Paper - Some uniform characteristics of the primate auricle (1922)
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Streeter GL. Some uniform characteristics of the primate auricle. (1922) Anat. Rec. 22(6): 335-350.
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Some Uniform Characteristics of the Primate Auricle
Carnegie Embryological Laboratory, Baltimore, Maryland
In the attachment of the auricle to the side of the head there are certain phj^sical requirements that must be met. The auricle must, of course, be held securely in place and at the same time it must possess a varying degree of motility and power of direction; its shape must be maintained for the efficient collection of sound vibrations, and provision must be made by some closure de\4ce to protect the external acoustic meatus from the entrance of foreign substances, particularly in burrowing and aquatic animals. In studying the striking differences in structure exhibited by the auricle in the various animal forms, one finds that most of their auricular individualities are in the nature of remarkable morphological adaptations to these varying phj^sical requirements. Perhaps the most constant requirement is that the auricle be securely attached. In consequence one would expect to find that portion of the auricle concerned in this function to be less variable in form than its other parts. This appears to be true, at any rate for the Primates, and it is the purpose of this communication to call attention to this relatively constant articular provision and its expression in the form of the auricle.
The existing terminology of the external ear is a purely descriptive one and is based upon the form usually met with in the human adult. During the period of its inauguration, scant attention was given to the embryonic stages and as little to the ear of other animals. It is therefore not surprising that one finds it more or less inadequate for any functional analysis of the auricle or for the study of any ear other than that of adult man. When an appropriate time comes the nomenclature of the external ear, as much as any other part of the body, will need a thorough reconsideration. For the present, I am departing verj' little from the prevalent terminology, and then only where it seems unavoidable. As can be seen in figure 2, the following substitutions have been made: fossa articularis superior (for fossa triangularis), fossa articidaris inferior (for cymba concha), plica principalis (for crus inferius anthelicis),^ and crus helicis to include all that part of the helix derived from the mandibular arch. Furthermore, on the median side of the cartilage, corresponding to the articular fossae, there are the superior and inferior articular eminences, partially separated by the groove of the principal fold. It is to these eminences that I would first of all direct the reader's attention.
The two articular eminences (fig. 1) are continuous with each other anteriorly and together constitute a relatively rigid, bowlshaped base from which the auricle is suspended. It is only this part of the auricular cartilage that offers a contact surface suitable for its attachment to the skull, and it may therefore be designated as the pars articularis. Nearly the whole of the inferior eminence contributes to this surface, whereas, of the superior eminence, only the forward and lower portions take part. It is true that the band-like, fenestrated cartilage surrounding the external acoustic meatus also has a bonj^ attachment, but -this is quite different in character and is to be compared rather to the tracheal rings, serving as a mechanism to prevent a collapse of the meatus. In structure and position it offers little if any support to the auricle.
The articular eminences are attached to the periosteum by a fibrous ligament sufficiently loose to permit free movement of the auricle. The extrinsic ear muscles are attached around the margins of the eminences. The superior auricular muscle is inserted into the superior eminence, the posterior auricular muscle into the conchal wall immediately adjoining the inferior eminence, while the anterior auricular muscle is inserted at the base of the spina. It is to be noted that the spina is in reality a process of the pars articularis, from that portion where the inferior articular eminence merges into the crus helicis.
- I am adopting the term plica principalis as used by Boas (Ohrknopel und iiusseres Ohr der Siiugetiere. Kopenhagen, 1912).
As for the crus helicis, this is not, strictly speaking, a part of the helix, from which it differs both embryologically and structurally, as I have pointed out elsewhere.- It merges with the helix as a continuous fold, but one can always recognize the point of junction of the two at about the level indicated in figure 2. The crus helicis constitutes, first of all, the lateral rim of the bowl-shaped pars articularis and only secondarily acts as a support to the anterior end of the helix.
Figs. 1 and 2 Human adult auricle. In figure 1 the auricular cartilage is viewed from the median side, showing the two eminences that constitute its main area of contact with the skull. In figure 2 can be seen the cavities (fossae articulares) of these eminences with the plica principalis projecting between them as a strengthening ridge. The articular fossae are continuous with and constitute a specialized part of the concha, the pars articularis. The crus helicis and the cartilaginous spina are also parts of this attachment mechanism.
So much can be readily seen in the adult ear. In the human embryo and fetus the entity of a pars articularis is even more pronounced. It is the first part of the auricular cartilage to acquire its distinctive form and is more or less bowl-shaped from the outset, forming a cap over the upper end of the first gill cleft. It has a marked oral process (spina) extending forward into the mandibular arch and its lateral rim (crus helicis) is well defined. The floor, projecting against the skull, is early subdivided into two fossae by the plica principalis, which can be recognized in the 43-mm. fetus and is well-pronounced in 50-mm. specimens. That portion of the floor corresponding to the inferior articular eminence is relatively larger, as compared with that of the superior, than obtains in the adult. The pars articularis is directly continuous with the concha and unquestionably should be considered a part of it. The latter spreads down towards the region of the antitragus and the meatus. In embryos less than 30 mm. long this portion of the concha is still fenestrated, which condition is more marked and remains permanent in the region of the meatus. The presence of the scapha, with its characteristic rolled edge (helix) is indicated early, but is relatively small and its growth slow until the later fetal stages.
