Paper - The ossification of the human frontal bone with special reference to its presumed pre- and post-frontal elements
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Inman VT. and Saunders JBdeCM. The ossification of the human frontal bone with special reference to its presumed pre- and post-frontal elements. (1937) J. Anat., 71(3): 384-394.
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The Ossification of the Human Frontal Bone with Special Reference to its Presumed Pre- and Post-Frontal Elements
By Verne T. Inman, M.D., Ph.D. And John B. De C. M. Saunders, M.B., F.R.C.S. (Edin.)
Division of Anatomy, University of California Medical School
In a recent number of the Journal of Anatomy there has appeared an article by Ashley-Montagu on the presumed existence of a post-frontal element in the skull of a young gorilla. Ashley-Montagu’s conclusions are based largely on the view that the post-frontal bone "is present in the developing skull of probably all primates and certainly of all men. “ It is not Widely known among anatomists”, he says, “that the post-frontal bone is a constant element of the developing human skull. For some extraordinary reason most modern textbooks of anatomy and embryology make no mention of the post-frontal element of the human foetal skull in spite of the fact that a constant and independent centre of ossification of this bone is present in every case, and the bone itself may generally be observed up to the end of the third foetal month, and not rarely in the skulls of newborn children.” .
Having available a considerable collection of material specifically prepared to demonstrate the ossification of the skull in man, we were prompted to seek for this element. We were surprised, in the light of the above definite statement, that we were unable to find the slightest trace of this element occurring as an independent centre, nor were we at any time able to find such other secondary centres of ossification as have been commonly described for the frontal bone. We noted, furthermore, that there was some disagreement among the authors of various standard textbooks, and it became increasingly apparent that in view of the great morphological and phylogenetic conclusions which have been based upon the supposed existence of such secondary centres, reinvestigation of the ossification of the frontal bone was necessary.
Gabriel Fallopio appears to have been one of the first to state clearly that the frontal bone constantly develops from two symmetrical halves. In his Observatimws Anatomicae (1561) he says: “Ita ut in omnibus puerulis qui tenerimi sunt frontis os ex duabus constare partibus reperierim.” Vesalius was also well aware of this mode of origin of the frontal bone, for not only does he discuss Fallopio’s observations in his Observationes Gabrielis Falloppii Ezvamen (1564), but the matter is mentioned in the Fabrica (1543). Jhering states that Galen was ignorant of this fact, but it would seem on examining his De usu partium and De admim'stratiom'bus anatomicis that he may have been aware of this in some instances.
Up to the time of Kerckring (1670) but few serious attempts had been made to examine the origin of the bones before birth. This author dissected and described specimens from approximately the second month of foetal life.
Albinus illustrates in his Icones ossium foetus humani (1737) that area which has since been called the post-frontal element and records an instance in which each half of the frontal bone was divided into three parts. This single observation was made on an aborted foetus. He says: “In abortu juniore, cui totum os magnitudinis unguis mediocris, utramque illam partem inveni divisam in tres; superiorem, quae ad frontem pertinet; inferiorem, quae ad foramen oculi, tertiam quae ad caput supercilii.”
Apart from the single observation quoted above from Albinus, the recognition of secondary or accessory centres of ossification dates from the work of Serres (1829).1 This author described accessory centres, (1) for the part of the pars orbitalis at the trochlear fossa and (2) for the zygomatic process. Serres considered them, in association with his studies on the comparative anatomy of the skull, as representing the pre- and post-frontal elements. Rambaud & Renault (1864) accept the findings of Serres, but do not appear to have seen these centres as independent ossicles. They state that at the fourth month the centres have already been joined by a suture which leaves only a shallow groove and that the centres have always so appeared at this time. These authors added a third centre for the nasal spine which, they state, appears late, sometime after the eighth year.
- 1 Many authors give as references to the work of Serres on accessory centres the C'.R. Acad. S’ci., Paris, 1819 and 1838. These abstracts contain no mention of specific centres. Rambaud & Renault also give as reference Serres, Des Lois de l’oatéoge’m'e, Paris, 1829. This work is unavailable to us. It would seem that the majority of authors have quoted Serres through Rambaud & Renault.
