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IV. Transverse section through the internal ear

male

Embryology - 20 Apr 2024 Expand to Translate
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Braune W. An atlas of topographical anatomy after plane sections of frozen bodies. (1877) Trans. by Edward Bellamy. Philadelphia: Lindsay and Blakiston.

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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding. (More? Embryology History \| Historic Embryology Papers)

THIS plate does not represent a directly transverse plane section, but an oblique one. In order to bring the relation of the ear well into view the section is taken obliquely backwards and upwards. It commenced close under the nose, and has involved in its course to the external meatus the inferior turbinated bone, the upper portion of the pharynx, the right Eustachian tube, the tympanum and the external meatus, and passing out through the pons Varolii, has divided the upper half of the cerebellum and the posterior lobe of the cerebrum above the external occipital protuberance. The preparation was made from the body of a young man, which presented nothing abnormal. As the section in the posterior half of the left side passes higher than in the right, it is nearer the roof of the left tympanum, whilst on the right it approaches its floor. I convinced myself, however, from many sections, that if there be any deviation of the saw from the proper direction, it is not possible to make a thoroughly symmetrical preparation.

In this plate the objects of chief importance are the relations of the right organ of hearing, the section of which has been so fortunate, that not only the external meatus and the tympanum, but also the first part of the Eustachian tube as well, have been divided. I was never again able to obtain the parts together to such an extent, although I made more than twelve sections in the same direction. Owing to individual variations in the base of the skull in the direction of the Eustachian tube, it is impossible to give any exact definitions for making such a section.

As the chief points of interest lie in the upper half of the section, it will be represented instead of the lower half as in preceding plates ; consequently one looks from below upwards into the skull, and the parts lying on the right side are really those of the left side and vice versa. In describing the individual organs, then, of the left ear, the left nasal cavity, and so on, the right side of the plate must be consulted.

The upper half of the external meatus, the relations of the cartilaginous, integumentary, and bony parts of the right ear are seen. The connections of the cartilaginous portion of the Eustachian tube with the cartilage of the pinna, and the fissures of the external meatus appear as gaps between the cartilaginous rings.

By this disposition a large amount of passive motion is allowed at the entrance to the ear, which is noticeable equally in the movements of mastication and in the contractions of the pinna. By drawing back the ear the curve of the meatus is diminished and the examination of the membrana tympani rendered more readily accessible.

It is worth noticing that the curvature of the external ineatus is not so abrupt as one might imagine from the examination of soft preparations or plates.

After examination of the living body as well as Pirogoffs plates (a a 0, fasc. i, Tab. 6) which also were prepared from frozen bodies, and in spite of the various differences which in this respect the external meatus offers, I must admit that the parts seen in the section of a body not thoroughly hardened, vary materially with their original position. It is well known that this canal is curved from before backwards and from above downwards, and is thus somewhat serpentine in its course. Naturally, the relations in this section could not be represented with perfect clearness, although it will be seen from the stronger shading of the internal parts, that the upper wall of the canal rises backwards and somewhat upwards, and consequently that the semi-canal in the region of the membrana tympani is deeper than it is wide externally.

The membrana tympani has been divided in its lower half, hence the ossicles are not interfered with.

The direction and position of this membrane should be noticed, as it lies in a plane which makes a very acute angle with the horizontal, and also the navel-shaped retraction towards the tympanum, and the portion of the malleus which is in relation with it. As the ossicles of hearing, on account of their small size, are very difficult to represent accurately, a woodcut is introduced at the end of this chapter, in which the parts are enlarged three times. Deep down in the tympanum passing from before backwards is seen a bony protuberance which belongs to the semi-circular and Fallopian canals, and behind the stapes, in the section of the temporal bone, is shown the canal, containing the facial nerve. This marked " cropping up " of this canal on the roof of the cavity of the tympanum is characteristic of the young individual. The young ear is especially suitable for the study of the organ. In the middle line from the stapes is the vestibule. The cochlea is not shown as it lies above the section.

