Paper - Blood supply of the otic capsule of a 150 mm (C.R.) human fetus
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Bast TH. Blood supply of the otic capsule of a 150 mm (C.R.) human fetus. (1931) Anat. Rec. 48: 141-151.
Gray's Anatomy (1918) describes the adult arteries:
Anson BJ. Karabin JE. and Martin J. Stapes, fissula ante fenestram and associated structures in man: II. From Fetus at Term to Adult of Seventy (1938) Arch. Otolaryng. 28: 676-697.
Cauldwell EW. and Anson BJ. Stapes, fissula ante fenestram and associated structures in man III. from embryos 6.7 to 50 mm in length. (1942) Arch. Otolaryng. 36: 891-925.
Anson BJ. and Cauldwell EW. Stapes, fissula ante fenestram and associated structures in man: IV. From fetuses 75 to 150 mm in length. (1943) Arch. Otolaryng. 37: 650-671.
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Blood Supply of the Otic Capsule of a 150 mm (C.R.) Human Fetus
T. H. Bast
Department of Anatomy, University of Wisconsin
The Publication Or This Research Was Financed By “The Doctor Carlos F. Macdonald Research Fund” For The Research Council Of The American Otological Society.
One coloured plate (one figure)
This account of the blood supply of the capsule of the human fetal ear is based on a wax-plate reconstruction of serial sections, each 25 in thick, of the petrous bone and surrounding structures of a 18.5 weeks, or 150 mm (crown—rump length), human fetus. In this reconstruction the following parts were modeled: periotic labyrinth, ossiﬁed portion of the otic capsule, nerves, arteries, veins, tensor tympani muscle, and stapedius muscle. Because of a natural injection, the arteries and veins were rather easily traced. Tracings were made of each section at a magniﬁcation of forty diameters. These tracings were transferred to 1-mm. wax plates. The parts desired were cut out and piled. The resulting model containing the above-mentioned parts is 18 inches X 20 inches X 28 inches in size. Because of the many small arteries, veins, and nerves, the model is too massive and complicated to be represented by drawings or even by stereoptican views. The accompanying partly diagrammatic ﬁgure was therefore drawn to show the periotic labyrinth, the ossiﬁed portion of the capsule, and only those arteries supplying the capsule. That part of the capsule not shown is still cartilage at this stage. It will be seen from the ﬁgure, therefore, that tl1e region of the capsule around the posterior and lateral canals is cartilaginous and is one of the last regions to ossify. The arteries to the most internal part of this portion of the capsule are quite numerous and are shown in the drawing.
As far as the author knows, there is no detailed account of the blood supply to the otic capsule. Wittmaach (’19) made the statement that in older capsules one can determine the centers of ossiiﬁcation by the arrangement of the spicules of bone and cartilaginous remnants. At these centers, he claims, the spicules of bone lie perpendicular to the surface of the capsule and thus represent the direction of entrance of the blood vessels. This statement, while it is true in part, does not tell the whole story, because in the older capsules, even in relatively young features like the one described here, the more prominent arteries to the capsule do not all enter the points where ossiﬁcation began. If arteries still enter at the ossiﬁcation centers, they are so small that in our model, which is an enlargement of the actual fetal ear by forty diameters, they were not seen. A few of the arteries in our a model enter at or near the points Where ossiﬁcation began, but most of them enter the capsule at some distance from these points. There is some evidence that the arteries shift with the growth of the bone around the centers, so that in older capsules the arteries do not enter at the point Where ossiﬁcation began. It seems, therefore, that Wittmaacih’s criteria for determining the position of ossiﬁcation centers in older capsules are not entirely satisfactory.
The direct object of this paper is to show the blood supply of the otic capsule and how this supply is related to the ossiﬁcation centers, in order to determine Whether or not there is any relation between blood supply and otosclerotic areas in adult ears. The order of appearance and thegrowth of the ossiﬁcation centers of the otic capsule were described in my previous publications (Bast, ’28, ’29).
Arteries to the otic capsule
The arteries to the otic capsule are:
- The inferior tympamlc artery branch of the ascendi-ng pharyngeal artery (pl. 1,sa.tymp.i»nf.).
- The posterior memIrtgeal artery, branch of the occipital artery (pl. 1, ai.me'n:.post.)v.
- The vrecarrerzit mastoid artery, branch of the occipital artery (pl. 1, a.rec.mast.).
- The stylomastoid artery, branch of the posterior auricué lar artery (pl. 1, a.stylo.).
- The accessory stylomastoid artery, branch of the auricular branch of the «posterior auricular artery (pl. 1, a.stylo.acc.).
- The subarcuate artery (pl. 1, a.sab.arca.).
