The auricle ([Fig. 168]) is composed of a thin curved plate of cartilage (cartilago auriculæ), to which numerous muscles are attached and which is covered on both sides by integument intimately attached to the cartilage. Distad the edges of the cartilage form a free border (b); proximad the plate is rolled together in such a way that its edges (g and h) overlap on the craniomedial side, thus enclosing a tube or funnel (e) (concha) which forms the enlarged outer portion of the auditory passage.

Within the cavity of the auricle, on its medial surface, somewhat below the level of the lowest portion of the margin of the auditory opening is seen a very prominent pedunculated process (d). This is really part of a transverse ridge, extending craniocaudad on the medial wall of the auricular cavity. This ridge is the antihelix (d); it divides the auricula into a distal and a proximal portion. The portion distad of the antihelix is the scapha (a); its surface is smooth except for two slight longitudinal ridges extending distad from near the two ends of the antihelix. That portion of the auricle that lies proximad of the antihelix forms a deep irregular cavity known as the concha (e). A deep furrow, corresponding to the antihelix, separates scapha from concha externally. The concha extends caudad as a deep, rounded pocket; externally this pocket forms a prominent convexity, the eminentia conchæ (j), on the caudal surface of the proximal portion of the external ear. Farther proximad the concha narrows like a funnel and becomes much compressed laterally, so that a section of its cavity forms a narrow craniocaudal slit.

The inner surface of the concha is marked by a number of very prominent ridges and projections. These are usually due to folds in the cartilage, so that external furrows and depressions correspond to the internal projections. The edges of these depressions serve for attachment of muscles ([Fig. 169]), by contraction of which the form of the concha is changed,—the folds of the cartilage thus serving as regions of greater flexibility.

Two or three centimeters proximad of its external opening the cartilage of the concha ends in a free edge which receives within itself the distal end of the cartilaginous external auditory meatus (e′). The two are united together by fibrous tissue. The cartilaginous auditory meatus (e′) extends as a nearly cylindrical tube mediad and slightly craniad to the tympanic membrane in the opening of the bulla tympani. This tube is lined with integument which continues over the tympanic membrane. The subcutaneous tissue of the tube contains the ceruminous glands, which secrete the ear-wax and open on the surface of the skin.

To understand the relations of the muscles of the external ear it is necessary to consider a little more fully the structure of the auricular cartilage ([Fig. 168]), especially as compared with the human ear. The ear of the cat differs from the human ear in the fact that the edges of the auricular cartilages are rolled together proximad so as to overlap. There is thus no gap between the tragus (g) and antitragus (h), as in man, but these two structures partly overlap.

[Fig. 168].—Cartilage of the External Ear, Craniomedial View.

a, scapha or pinna; b, helix; c, spine of the helix; d, antihelix; e, concha; e′, cartilaginous auditory meatus; f, crista helicis; g, tragus; h, antitragus; i, processus uncinatus; j, eminentia conchæ.

The free edge (b) of the auricular cartilage corresponds partly to the helix of man. At its mediocranial angle the proximal part of the auricle is composed of muscle and integument only, so that the cartilage when isolated presents here a deep notch distad of which is a prominent projecting angle, the spine (c) of the helix, which serves for attachment of the adductor auris superior muscle ([Fig. 169], 1). Proximad of the spine, separated from it by the deep notch, the cranial edge (g) of the auricular cartilage approaches the caudal edge (h), finally overlapping it, so that the cavity of the auricle is now completely surrounded. This overlapping portion of the cartilage is on the medial side of the cavity, and in the natural state is covered by muscles and integument, so as not to be apparent.

That portion of the cranial edge of the cartilage that overlaps the caudal edge forms a cartilaginous plate which projects proximad as a blunt point; this plate is the tragus (g). Along the inner edge of the tragus is a ridge which forms a continuation of the helix; this is the crista helicis (f). The portion of the caudal edge of the cartilage that is partly overlapped by the tragus is an irregular flat plate known as the antitragus (h). At its caudolateral margin the antitragus extends distad as a thin, pointed, cartilaginous spine, the processus uncinatus (i). This supports a sheet of integument which bears two longitudinal ridges projecting into the cavity of the ear, with a broad groove between them.