(2) The sense capsules.
The sense capsules are cartilaginous or bony structures which surround the olfactory and auditory organs and are closely united to the cranium.
Fig. 25. A dorsal and B ventral view of the cranium of a common Frog (Rana temporaria) from which the membrane bones have mostly been removed. × 2 (after Parker).
| 1. sphenethmoid. | 8. pro-otic. |
| 2. palatine. | 9. anterior fontanelle. |
| 3. pterygoid. | 10. right posterior fontanelle. |
| 4. quadrate. | 11. quadratojugal. |
| 5. columella. | 12. nasal capsule. |
| 6. exoccipital. | II. V. VI. IX. X. foramina for |
| 7. ventral cartilaginous wall of | exit of cranial nerves. |
| cranium. |
The auditory capsules are fused with the sides of the posterior end of the cranium just in front of the exoccipitals. They are largely cartilaginous, but include in their anterior walls a pair of irregular cartilage bones, the pro-otics (figs. 24 and 25, 8). The cartilaginous area lying ventral to the pro-otic and external to the exoccipital is pierced by a rather prominent hole, the fenestra ovalis, which forms a communication between the internal ear cavity, and a space the tympanic cavity, which lies at the side of the head, and is bounded externally by the tympanic membrane. The fenestra ovalis is occupied by a minute cartilaginous structure, the stapes, and articulated partly to this and partly to a slight recess in the pro-otic is the columella (fig. 25, B, 5), a rod in part bony and in part cartilaginous, whose outer end is attached to the tympanic membrane. The columella and stapes are together homologous with the mammalian auditory ossicles and with the hyomandibular of Elasmobranchs. Sometimes the term columella is used to include the whole ossicular chain,—the columella together with the stapes.
The olfactory or nasal capsules (fig. 25, B, 12) are fused with the anterior end of the cranium and differ from the auditory capsules in being to a great extent unossified. There are however two pairs of membrane bones developed in connection with them, the vomers and the nasals. They are drawn out into three pairs of cartilaginous processes, on the dorsal surface into the prenasal and alinasal processes which bound the external nares, and on the ventral surface towards the middle line into the forwardly-projecting rhinal processes.
The nasals (figs. 24 and 26, 11) form a pair of triangular bones lying dorsolaterally in front of the fronto-parietals. Their bases are turned towards one another and their apices are directed outwards and backwards. They correspond in position with the prefrontals of the reptilian skull as well as with the nasals.
The vomers are a pair of irregular bones lying on the ventral surface of the olfactory capsules. Each bears on its inner and posterior angle a group of minute pointed teeth, while its outer border is drawn out into three or four small slightly diverging processes, the two posterior of which form the inner boundary of the posterior nares (fig. 24, B, 15).