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Folia Morphol. Vol. 67, No. 2, pp. 111–119 Copyright © 2008 Via Medica O R I G I N A L A R T I C L E ISSN 0015–5659 www.fm.viamedica.pl Topographical anatomy and measurements of selected parameters of the rat temporal bone J. Wysocki Clinic of Otolaryngology, Medical Faculty No. 2, Medical University of Warsaw, Kajetany, Poland Institute of Physiology and Pathology of Hearing, Warsaw, Poland [Received 22 October 2008; Accepted 22 November 2008] On the basis of dissection of 24 bones of 12 black rats a systematic anatomical description was made and measurements of selected size parameters of the tem- poral bone were taken. Besides the main air space in the middle ear, the tym- panic bulla, there are also additional air cells, namely the anterior and posterior epitympanic recesses, containing the head of the malleus and the body of the incus. On the side of the epitympanic recesses the following are easily accessi- ble: the malleus head and the core of the incus, the superior and lateral semicir- cular canals and the facial nerve. On the side of the ventral tympanic bulla it is easy access to both windows and the cochlea. The semicircular canals are rela- tively large, the lateral canal being the largest and the posterior the smallest. The length of the spiral canal of the cochlea does not exceed 11 mm. It is worth mentioning that both the vertical and horizontal dimensions of the scala vestibuli and scala tympani do not even exceed 0.7 mm in the basal turn, and are signif- icantly decreased to tenths of a millimetre in further turns. This needs to be taken into consideration during all experiments requiring the introduction of examin- ing instruments into the cochlear scala. (Folia Morphol 2008; 67: 111–119) Key words: temporal bone, anatomy, measurements, rat INTRODUCTION pecially rodents, cover a wide spectrum of prob- The temporal bone containing the vestibuloco- lems in the field of experimental and clinical med- chlear organ is significant in the pathology of hu- icine [1, 4, 6], there are only a few publications mans and animals. Accurate anatomical descrip- dealing with the anatomy of the rat ear [3, 5, 7]. tion of the human ear with all its structures is found The aim of undertaking this study has been to come in every anatomy textbook and in specialist mono- up with a systematic anatomical description of the graphs. Analogical descriptions of the ear structures temporal bone, including the topographic aspects, in of animals, found in veterinary anatomy textbooks, selected animal species for clinical purposes. are scarce, schematic and do not go beyond a rough enumeration of the structures, often with no data MATERIAL AND METHODS on topography and variability. The same applies Twenty four temporal bones (12 left and 12 right, to monographs devoted to particular species of lab- 6 male and 6 female specimens) derived from oratory animal [9, 10]. While experiments on the 12 black rats (Rattus rattus, Linnaeus, 1758) and vestibulocochlear organ in laboratory animals, es- weighing 175–200 g were used for the study. The Address for correspondence: J. Wysocki, Institute of Physiology and Pathology of Hearing, Warsaw, 1 Zgrupowania AK Kampinos, 01–943 Warsaw, Poland, tel./fax: +48 22 835 52 14, e-mail: [email protected] 111 Folia Morphol., 2008, Vol. 67, No. 2 animals were obtained from the Drug Institute in Warsaw, Poland and were not deliberately deprived of life, but rather routinely euthanised after scien- tific experiments, and so it was not necessary to ac- quire permission from the Bioethics Committee. The description was complemented by particu- lar measurements of selected size and volume pa- rameters of the temporal bone in order to provide Figure 1. Scheme of measurements conducted on the whole indispensable data to scientists planning experi- skull of the rat; 1 — full length of the skull, 2 — length of the ments on animals. The terminology applied is con- neurocranial base. sistent with official veterinary anatomical nomen- clature [8]. The investigation adopted traditional techniques for anatomical preparation, observation, photogra- A phy and measurements. The bones were fixed in 10% formaldehyde solution. Next they were filled by in- jection with liquid latex as previously described [11] and then prepared under an operating microscope. Measurements were made using an eyepiece (ocu- lar) with a 0.05 mm to 0.1 mm accuracy gauge (de- pending on the size of the objective), according to the schematic drawnings presented (Fig. 1, 2). The full length of the skull was measured as a distance between the opistocranion and prosthion points. The length of the neurocranial base was evaluated as a distance between the hormion and opisthion points. Student’s t test was performed for evalua- B tion of differences related to sex and body side. RESULTS The temporal bone and