A fourth centre, for the posterior part of the pars orbitalis, is given by Von Spec (1896) in von Bardeleben’s H andbmth. The existence of such a centre appears to be based on an observation originally made by Hyrtl (1860) that a Wormian bone is sometimes found in this region. Von Spee also observed such a Wormian bone and accepts Hyrtl’s conclusion that this has arisen from an independent centre of ossification. It has frequently been pointed out since (Patten, 1912) that the presence of a Wormian bone cannot be accepted as evidence of a true secondary centre.
Since the publication of Rambaud & Renault’s monograph, a number of special papers have appeared on the ossification of the frontal bone and on its pre- and post-frontal elements. Lecourtois (1869) published a study based upon foetal skulls of 5-6 months, in which he expresses agreement with the findings of Serres and of Rambaud & Renault.
In 1872, von Jhering published an important paper which has since been widely quoted. J hering, after reviewing the work of earlier authors, confirms the conclusions arrived at by Rambaud & Renault. He examined, however, no specimen earlier than the 4th month and at no time saw the assumed accessory centres as independent ossicles. Maggi (1897, 1898) published two extensive articles on the morphology of the cranial and cranio-facial bones in which he concludes that the existence of accessory centres for the frontal bone has been satisfactorily demonstrated, and that two of these centres are homologous with the pre- and post-frontal elements of other forms. His observations on man were based on the examination of foetal skulls from the 5th month and on the work of Rambaud & Renault. finally Von Bardeleben (1896), Augier (1912) and Ashley-Montagu (1931) have Written articles which, on analysis, present no original observations on the development of the frontal bone, but whose conclusions are either directly or indirectly based on the acceptance of the work of Serres and of Rambaud & Renault.
It is not surprising in consequence, that the standard textbooks of anatomy and of embryology almost uniformly describe and illustrate, apart from the primary centres, three pairs of accessory centres: (1) for the frontal spine, (2) variously for the medial angular process or trochlear pit (pre-frontal element), and (3) for the region of the zygomatic process (post-frontal element). These accessory centres, apart from that for the frontal spine, are, in the general concensus of opinion, assumed to appear about the 4th month and to fuse at the 7th. The centre for the nasal spine is recognized as appearing late, at or before the 10th year. In addition Quain and Von Spee (von Bardeleben’s H andbuch) mention the occasional occurrence of a fourth pair for the posteromedial part of the orbital plate. The statements in Quain, Cunningham, Gray, Morris, Piersol, Gegenbaur, Merkel, von Bardeleben, Testut, and Poirier, are, as in the special articles noted above, a continuation of the opinion of Serres and of Rambaud & Renault.
The only dissenting opinions which we have been able to find in the literature are those of Toldt and Le Double. Toldt, while recognizing the areas described by Serres and by Rambaud & Renault, is unable to accept these as indicating the existence of accessory centres. In Maschka’s Hcmdbuch d. gerichtlichen Medizin he says: “Ich habe etwas Derartiges beim Menschen nie gesehen und muss daher die Angaben von Serres, Rambaud u. Renault, welche durch J hering auch in die deutsche Literatur iibergegangen sind, wonach beim Menschen ein urspriinglich selbstandiges Os frontale posterius bestehe, in Abrede stellen. Ebenso wenig kann ich fiir den Menschen die Existenz eines zu irgend einer Zeit isolierten Os frontale anterius einraumen.”
Le Double discusses extensively the ontogeny and phylogeny of the preand post-frontal elements and points out the lack of agreement among anatomists. The existence of accessory centres in man has arisen, in his opinion, from misinterpretation of the findings.
This survey would indicate that the present belief that accessory centres for the frontal bone exist in man depends in large measure on the observations and interpretation of Serres and of Rambaud & Renault. In spite of the denial of their existence by Toldt, whose opinion is unfortunately buried in a textbook of forensic medicine, and the questioning of Le Double, all modern textbooks of anatomy continue to illustrate and describe such centres. Furthermore their existence forms the chief basis for their acceptance as pre— and post—frontal elements.