Coarse and Termination of Arteries
1. The inferior tympanic artery is a branch of the ascending pharyngeal artery. As the ascending pharyngeal artery nears the apex of the petrous bone it divides into two main terminal branches. The one runs parallel to the internal carotid artery and slightly anterior to it. The termination of this artery is not seen in our model, but it no doubt is the meningeal branch. The other or inferior tympanic artery passes posterior to the internal carotid to a point just medial and inferior to where the inferior petrosal sinus empties into the jugular vein. Here it sends a meningeal branch posteriorly to the dura in the region just medial to Where the vagus nerve penetrates the dnra.
The inferior tympanic artery then swings laterally and upward to the canal of the glossopharyngeal nerve. Here it sends one branch posteriorly to the bony otic capsule produced by ossiﬁcation center no. 2, which it enters at a point just lateral to the ganglia of the glossopharyngeal nerve. Another branch is sent to region of ossiﬁcation center no. 4 and the anterior end of ossiﬁcation process no. 2. The artery then accompanies J acobson’s nerve (tympanic branch of the glossopharyngeal nerve), swings anteriorly Over the promontory, sends several small branches to the ﬁssula ante fenestram (pl. 1, a».x.), and then breaks up into a. number of branches to the tympanum, where they anastomose with other vessels, namely, branches of the stylomastoid, tympanic branch of internal carotid and internal maxillary. One small branch accompanies the small superﬁcial petrosal nerve just above the belly of the tensor tympani muscle, which it supplies. I
2. The posterior meningeal artery is a branch of the occipital artery. It leaves the occipital below and anterior to the posterior semicircular canal, swings posteriorly, and enters the dura in the region of the union of the sigmoid and the lateral sinus. It ascends with. the lateral sinus. Some of its branches supply the dura around the endolymphatic sac. One branch enters the lower part of the expanded ossiﬁcation center no. 10 (pl. 1, 10, or Bast, ’28, ’29). Another branch accompanies the endolymphatic duct for a short distance. In our model the full extent of this branch was lost. Two veins accompany theartery along the endolymphatic duct. They arise from a plexus of veins around the vestibule and drain into a larger vein which receives numerous radicles from the very rich venous plexus around the endolymphatic sac. This larger vein then empties into the lateral sinus just below the endolymphatic sac.
3. The recurrent mastoid is another branch of the occipital. The occipital artery gives off a branch in the mastoid region just posterolateral to the posterior semicircular canal. The course and destination of this artery are not shown. This artery, however, a short distance from its origin gives off a branch, the recurrent mastoid, which bends downward and runs parallel to the occipital for a short distance and then penetrates the otic capsule slightly posterior to the lower bend of the posterior. semicircular canal, or in the region where the mastoid process later develops. (In the. fetus, here described this part of the capsule is» still cartilaginous.) We are not certain whether this artery corresponds to any artery described for the adult. In the fetus, however, it seems to play an important part in the excavating process of the massive cartilaginous capsule in the region of the semicircular canals. The other arteries which supply the cartilaginous capsule in the canal region arethe branches of the subarcuate, the stylomastoid, and the accessory stylomastoid. This excavating process ismost marked at the centers of the arcs described by the canals. -The importance of this process in the formation of the bony capsule and the pneumatization of the growing mastoid is now being studied.
4. The stylomastoid artery leaves the posterior auricular artery below and near the inner margin of the external auditory meatus. It swings downward and posteriorly, ascends in a lateral direction, accompanying Arnold’s nerve for a short distance, and enters the stylomastoid foramen and accompanies the facial nerve and the belly of the stapedius muscle in the lower part of the facial canal. The artery sends branches to the stapedius muscle and, a little higher up, branches to the bony capsule produced by ossiﬁcation centerno. 3. In the upper part of the tympa.nic cavity it anastomoses with the tympanic branch of the ascending pharyngeal, the tympanic branch of the vidian artery, and the tympanic branch which accompanies the lesser superﬁcial petrosal nerve in the-upper part of the canal of the tensor tympani muscle. From this plexus branches enter that part of the bony otic capsule forming the upper part of the facial canal which is the product of ossiﬁcation centers 3, 5, and 7. Other branches supply the capsule above the cochlea. This part of the capsule was developed from ossiﬁcation center no-.
5. The accessory stylomastoid artery is formed by the fusion of two small branches from the auricular branch of the posterior auricular artery. Both of these small arteries leave the auricular near the place where the chorda tympani leaves the facial nerve. . (It must be remembered that relationships in the fetus are diﬁerent from the adult. The chorda tympani, for example, in this fetus leaves the facial nerve after it has emerged from the stylomastoid foramen.) These two arteries follow the facial nerve to its bend, then fuse, a.nd enter the stylomastoid foramen as one artery. In the lower part of the facial canal it sends branches to the cartilaginous otic capsule around the lateral semicircular canal, and the rest of the artery is lost in the facial canal.
