terms have been used for the ear, and these required different terms for muscles, vessels, and nerves, the application of these terms in the literature is not always consistent. Directional terms for structures of the internal and external ear as given in Nomina Anatomica Veterinaria (2005) include the following:

Inner ear: Anterior, posterior, or lateral for semicircular ducts, canals, and ampullae; superior or inferior for the area vestibularis

Middle ear: Rostral or caudal on the tympanic membrane; medial or lateral in the auditory tube; rostral or lateral for the processes of the malleus; rostral or caudal in the cavum tympani

External ear: Directional terms apply to the ear with the concave surface of the auricle facing rostrally. Thus the concave surface is the rostral surface and the convex surface is the caudal surface and there are medial and lateral borders. There are rostral and caudal auricular nerves. The caudal auricular artery has lateral, intermediate, and medial branches.