Material and Methods
Our observations are based on a total of ninety-eight specimens ranging from the 6th week of intra-uterine life to the 10th month, post-natal. All the material was Caucasian. The majority of the specimens were prepared to demonstrate ossification by a technique developed in this department. This method is much more satisfactory than that of Schultze in that it enables one to follow the progress of ossification to term. The method, published elsewhere by R. Skarda (1934), brieﬂy consists in the rapid dissection of fresh foetal material which has previously been thoroughly freed from blood by washing via the umbilical vessels. The specimens are then hardened in 95 per cent alcohol for 48 hours and cleared in 1 per cent solution of NaOH and finally permanently mounted in 2 per cent formalin solution. The clearing process is carried out in darkness which improves the development of the final preparation. This technique results in preparations of great beauty. The bone is pure white, and its finest ramifications can be traced with case when viewed in good light under the dissecting microscope. The cartilage remains opalescent blue in colour. Shrinkage is approximately 2 per cent and distortion negligible. The specimens are mounted intact. This is a great advantage in that relations are maintained and there is little danger of creating artifacts. Furthermore the fact that the specimens are wet avoids cracking of the delicate bone which has given rise to so many of the erroneous observations of earlier workers on ossification.
The above technique demands absolutely fresh material. Foetuses of 6 months or over, if not fresh, tend to be opaque, and the ramifications of the delicate trabeculae cannot be made out with the same clarity. This can be remedied by carrying the preparation a stage further. The less satisfactory material is carefully dehydrated by carrying it successively from 50 per cent to absolute alcohol in steps of 5 degrees and then transferring to pure benzol. Specimens prepared in this second fashion show distinctly more shrinkage, and although the most delicate bone centres can be seen and readily followed, they do not have the same elegance as the others, nor do they have the same permanency.
In addition, dissections were made of formalinized specimens and a few dried foetal skulls examined.
Ossification of the Frontal Bone
The primary centres
It is universally agreed that the frontal bone ossifies from two primary centres which appear in membrane between the fortieth and fiftieth day. There is some disagreement however with regard to the precise position in which these centres arise. The majority of investigators and most standard textbooks describe ossification as beginning in the position of the future frontal eminences; the minority place the centres at the superciliary region.
We have observed the frontal bone appearing as two finely reticulated areas of ossification in specimens of 32 and 36 mm. sitting height (s.H.). In these and in all other specimens of approximately the same age ossification involves an area whose long axis corresponds approximately to the position of the superciliary arch, and which forms the lower part of the frontal squame and the more anterior portion of the orbital plate and a. primitive medial angular process. In our younger specimens this ossific centre is about 2 by 3 mm. The ossific meshwork is of regular and even density throughout this area. except for a slight thickening and condensation which forms a line in the position of the primitive supraorbital margin. In slightly older specimens a few fine, less dense trabeculae project radially from the periphery of the centre. These trabeculae are best defined along the upper margin of the developing pars frontalis.
After the appearance of the primary centres, ossification spreads more rapidly in the pars frontalis than in the pars orbitalis. Ossification proceeds in the pars frontalis by the formation of heavy islands of bone which constitute a crescentic area whose maximum depth corresponds to the future posterosuperior angle (see Pl. I, fig. 1). These islands fuse to form a series of primary radiating trabeculae (Pl. I, fig. 2). The appearance of secondary trabeculae serves to link the primary trabeculae, expanding the even meshwork of the primary centre. This expansion of the meshwork of the primary centre behind the advancing zone of ossification gives rise to the appearance of a centre in the position of the frontal eminence from which the primary trabeculae radiate, an effect which is often seen in specimens of 4 months and older. In fact, however, this false centre of radiation rises higher and higher up on the frontal squame with increasing age.
In the pars orbitalis the progress of ossification is less rapid. The radiating trabeculae so characteristic of the pars frontalis are very much smaller and less obvious and an expanding meshwork of trabeculae is maintained.
These observations show that ossification begins in the region of the superciliary arch and simultaneously gives rise to a portion of both the pars frontalis and the pars orbitalis and to the primitive medial angular process. The common statement that ossification commences at the site of the future frontal eminence is erroneous, and is undoubtedly the outcome of observations made on older specimens.
The secondary centres
In the ninety-eight specimens examined we have at no time been able to find any true accessory centres of ossification. No centres appear in those regions in which they have been stated to occur. In all instances the bony trabeculae were found to be continuous and unbroken. Furthermore, the regions in which such accessory centres are presumed to arise do not correspond to those in which secondary island formation is found. The specimens on which we base our observations are of such clarity that there is little possibility that such centres, had they existed, could have been overlooked. None the less, if dried preparations of foetal skulls between the third and seventh months, and in some at term, are examined, there are appearances which suggest the possible existence of secondary centres. It is these appearances which have, no doubt, been accepted by many authors as indicating the existence of accessory centres. We shall therefore consider the changes which occur in those areas which have given rise to error in interpretation.
(1) The presumed centre for the zygomatic process or post-frontal element.
