The tympanum is not completely closed. It communicates with the outside world through a "secret passage".
This secret passage is the eustachian tube. Its opening in the tympanum is calledTympanumMouth.Entering from the mouth of the tympanic cavity, the passage here is mainly composed of bones. The passage is inclined downward and gradually becomes thinner. When the internal diameter of the isthmus is only one or two millimeters, this is the narrowest place.Then move on, gradually open up, "building materials" also changed to cartilage based.The end of the passage is in the nasopharynx.
The whole channel is about 35 mm long.The height difference between the upper and lower passages is 15-25mm, so the liquid in the nasopharynx is not easy to "flow upstream" into the middle ear.
The main function of the eustachian tube is to regulate the pressure in the tympanum. In addition, it has the function of drainage to discharge the secretions in the tympanum.The eustachian tube near the tympanic cavity is often open;The eustachian tube near the nasopharynx can be shrunk or expanded. It is usually closed, but it can only be opened when swallowing, yawning, singing or blowing nose.
The eustachian tube is the pipe connecting the tympanum and the nasopharyngeal cavity, and is an important part of the middle ear sound transmission mechanism.The function of this tube is closely related to human adaptation to flight.Armstrong first clarified the significance of eustachian tube in the pathogenesis of aviation otitis media in 1938.
Structure and function: The length of Chinese eustachian tube is 31~35mm.The tube runs from the front wall of the tympanic chamber forward, inward and downward to the nasopharynx.The tube is divided into "bone" and "cartilage", which are slightly conical.The joint is the narrowest part of the whole pipe, called "isthmus".The two parts of the pipe generally form an included angle of about 145 ° at the joint.Anatomical variations at this location, such as the junction is not on the same axis, forming a "fork junction", the inner diameter of the isthmus is too thin or the mucosa flap is redundant, are likely to cause dysfunction of the eustachian tube.
The bone part is the lateral 1/3 segment of the eustachian tube, which starts from the anterior lower wall of the tympanic chamber, and the inner diameter of the tympanic orifice is about 3~4mm.This section is located between the temporal bone scale and the petrous bone, and is a permanent pipeline.The cartilaginous part forms the inner 2/3 segment of the tube. The posterior wall, the top wall and the upper part of the anterior wall are all cartilage walls, while the lower part of the anterior wall is the fibrous soft tissue wall.It is precisely because of the above structural characteristics of the cartilage that this tube has the function of "one-way valve" (or "flapping valve").The part that acts as the valve is the cartilage about 1cm away from the isthmus.
The mucosa of this tube is covered by pseudostratified ciliated columnar epithelium, and the cilia move towards the nasopharynx.There are a lot of collagen fibers in the submucosal basement layer, a lot of serous glands and mucus glands near the pharynx mouth, and a certain amount of fat tissue.The pharyngeal recess behind the pharyngeal orifice and the proliferative body above the pharyngeal orifice of the tube are lymphoid tissue;The tube itself, including the pharynx, has no lymphoid tissue.
This tube plays an important role in ensuring the sound transmission function of the middle ear.Since the tympanic cavity is an air containing cavity, it is necessary to keep the pressure inside and outside the cavity balanced so that the tympanic membrane and round window membrane can be in the best vibration position and normal hearing can be obtained.Active or passive methods can be used to balance the pressure difference between inside and outside the tympanum.The so-called "active method" is to stretch the membrane wall of the tube by muscle contraction. At this time, the tensor veli palatini plays a leading role;The superior constrictor of pharynx and other related muscles also play an auxiliary role.The physiological actions that cause these muscle contractions include swallowing, sneezing or yawning. Individual people can also pull out the membrane wall by moving the mandibular joint or rolling the tongue to contract the soft palate.Swallowing is the most effective one, which is often used in flight."Passive tube opening method" includes nose pinching and air blowing, swallowing and tube blowing.The first method does not need any equipment, so it is also called "home ventilation method", which is commonly used in the flight glide.In addition, the Frenzel method is a combination of active and passive methods, which is used to roll the tongue and contract the soft palate while pinching the nose and blowing air.Swallowing tube opening method is suitable for pilots of single seat military aircraft.
