Labeled Coronal Slices

In Talpa, many of the basicranial bones are pneumatized by air-filled extensions of the middle ear cavity. These areas of pneumatization in the mole are sometimes described as hypotympanic sinuses. Significantly, latex injected into one middle ear cavity penetrates contralaterally in Talpa: there is an interbullar connection (Pye & Hinchcliffe, 1968).

• Slice 146 is through the snout region of Talpa, with the skull above and the two lower jaws (mandibles) in articulation ventrolaterally. The slice shows the vault-shaped nasal cavity framed laterally by the paired maxillae, and roofed by the nasal bones on either side of the dorsal midline. The maxillae bear the teeth. The two halves of the nasal cavity are separated in the midline by the vertically aligned internasal septum formed by the mesethmoid bone. This flange is flanked by paired descending flanges from the nasal bones. The nasal cavity in this slice is floored on either side by the maxillae. The section passes through the skull just anterior to the infraorbital canals that are positioned just in front of the orbits. It shows the large scroll-like bones inside the cavum nasi (nasal cavity). These scroll-like bones are the maxillary turbinates and their origin from the internal walls of the maxillae can be seen. The twin-cusped form of the upper premolars (dilambdodont pattern) can be clearly distinguished, as can the way they form a V-shaped basin against which the pointed cusp of the lower premolars shear. This is ideal for puncture-crushing tough beetles and the like.
• Slice 228 is at the level of the anterior (maxillary) root of the zygomatic arch and is just posterior to the infraorbital canal (which can be seen in coronal slices 204-220). This slice illustrates the very wide, rounded form of the snout in this area of the mole’s skull. This is unusual in mammals, where the normal arrangement is for this region to be constricted (flattened) in order to make room for the eyeball. In moles, where the eyeball is greatly reduced in size and function, the braincase does not need to be constricted and it takes on this rounded form. The turbinates are large and complex; their origins can be seen as the bones in the dorsolateral corners of the skull vault and the two curving septa on each side of the midline septum formed by the mesethmoid bone. These are therefore the turbinates of the frontal and the mesethmoid bones, respectively. The lower part of the midline septum is formed by the vomer; it does not give rise to any turbinates. The floor of the skull is composed of the palatal plates of the palatine bones on each side of the midline. The roof of the skull is formed by the frontals and the sides are the left and right alisphenoid bones. The molar teeth are situated in the posteriormost region of the left and right maxillary bones. Note the left and right mandibles and the deep sockets (alveoli) of the molar teeth implantation. Below them runs the inferior alveolar canal for the mandibular branch of the trigeminal nerve and accompanying blood vessels; these supply the dental tissues and the skin of the lower jaw.
• Slice 289 is at the level of the tall coronoid processes of the mandible, just behind the eyeball. Each coronoid process is seen as a tall vertical plate at the side of, and away from, the cranium. The small structure in cross-section on each side of the lateral face of the coronoid process is the thin zygomatic arch. Also note the inferior alveolar canal in the lower jaw. The skull shows highly convoluted turbinates, which occupy the entire volume of the skull at this region. The roof, walls, floor and ventral midline of the cranial cavity at this level are formed by the frontals, alisphenoid, pterygoids and basisphenoid respectively. The vertical plate of the interorbital septum is in connection with the mesethmoid anteriorly but here is composed of a vertically directed flange of the narrow median basisphenoid. A contact between the basisphenoid and the mesethmoid is considered to be primitive for mammals. The scroll-like turbinates in this section come from the frontals, alisphenoids and pterygoids. Their size and location so far back in Talpa's skull is unusual in mammals, but their constituent bones (frontals, alisphenoids and pterygoids) are even more unusual.
• Slice 319 is taken in front of the level of the posterior root of the zygomatic arch. The latter can be seen as a tiny rounded element located at the tip of the vertical posterior plate of each of the lower jaws, lateral to the cranial vault. The sections of the lower jaws here almost form a narrow L-shape. The roof of the skull is formed by the parietals on each side of the midline; their median sagittal suture is just visible. The middle levels of the cranial walls are formed by thin processes of the squamosal and petrosal bones, but the main part of the side wall is formed by dorsal extensions of the pterygoid bones. The medial septum and the median floor of the cranium are formed by the basisphenoid bone. The curious semi-triangular structures at each ventrolateral corner are the pterygoid bones; they show an unusual spider-web arrangement of internal trabeculation. The trabeculae themselves are curving, branched folds of compact bony laminae rather than discrete spicules. Between the trabeculae are air spaces in communication with the tympanic cavity. The floor of the cranial cavity proper – at the mid-level of the skull in this section – is made up of the petrosal.
• Slice 434 is at a level just posterior to the external auditory meatus (external ear tube). The spongy structures in the ventral midline are the trabeculae of the basisphenoid. The spiral shapes on either side of this structure are the left and right cochleae of the inner ears. The semicircular canals cannot be made out in this view (they are further posterior, in coronal slices 461-501). The cochleae are surrounded by air cells of the petrous temporal bones. These cells are diverticulae of the middle ear cavity, providing a connection between the left and right middle ear cavities. This arrangement is extremely rare in mammals and is mainly to be found in moles. Its functional significance is poorly understood at present. The diverticulae are filled with bony trabeculae (rods) but are not connected to the nasal cavity. All the thickened bone below the brain is pneumatised basisphenoid, plus contributions from alisphenoid and basioccipital (though not in this section). The wide, flat, capacious shape of the cranial vault is formed by the parietal bones. The bulging prominences at the mid-horizontal level of the cranial vault are composed of the squamosals, while the lower parts (containing the diverticulae) are the petrous temporals. Between the trabeculae are air spaces in communication with the tympanic cavity.