Internally and in front of the cavity of the tympanum is the internal carotid artery, shown from its entrance to its first curvature, and subsequently divided transversely. In front of the carotid is the Eustachian canal, flat anteriorly, and passing deep down posteriorly. It runs consequently more vertically backwards than the plane of section passing from the nasal to the auricular aperture. The section passes through its pharyngeal opening, laying that portion of its canal free, but not its bony portion. Thus, only a small portion of the lateral tubal cartilage (Riidinger's hook) is divided anteriorly, whilst a long strip of the median cartilage is exposed. Laterally the canal exhibits a mucous membrane rich in glands and cellular tissue. A portion of the tensor palati can be seen, and its origin can be traced backwards to the spine of the sphenoid. No portion of the levator palati is shown, as the section passed above its origin. It has been repeatedly proved that the tensor palati is at the same time a dilator of the Eustachian tube, and an elevator of the lateral cartilage, thus opening the canal.

The excellent representation of the Eustachian tube in Riidinger's atlas (' Atlas des menschlichen Gehororganes,' Miinchen, 1867) should be compared with this.

The projecting lip is clearly seen where the middle of the tube stands out at a point about '6 of an inch from the posterior wall of the pharynx, and behind it is the fossa of Rosenmiiller (recessus infundibuliformis of Tourtual).

The mucous membrane of the pharynx is rich in glands, and is continuous with that of the Eustachian tube and nasal cavities. It presents numerous blind crypts and depressions, which can be only hinted at in the plate. The mucous membrane has fallen into the section at the point where it passes over to the roof of the pharynx, above the rectus capitis anticus.

Fig. 1. Section of right ear, enlarged three times. Seen from below.

1. Internal carotid artery. 2. Vestibule. 3. Facial nerve. 4. Corda tympani. 5. Stapedius. 6. Tensor tympani.

From the relative position of the Eustachian tube to the pterygoid process and inferior turbinated bone, it is evident that oedema of the mucous membrane may easily close up its opening. Such a swelling may happen from cold, and nasal polypi are often the cause of difficulty of hearing.

As regards the left ear there is little to say, as the saw passed at a considerably higher level than on the right side. The cavity of the tympanum is laid open nearer its roof, and in front of its connection with the posterior portion of the Eustachian tube, at the middle of which a bristle has been introduced into the canal for the tensor tympani. Further forward is the upper half of the cartilaginous part of the canal. By the laying open of the left meatus auditorius internus, the auditory nerve is well shown. That portion of the nerve which goes to the cochlea is divided, while the vestibular nerve passes with the facial through the superior fovea (and in the plate disappears deep down). The section of the cochlea, the direction of its base to the meatus, and the exposed vestibule are clearly seen, and agree with Riidinger's statements.

There is nothing to add as regards the brain. The pons Varolii in section shows the fibres of the pyramid passing through it. The anterior portion passing from the fourth ventricle to the aqueduct of Sylvius is met with ; behind it is a part of the vermiform process.

As the section has passed through the skull above the jugular foramen, but very little of the internal jugular vein and eighth pair of nerves are seen.

On the anterior border of the pons Varolii are the divided fibres of the sixth nerve. The third division of the fifth is met with on both sides, just below the foramen ovale.

The branches of the second division of the fifth lying in the section are the palatine, which lie below the spheno-palatine foramen, and the dental branch which is on the maxillary tubercle.

The other structures and tissues on the plate will be alluded to individually.

Historic Disclaimer - information about historic embryology pages
Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding. (More? Embryology History \| Historic Embryology Papers)

Reference

Braune W. An atlas of topographical anatomy after plane sections of frozen bodies. (1877) Trans. by Edward Bellamy. Philadelphia: Lindsay and Blakiston.

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Cite this page: Hill, M.A. (2024, April 20) Embryology Book - An Atlas of Topographical Anatomy 4. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_An_Atlas_of_Topographical_Anatomy_4

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