skull The rat’s temporal bone is an element of the lat- eral and ventral wall of the skull (Fig. 3–6). It con- sists of the squamous, tympanic, petrous and small mastoid. The first three parts are almost equal in size, although they obviously differ in external and internal topographical features. After maceration, the temporal bone falls apart into the three elements and therefore complete synostosis does not occur Figure 2. A. Scheme of arrangement of the structures of the during life. There is then a temporal skull complex tympanic cavity of the rat; 1 — muscular process of the tympanic bulla, 2 — anterior epitympanic recess, 3 — posterior rather than a temporal bone. epitympanic recess, 4 — oval window, 5 — separated air cell, The squamous part forms a considerable frag- 6 — jugular foramen, 7 — round window niche, 8 — promontory, ment of the lateral wall of the skull and joins the 9 — cave for the tensor tympani muscle, 10 — internal orifice of the Eustachian tube, 11 — borders of the opened tympanic bulla; zygomatic bone, creating part of the zygomatic arch. B. Scheme of measurements conducted on the tympanic cavity The tympanic part is much better developed and of the rat’s ear; 1 — maximal length of the tympanic cavity, limits the lateral, inferior and small fragment of 2 — length of the semicanal of the Eustachian tube, 3 — length of the anterior epitympanic recess, 4 — height of the anterior medial wall of the tympanic cavity. It consists of the epitympanic recess, 5 — length of the posterior epitympanic tympanic ring and the tympanic bulla, and consti- recess, 6 — height of the posterior epitympanic recess, 7 — minimal distance from the round window to the internal orifice tutes the ventro-latero-rostral part of the temporal of the Eustachian tube, 8 — minimal distance from the round bone. There is no isolated external acoustic meatus. window to the carotid canal, 9 — minimal distance from the The tympanic part bulges inferolaterally, where the round window to the jugular foramen. 112 J. Wysocki, Rat temporal bone... Figure 3. View of the left lateral wall of the rat skull. Scale bar Figure 5. Specimen of rat head. View of the lateral supra and 1 mm; 1 — frontal bone, 2 — squamous part of the temporal infratentorial cavity of the skull. Left side. Scale bar 0.5 mm; bone, 3 — parietal bone, 4 — occipital process of the squamous 1 — dura mater of the sigmoid sinus, 2 — dorsal petrous sinus, part, 5 — squamous part of the occipital bone, 6 — left occipital 3 — groove and canal of the trigeminal nerve, 4 — neural area, condyle, 7 — paracondylar process, 8 — mastoid process, 5 — external opening of the cochlear canaliculus, 6 — subarcu- 9 — auditory bulla, 10 — zygomatic process of the squamous ate fossa. part, 11 — zygomatic bone, 12 — handle of the malleus, 13 — postglenoid foramen, 14 — stylomastoid foramen. Figure 4. Ventral wall of the rat skull. Scale bar 1 mm; 1 — fora- men magnum, 2 — hypoglossal nerve canal, 3 — jugular foramen, 4 — paracondylar process, 5 — auditory bulla, 6 — zygomatic process, 7 — middle lacerated foramen, 8 — oval foramen, 9 — muscular process, 10 — pharyngeal opening of the auditory Figure 6. Specimen of rat head. Left side. Scale bar 0.5 mm; tube, 11 — external opening of the internal carotid canal, 1 — temporal muscle, 2 — external acoustic meatus, 3 — interior 12 — opening of the tympanic canaliculus (external opening of the external acoustic meatus, 4 — facial nerve after leaving the of the stapedial artery canal). canal, 5 — branches of the facial nerve, 6 — masseter muscle, 7 — digastric muscle. tympanic membrane turns into the tympanic bulla poral canal containing the petrosquamous sinus, with no distinct border line. Rostrally the bulla grad- which opens with the postglenoid foramen. The ually narrows and turns into a short sharp muscular petrous part takes the form of a bony lamina with process, which is the attachment of the palate mus- complicated external carving and internal structure. cle. It is well developed in the medial part of the It does not have the shape of triparietal pyramid as bulla, so that it forms an almost complete circum- observed in other mammal species but is rather sim- ference of the auditory tube canal, complemented ilar to an irregular cuboid. In fact it is a flat lamina, only by a small fragment of the petrous part. which is only thickened in some areas, as the cap- The petrous part forms the middle and posterior sule of the labyrinth is particularly thin and delicate cranial fossa and contains the inner ear. On the bor- in rats. The medial wall of the petrous part forms der of the squamous and petrous parts lies the tem- the lateral wall of the infratentorial cavity of the skull 113 Folia Morphol., 2008, Vol. 67, No. 2 and the anterior wall forms the ventral wall of the supratentorial cavity (Fig.

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