Morphological And Morphometrical Aspects of The Auditory Ossicles In Roe Deer (Capreolus Capreolus)

The study provides a series of distinctive aspects of the auditory ossicles alongside comparative morphometric data, bringing facts in respect to morphology and morphofunctionality of the auditory ossicles in this little-studied species. The most important features noted are evident conical shape of muscular process of malleus and triangular aspect of the handle of malleus. For the incus, a short body of the bone and the direct continuation, with no clear distinction as an individualized piece, for the lenticular process. As for the stapes, the clear proling of the muscular tubercle for the stapedial muscle and elliptic shape of the foot of the stapes is noted. A series of comparative measurements and indices are also calculated in the attempt of proling differences from the domestic counter species- sheep and goat.


Introduction
Taxonomically, the roe deer belongs to the Atryodactyla order, Subfamily Capreolinae, genus Capreolus, species Capreolus Capreolus, with more than 55 subspecies. The ear is an important sensory organ, comprising structures that originate from the neural crests, endodermal, mesodermal, and ectodermal layers. Anatomically, the ear consists of three distinctive sectors: the external ear (auris externa), the middle ear (auris media) and the internal ear (auris interna). Placed in the dorsal part of the tympanic cavity (cavum tympany), within the temporal bone, the petrosal part (os temporale, pars petrosa), the assembly of the auditory ossicles (ossicula auditus) play an important role in the sound transmission from the tympanic membrane (membrana tympany) to the internal ear (vestibular fenestra-fenestra vestibuli) ( The present study aims to provide morphological and morphometrical data concerning the auditory ossicles. Given the so-called "transitional type" of the ear and the malleus-incus microtype (Péus et al. 2020) for the studied species, special emphasis is given to some correlative facts between mechanical and physical aspects and some of the metrical features of the ear ossicles to the speci city of sound transmission.

Material And Methods
The biological material was represented by 6 roe deer crania originating from the collection of comparative specimens included in the Anatomy Department's reserve. A bone rongeur was also used to create a small window to allow the approach of the middle ear cavity and the acoustic bone assembly.
Ossicles were gently cleansed and prepared for in-depth macroscopical analysis.
Based on the digital images collected (Olympos MTX stereomicroscope and camera), a graphical digital tablet (Wacom Intuos Art & Touch Small) (Wacom Corporation Intl), Corel Painter Essentials 5 suite (Corel Tm ) and ImageJ ,a graphical redraw and further measurements of the ossicular assembly were made.
Measurements on the auditory ossicles were made in accordance with data suggested by a series of authors (Kurtul et al. 2003

Results
The largest of the ossciles-the malleus (Malleus) is the laterally placed one, connected to the smaller incus (Incus) and then, the most medially placed, the stapes (Stapes).
The Malleus consists of a head (Caput mallei) connected by a neck (Collum mallei) to the handle (Manubrium mallei). The head of the malleus in Capreollus capreollus is ovally-shaped, slightly obliquely placed medio-laterally, cranio-caudally. On its dorso-caudal border, an articular surface (Facies articularis) with a relatively circular shape and somehow divided into 2 distinctive, uneven surfaces is present. The angulation between the 2 surfaces was measured to be approximately 135 degrees. The neck of the malleus is well pro led, obliquely placed in ventro-lateral and caudal direction, slightly attened, with an overall curly appearance. Its dorsal border is convex while in the distal part slightly concave. The neck has three processes: the anterior process (Processus rostralis) in ventro-cranial direction, the muscular process (Processus muscularis), and the lateral process (Processus lateralis). A thin ossicular lamina that extends in a pointed direction, overpassing the overall shape of the ossicle, imparts the overall shape of a triangular pennant. The conical muscular process detaches from the medial surface of the neck, at the same level as the anterior process is detaching. The lateral process detaches from the lateral surface of the neck, close to the terminal part of this segment, marking the passage to the manubrium. This process has a triangular shape and can be easily identi ed through the tympanic ring. The last segment-the handle of malleus (Manubrium mallei) continues ventrally. It has an overall tri-faceted process that terminates attened, pointed, almost rounded distally, exceeding 50% out of the total length of the malleus.
The incus (Incus) has a body (Corpus incudis) and two crura (Crus breve, Crus longum), the body being the most developed segment of this ossicle. Overall, the ossicle has the shape of a human bicuspid tooth.
The body possesses a visible articular process-Processus articularison its cranial aspect with a saddlelike aspect, slightly excavated, as the negative of the surface described in malleus. The two facing surfaces are placed at an approximate angle of 60-70 degrees. The crura of the incus detach from the aboral part of the body of the ossicle. The short crus (Crus breve), conical and thicker at its base points in dorso-medial, oblique direction. The long crus (Crus longum) detach from the ventral border of the body to bend medially to continue with the lenticular process (Processus lenticularis). This process is cylindrically shaped, quite distinctive at the tip of the crus, with an oval articular surface for the junction with the head of stapes The stapes (Stapes) is located in between the incus and the oval window. Almost triangular, with an oblique, ventro-medial and cranial disposition, it is composed of a head (Caput stapedis), dorsally placed, a base (Basis stapedis), ventro-medial and cranially placed linked by 2 crurae-the anterior crus (Crus rostrale) and the posterior crus (Crus caudale). The head is reduced, with a hemispherical articular surface for the lenticulate bone. Its caudal border bears a reduced tuberosity that serves as an insertion point for the stapedial muscle (m.stapedius)-the muscular process. The two crura, cylindrical and slightly divergent connect to the basal part of the bone. The posterior crus is thicker than the anterior one. The caudal border of the posterior crus seems concave while the anterior border is straight. The anterior crus, much thinner, presents a posterior border that is slightly concave while its anterior border is convex. This makes the anterior crus as being somehow curved as appearance while the posterior one seems quite straight. The base (footplate) of the stapes is elliptically shaped, with a medial surface that serves as attachment point to the oval window.