The primary centre of ossification gives rise to a part of the frontal and orbital plates and a primitive medial angular process. The bony superior orbital margin seen at this period does not correspond to that of the adult. The lateral two-thirds from the position of the future supra-orbital notch to the zygomatic process is due to an entirely new formation. At 42 mm. s.H. a linear thickening of osseous material appears in the lateral two-thirds of the orbital margin at the junction between the pars orbitalis and pars frontalis. This thickening becomes more and more apparent as it is followed laterally, and has resulted in the formation at birth of almost the entire supero-lateral wall of the orbit as well as giving rise to the zygomatic process, and thereby completes the separation of the orbital cavity from the temporal fossa. At 49 mm. S.H. it has spread to form a recognizable zygomatic process which is more apparent at 59 mm. s.H. (see Pl. I, fig. 2). A considerable interval exists between this process and the frontal process of the zygomatic bone; an interval which persists until the foetus has attained a length of 250 mm. The closure of this interval. is due to a further extension of ossification in the frontal process of the zygomatic bone rather than to any further ossification in the zygomatic process of the frontal, for at 155 mm. s.H. the proportions of the zygomatic process of the frontal have almost attained those seen at birth.
From 49 mm. s.H. onwards the base of the zygomatic process of the frontal, including the lateral two-thirds of the orbital margin, progressively thickens. This thickening extends dorsally on to the lateral surface of the pars frontalis but does not involve its postero-lateral angle. This angle is, as a result of this thickening, bounded antero-superiorly by a ridge. The ridge and the area bounded by it becomes, with further growth of the skull, more and more apparent until, in any specimen from the third month on, a pseudo-suture appears to separate off this angle, giving rise to the appearance of an autonomous centre. In a macerated skull at 7 months, the ridge extends upwards and dorsally from the posterior border of the zygomatic process (lateral angular process) to a point some 5 mm. above the pterion (Text-fig. 2, l.t.). This ridge delimits a quadrilateral area of bone, approximately 5mm. in height and 15 mm. in length, which includes the postero-lateral angle (Text-fig. 2, p. f.). Dissections and histological sections demonstrate that this ridge gives attachment ‘to the temporal muscle and is none other than the linea temporalis. This is confirmed by the finding, in a few specimens, of a continuation of the ridge on to the antero-inferior angle of the parietal bone.
Text-fig. 1. Postero-inferior view of the left disarticulated frontal bone of 7 months’ old human foetus. P.F. pars frontalis; P.0. pars orbitalis; l.t. linea temporalis; p. “presumed” post-frontal element; f. superior limb of fissure separating supero-lateral process from the pars orbitalis.
Text-fig. 2. Lateral View of the facial region of a full-term human foetal skull. p. “presumed” post-frontal element; l.t. linea temporalis; pr. “presumed” pre-frontal element; 7'. ridge separating the medial angular process from the pars frontalis.
In the disarticulated frontal bone, the new process which springs from the margin between the pars frontalis and orbitalis, and which completes the supero-lateral wall of the orbit and gives rise to the zygomatic process, is separated postero-laterally, from the rest of the pars orbitalis, by a deep fissure (Text-fig. 1, f.). This fissure gives attachment to the membrane filling the lateral fontanelle. The inferior limb of the fissure extends on to the roof of the orbital cavity, in the ﬂoor of the lachrymal fossa. The inferior limb of the fissure is obliterated, medio-laterally, at birth, but usually persists as a linear series of small foramina and is often evident in the adult. The presence of this fissure in the later months of intra-uterine life further falsely accentuates the appearance of an independent centre of ossification in this area.
The area outlined by the linea temporalis and the fissure described above constitutes what has been designated as the post-frontal element of the human 390 Verne T. Inman and frontal bone, and has been regarded as arising from an autonomous secondary centre of ossification.
(2) The presumed centre for the medial angular process (trochlear pit) or prefrontal element.
A primitive medial angular process is ossified simultaneously with the appearance of the primary ossific centre. This process appears to be a continuation of the superciliary margin medially and forms a small portion of the anterior part of the medial wall of the orbit. At 49 mm. S.H. only the more anterior portion of the pars orbitalis is osseous. At this time ossification is relatively more extensive in the medial angular process.
Ossification spreads much more slowly in the orbital roof and in consequence an unossified interval persists, until the 4th month, between that part of the medial angular process which lies in the medial wall of the orbit and the roof.