- Streeter: Embryological significance of the crus helicis. Anat. Rec, vol. 18, 1920.
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Turning from these considerations to a survey of the auricle of other Primates, we find a striking constancy in the form of that portion concerned in its attachment — the pars articularis. A representative series of Primates is shown in figures 3 to 14. For convenience in arrangement, these figures are drawn at about the same size and so are at different enlargements. This treatment tends to minimize the marked differences in size actually prevailing in the scapha and helix in these different forms, and at the same time tends to give the impression of a greater variation in size in the other parts than really exists. In contrast to the variable scapha and helix, the parts composing the pars articularis (crus helicis, superior and inferior fossae, and the principal fold) exhibit about the same form and relations in each figure.
Figs. 3 to 14 In these sketches the helix and scapha are dotted and the crus helicis is indicated by closer dots. Abbreviations: ANTH., anthelix; AT., antitragus; F.A./., fossa articularis inferior (cymba concha); F.A.S., fossa articularis superior (fossa triangularis); H., helix and scapha; P.P., plica principalis (crus inferius anthelicis) ; T., tragus. Most of these sketches were made from preserved specimens kindly lent to me by Dr. Adolph H. Schultz, and the species as identified by him are herewith given. (In the four specimens obtained elsewhere the source is mentioned.) Figure 3, Troglodytes niger, copied from figure V, page 214, of Boas (12) ; figure 4, Gorilla gorilla, from photograph issued by the New York Zoological Society; figure 5, Pongo pygmaeus; figure 6, Hylobates concolor; figure 7, Nasalis larvatus; figure S, Pithecus philippinensis; figure 9, Papio porcarius; figure 10, Ateleus variegatus; figure 11, Alouatta seniculus; figure 12, Hapale rufimanus, copied from figure 243, Tafcl 23, of Boas ('12) ; figure 13, Lemur variegatus; figure 14, Tarsius spectrum, from specimen belonging to Dr. H. Woollard.
The principal fold is a little more variable m size than the other parts of the attachment mechanism, although its relations are essentially the same in each case. Where it is particularly well marked it comes to the level of and fuses with the rim of the concha (anthelix), as usually occurs in man. It was this relation which led to the term 'crus inferius anthelicis.' Where it is not so well developed it does not reach the conchal brim and is thus not continuous with the anthelix. Certainly, in most Primates one cannot properly speak of it as a crus of the anthelix. Since the figures show the chief points that I wish to bring out, it will not be necessary to describe them here individually. It may, however, be well to call attention to the Tarsius specimen' as the most discordant one in the series. One of its peculiarities is the extraordinary development of what appears to be the principal fold. Instead of constituting a simple strengthening ridge, the fold projects from the conchal floor as a free flap, somewhat of the nature of a similar structure seen in certain bats.
In view of the above observations, we can briefly analyze the form of the primate auricle somewhat as follows: The auricle consists of a primary part (concha), a rigid, shell-like support which is relatively constant in form, and a secondary part (scapha-helix-lobule) which flares from the caudal rim (anthelix) of the concha and is exceedingly variable both in size and form.
The concha in turn may be subdivided into a lower half (cavum conchae), which serves as an approach to the meatus and at the same time provides a closure mechanism by the specialization of its antero-inferior walls, and an upper half (pars articularis) , which constitutes an attachment base from which the auricle as a whole is suspended.
' For the privilege of studying this valuable specimen I am indebted to Dr. H. Woollard, of University College, London.
The pars articularis comprises several elements, all of which contribute to its effectiveness, a) The superior and inferior articular eminences offer jointly an attachment surface which comes in direct contact with the skull. These eminences are separated by a groove or strengthening fold which in a lateral view projects as the plica principalis, separating the two fossae that correspond to the eminences, b) The spinous process (spina) offers a point for Ugament and muscle insertion, c) The crus helicis forms the anterolateral rim of the pars articularis, its upper end supporting and merging into the helix. Below, it marks the transition of the pars articularis into the lower part of the concha. Including, as we thus do, the pars articularis in the concha, the so-called superior crus of the anthelix becomes simply the upper end of the anthelbc, or, in other words, the superior rim of the concha.
Cite this page: Hill, M.A. (2020, October 25) Embryology Paper - Some uniform characteristics of the primate auricle (1922). Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Some_uniform_characteristics_of_the_primate_auricle_(1922)
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