Discussion
The morphological aspects in case of the malleus in Capreollus capreollus are similar to those described in buffalo (Nourinezhad et  The middle ear, in its morphology, can be regarded as a pressure ampli er. The ossicular arrangement, its joints, ligaments, and muscles do nothing but change the e ciency of the sound transmission (Huttenbrink 1992;Chhan et al. 2016) and generate mechanical advantage, from the tympanic membrane to the oval window towards the cochlear system (Uziel et al. 2009). The malleus and the incus can be regarded as a type 1 lever that has a counter-clock rotational movement as the tympanic membrane moves inward, pressing then against the internal crus of the stapes onto the oval window, with a cited ratio up to 19:1 effective tympanum to oval window (Pujol 1989). Another term used for the description of this assembly is "hinge-like rotational motion" (Péus et al. 2020) with leveraging capabilities. There is a direct relation between the surface area of the tympanic membrane, the surface of the footplate of the stapes, and the lever system of the middle ear, with a cited ratio between the two arms of the lever of aprox 1.3 (Uziel et al. 2009). By adopting this lever system to the actual measurements, mainly the ones that point to the elements forming the two arms of the lever, we can establish comparative data among related species that have available data (maleo-incal complex and stapes). More than that, some researchers suggest a complex calculation for the expected gain by this hinge-like rotation by multiplying the area ratio between the tympanic membrane (pars tensa) and oval window by the lever ratio of the malleo-incal complex.
A set of measurements (with different reference points) point us to a landmark that is usually not established morphologically but through CT and 3D volume reconstruction procedures-the establishment of the principal axes, the moments of inertia and the C.O.M (Center of mass)(Péus et al. 2020). Our approach did not use this kind of investigation, but due to the high resemblance to the sheep model that sources illustrated and cited, a similar point was established by using the three principal axes suggested (Péus et al. 2020).
To evaluate similarly the available data for the assessment of the lever ratio in the maleo-incal complex we have re-processed the data for sheep (Péus et al. 2017), goat (Martonos et al. 2021) along the ones from roe deer.The compared ratios are among the L manu-L mal, L head-L umbo (for malleus) and the incus the L sp-L lp ratio. As suggested by the comparative gures calculated based on the two distinctive sets of measurements, some interesting facts arose here. The simple calculations of the total lengths of the malleus vs incus show some higher ratio for the goat ossicles, as the ones for sheep and roedeer are not so distant. Values for the length of the long crus of incus as opposed to the length of the stapes are showing higher values for goat, while the pair sheep-roe deer is similar. The malleus vs stapes total lengths are similar in all three compared sets of specimens.
In terms of the measurements used in the assessment of some functional ratios (Péus et al. 2020), a series of facts are revealed. Although the CenterOfMass point was an estimate in our approach for goat and roe deer samples, some gures are pointing to a much clearer differentiation of ratios in the case of the roe-deer data from the ones calculated for sheep and goats. This distinction may be a better indicator for the accuracy of the measurements taken in consideration in direct relation to the functional aspects that point to the mechanics of hearing in these species (lever ratios). Special attention is drawn towards the ratio of length of malleus and the length of the long crus of incus that shows a much smaller ratio for roe deer, as a possible differentiation that might be the result of another pattern in the transmission of vibrations along the complex system of the middle ear.

Conclusions
The present study brings into light some new elements of the middle ear anatomy for a species relatively little studied. This makes this main purpose of the study-the complete morphological description of the ear ossicles, alongside some morphometrical data. All these, combined, with series of graphical elements may serve as a useful didactical tool in the study of the comparative morphology of the middle ear and to point to some new directions in this study.