At 59 mm. S.H. the medial angular process commences to show some thickening of its reticular network, Whereas at this time scarcely a third of the orbital plate is ossified.
At 80 mm. S.H. ossification has spread to complete the major part of the orbital plate and it commences to fuse with the medial angular process, obliterating the interval between these structures. This line of fusion is indicated at birth either by a groove, or by a series of foramina resembling a suture, which rums obliquely across the trochlear pit to the anterior end of the fronto-ethmoidal suture (Text-fig. 2).
At 59 mm. S.H. a slight thickening extends medially from the position of the future supra-orbital notch across the base of the medial angular process to the metopic suture (Pl. I, fig. 2). This thickening indicates for the first time the boundary of the permanent orbital rim. At 99 mm. S.I-I. the trabeculae have thickened to such an extent as to produce a ridge which separates the medial angular process from the pars frontalis, a ridge which is very apparent at term (Text-fig. 2, r.). As a result, the superior orbital margin of the foetus is Well defined medially. A new margin is therefore formed anterior to the primitive margin which, if traced downwards, becomes continuous with the crista lacrimalis anterior. Between these two margins lies the fossa sacci lacrimalis and the lacrimal sac.
In the later months of foetal life a quadrilateral area of bone is defined in the trochlear region of the macerated frontal bone. This area is bounded anteriorly by the ridge of bone which crosses the base of the medial angular process, superiorly by a groove or series of foramina, inferiorly by the frontomaxillary, fronto-lacrimal and anterior part of the fronto-ethmoidal sutures (Text-fig. 2, pr.f.). Its surface is rough and irregular. It is this area which has been almost universally regarded as the accessory centre for the trochlear pit or medial angular process and which has been designated as the pre-frontal element of man.
The centre for the nasal
At no time prior to birth have we ever observed the presence of a secondary centre for the nasal spine. This process remains unossified for some time after birth. Whether the spine ossifies from secondary centres at or about the 12th year, as is usually stated, or whether it ossifies as an extension from the frontal bone, we are not in a position to say.
Radiograms, both ﬂat plates and stereograms, of children up to the age of puberty, have yielded little information. We have seen the nasal spine in both male and female subject at 10 years, when it appears, so far as We can tell, to be in continuity with the rest of the frontal bone. Earlier than this We have been unable to make out the process and have not seen it as a separate centre.
Centre for the posterior part of the orbital plate
As already mentioned, the orbital plate is ossified in the anterior third of its extent simultaneously with the appearance of the frontal centre. The further spread of ossification to complete the formation of this plate is delayed for some time, excepting that part which abuts against thefuture cribiform plate. At 78 mm. S.H. ossification of the pars orbitalis is almost complete, but for a short time an inconspicuous incisura exists between the main lateral portion of this plate and the earlier formed medial part. These soon fuse. As at the periphery of the superior angle of the pars frontalis, a few islands of osseous material may precede the formation of the reticulum. None of these can be regarded as a true secondary centre, but their independent development may give rise to a Wormian bone such as that described by Von Spee. The manner in which ossification proceeds in the pars orbitalis, with the production for a time of an incisura, might give rise, unless carefully followed in many specimens, to the appearance of what we must call a pseudo-centre.
It is perfectly apparent that the conception of the development and ossification of the frontal bone found in almost all modern textbooks is fundamentally a repetition of the views originally expressed by Serres, and by Rambaud & Renault, which have been widely disseminated by Jhering’s publication. The illustrations which continue to promulgate these misconceptions appear to be, for the most part, based upon those which accompany the treatise of Rambaud & Renault. The observations of Toldt and the opinion of Le Double seem to have had no inﬂuence on, and to have affected in no way, the classical description.
It is universally recognized that the frontal bone ossifies from two primary centres. Mall gives the 56th day as the time of their appearance, a little later than that given by Toldt. Our findings are essentially in agreement with those of Mall. The statement that the primary centre of ossification appears at the position of the future frontal eminence is, although exceedingly common, incorrect. Almost all original observers recognize that ossification commences in the superciliary region, forming simultaneously part of the frontal squame and orbital plate.
We have been unable to find any evidence, at any time in the prenatal period, of true accessory centres of ossification for the frontal bone.
