296 



DISCOVERY REPORTS 



LOCATION OF THE WAX PLUG 



It has long been supposed that the external meatus of the larger Cetacea is closed for a short distance 

 immediately below the blubber, and that more proximally its lumen opens out again and widens 

 gradually until it terminates in the glove-finger-like structure, which protrudes from the tympanic 

 bulla. That this statement is inaccurate, and that the structure identified by Lillie and others as the 

 epidermis of the meatus is only the corium of the latter, will be seen during the course of the descrip- 

 tion of the wax plug (see Text-fig. 1). It is not surprising that this part of the meatus has escaped the 

 attention of most Cetacean anatomists since Lillie, for it lies in a deep groove between the squamosal 

 and paroccipital process, and is invested in nearly a foot thickness of tough, fibrous tissue. More 

 distally, near the ' blind ' section, the meatus protrudes beyond the lateral limit of the bones, and its 

 lumen is flattened dorso-ventrally so that the dorsal and ventral walls are nearly in apposition. The 

 corium of the superior wall of the proximal part of the meatus conforms with the shape of its adjacent 



Text-fig. 1. Diagram showing the inter-relationships of the tympanic cavity, external meatus and epidermis in a typical 

 Mysticete. The space occupied by the wax plug is cross-hatched. TC, tympanic cavity; TB, tympanic bulla; TM, tympanic 

 membrane; TA, tympanic annulus; GF, glove-finger; C(ZC), core of the wax plug or the zona cornea of the epidermis of the 

 glove-finger; O(SC), outer covering of the wax plug or the stratum corneum of the epidermis of the external auditory meatus; 

 L, lumen of the external auditory meatus; C, corium of the dermis; ZG, zona germinativa of the epidermis; ZC, zona cornea 

 of the epidermis; EM, external auditory meatus; BS, 'blind' section; B, blubber. 



bones, and is sharply convex on its dorsal aspect. Ventrally the corium of this part of the meatus is not 

 covered by bone and is thus, in the living animal, subjected to pressure transmitted through the fibrous 

 tissues. The ventral aspect of the corium is consequently slightly concave and its upper surface bulges 

 towards the lumen of the meatus. 



The ' glove-finger ' has frequently been referred to as the tympanic membrane, but, for reasons which 

 they state, Fraser and Purves (1954) disagree with this interpretation. The homologue of the tym- 

 panic membrane is the triangular ligament which is inserted into the manubrium of the malleus. This 

 ligament has a deep but very narrow concavity on its lateral aspect which is continuous with the 

 external meatus, and is thus not fundamentally different from the tympanic membrane of the Odonto- 

 ceti and other mammals. Between the glove-finger and the tympanic annulus there is another circular 

 concavity which is also continuous with the meatus. The concavity in the tympanic ligament, that 

 which encircles the glove-finger and the whole length of the auditory meatus from the tympanic 

 annulus to the constricted lateral extremity, is filled by the wax plug. A section (PI. XV, fig. 2) taken 

 near the centre of the so-called blind section in Balaenoptera acutorostrata shows that in this species 

 the lumen is not entirely closed, and that it is surrounded by concretions of a waxy consistency. The 

 plug is, therefore, not limited to the dimensions given by Lillie and Turner, but may be a metre or 