6. The subarcuate artery in our model runs parallel to the internal auditory artery and auditory nerve up to the internal auditory meatus, then swings laterally to enter the subarcuate fossa. From our model, because of itsslimited extent, it could not be determined whether this artery is a branch of the internal auditory or the basilar artery. It is a relatively large Vessel slightly smaller than the internalauditory. Just before entering the subarcuate fossa it gives off a branch which by means of several twigs enters the bony capsule formed by ossiﬁcation center no. 10 around the posterior part of the superior‘ semicircular canal. These vessels do not enter at the point where ossiﬁcation began, but «more medially. If they‘ did originally enter the center of ossiﬁcation, they haye shifted with the expanding ossiﬁcation.
The rest of the artery enters the subarcuate fossa, penetrates into the massive cartilaginous capsule which surrounds the canals and ﬁlls the space between them. As already stated in myprevious accounts on the ossiﬁcation of the otic capsule, this part of the otic capsule is the last to ossify. The cartilage here differs from the cartilage which constituted the rest of the otic capsule ‘in younger fetuses, in that it has a very rich blood supply. We have already enumerated the arteries which supply this portion of the capsule in the paragraph on the recurrent mastoid artery. The Veins which drain this portion of the capsule are Very numerous and empty into the sigmoid and lateral sinuses.
Drainage of the endolymphatic sac
The venous drainage of the capsule will not be described in detail here, because in general the Veins accompany the arteries, but are much more numerous. The venous drainage of the endolym,phatic sac, however, is of interest. The sac which lies in the dura, posterior to the lateral sinus and common crus of the superior and posterior semicircular canals, is completely surrounded, especially in its lower part, by a dense network of venous sinuses which drain directly into the lateral sinus. The arterial supply to the sac, as already stated, is from the posterior meningeal artery.
From the model of a 150 mm (C.R.) human fetus the following blood supply to the oticcapsule has been determined.
1. Arteries to the ossiﬁed part of the otic capsule
- a. The inferior tympanic to regions developed from ossiﬁcation centers 2 and 4. Center no. 1, no doubt, is supplied by this artery, although no deﬁnite branches are shown in our model, but the artery courses over it.
- b.The posterior meningeal to the lower part of region developed from ossiﬁcation center no. 10.
- c. The stylomastoid to regions developed from ossiﬁcation centers 3 and 5.
- d. The arterial tympanic plexus sends branches to those portions of the capsule developed from ossiﬁcation centers 3, 5, 7, 8, and very likely to the small accessory centers 11, 12, and 13.
- e. The subarcuate artery supplies the capsule developed from center no. 10.
2. Arteries to the cartilaginous part of the cepsate
- a. The subarcuate artery to the region between the superior and lateral semicircular canals.
- b. The stylomastoid artery and the accessory stylomastoid to the region anterior to the lateral posterior canal.
- c. The recurrent mastoid artery to the region below and posterior to the posterior canal.
3. The endolymphatic sac is supplied by the posterior meningeal artery and has a very rich venous drainage into the lateral sinus.
4. The ﬁssula ante fenestram is supplied from the middle ear side by small twigs of the inferior tympanic artery.
Bast T. H. 1928 The ossification of the labyrinthine capsule. 1, Trans. Amer. Otolog. Soc. 2, The Laryngoseope, October.
- Bast TH. Ossification of the otic capsule in human fetuses. (1930) Contrib. Embryol., Carnegie Inst. Wash. 121, Publ. 407, 53-82.
WITTMAACH, KARL 1919 Die Otoskle-rose auf. Grund eigener Forschunge-11. Fischer, Jena.
This is a drawing of part of a model of the internal ear of a 150 mm (C.R.) human fetus, age about eighteen and one-half weeks.
It is drawn to show the arterial blood supply to the otic capsule. The periotic labyrinth is shown in yellow, the ossiﬁed portion of the capsule in black, and the arteries in red. The numbers 1, 2, 3, 4, 8, and 10 represent the approximate point of origin of the corresponding ossiﬁcation centers (Bast, ’30). The part of the capsule which is still cartilage is not drawn.
oaa.fac., canalis facialis
can.s/ap., cana.lis superioris
ccm.lat., canalis lateralis
caa.post., canalis posterioris
sa.c.¢mdol., saccus endolymphatieus
aqaaed.coch., aquaeduetus cochlea:-is
fe/n.rot., fenestra rotunda seu cochleae
fe-n,.o*val., fenestra ovalis seu vestibuli
an, communication of ﬁssula ante fenestram with middle ear
a-.a:., arteries entering the ﬁssula ante fenestram
a.aa-r.post., arteria auricularis posterior
a.ca.r.ea:t., arteria earotis externa a.car.mt., arteria earotis interna a.men.post., arteria meningea posterior a.occip., arteria Oeeipitalis a.phary.a3ce'n.d., arteria pharyngea ascendens a.'reo.mast., art-eria mastoidea recurrens a.styl0., arteria stylomastoidea a.styZ0.acc., arteria stylomastoide-a accessoria
ct.sub.m*c'u., arteria subarcuata a.tymp.in.f., arteria tympanica inferior
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