It has been stated that a secondary centre appears in the region of the zygomatic process or in the neighbourhood of the pterion. This centre is regarded by many observers as representing the post-frontal bone of premammalian skulls. We have shown that there is no such centre. What has been accepted as such is an area ossified by extension from the primary centre and modified by the development of the lateral two-thirds of the orbital margin and bounded externally by the temporal line, and internally by a fissure or series of foramina. This fissure extends on to the roof of the orbit and lies in the ﬂoor of the lacrimal fossa. It may persist as a series of more or less extensive foramina which occur in the region where the cribra orbitalia of Welcker (1887) are most commonly found. While Welcker denies that cribra are to be found in the newborn, he finds them in the skulls of young children. Both Ahrens (1905) and Koganei (1912) state that they are found most frequently in the young.
We have noted in young skulls with weakly developed cribra that the cribra lie in the precise region of the fissure and we suggest that there is some relationship between the development of the cribra and the blood vessels which enter this region, as has been demonstrated for the cribra cranii.
The lateral two-thirds of the permanent supra-orbital margin arise, as detailed above, as a separate process which serves to close the orbit from the temporal fossa. It also completes the fossa lacrimalis and, appearing late, is not completely fused with the rest of the bone until after birth.
This process leads to the formation in the adult skull of what has been described by Schwalbe (1906) as the planum or trigonum supra—orbitale. An oblique groove separating the trigonum supra-orbitale from the rest of the frontal bone is chieﬂy characteristic of recent man and occurs in some individuals of all primates. This groove extends from the supra-orbital notch upwards and laterally towards the temporal ridge, and has been named the sulcus supra-orbitalis.
In those forms such as the gorilla, chimpanzee, and the Neanderthal race in which the various supra-orbital elements are blended to form a distinctive torus supra-orbitalis, the sulcus supra-orbitalis may be obliterated, but when found is regarded by Cunningham (1908) as a juvenile character. Although the sulcus is most frequently related to the supra-orbital notch and may contain a branch of the supra-orbital nerve in part or in its entire extent, it cannot in any way be regarded primarily as a nerve pathway.
It has not been recognized that the sulcus supra-orbitalis defines developmentally the level of the primitive orbital margin and that variations in the dimensions of the orbital rim are dependent upon the degree of development of the supero-lateral process.
In the hypertelerotic skull described by Greig (1924-), under-development of this process has given to the orbital cavity an exaggerated lateral inclination, and suppression of its medial half has profoundly modified the orbital margin which is megaseme. Greig has pointed out that in hypertelorism there is retention of many foetal characteristics and among them we note retention of the primitive supra-orbital margin.
A study of the development of the eyebrow region and of the supra-orbital margin indicates that the essential primordia in man and the primates are identical. The differences which exist between adult forms are the outcome of differences in developmental direction alone.
We have demonstrated that there is no accessory-centre for the medial angular process or trochlear pit and we have described the area which has been erroneously considered as the pre-frontal element.
Opinions differ as to whether the anterior or posterior lacrimal crest should be considered as forming, for purposes of mensuration, the exact medial margin of the orbit. Some have given the posterior crest as the true limit of the orbital contents on the ground that it gives attachment to the septum orbitale. Others have taken the anterior crest. It would appear that the crista lacrimalis posterior is a continuation of the primitive orbital margin on the medial side. The lacrimal sac is therefore at first an extra-orbital structure and only with the formation of the new rim margin does it become potentially intra-orbital. This change in relationship is of ontological and phylogenetic interest. Flower (1907) has shown that in most mammals and of the primates in the lemur alone, the lacrimal fossa is extra-orbital. In all other primates the sac is considered to lie within the orbit.
- An historical review of the literature on the ossification of the frontal bone and its accessory centres is given.
- Observations were made upon a total of ninety-eight foetal skulls ranging in age from the 6th week of intrauterine life to the 10th month postnatal.
- Primary centres of the frontal bone make their appearance in the superciliary region and the details of ossifications are given.
- No secondary centres of ossifications have been found at any time in the frontal bone. The changes which have given rise to the error that such centres exist are considered.
- Attempts which have been made to homologize portions of the frontal bone with the pre- and post-frontal elements of premammalian skulls have been based upon the presumed existence of secondary centres. As no such centres exist in the human skull, such conclusions as have been made, on this basis, must be discarded.
- The significance of the trigonum supra-orbitalis of Schwalbe and of the sulcus supra-orbitalis as well as of other details about the orbit are discussed. 394 Verne T. Inman and John B. de 0; M. Saunders
In conclusion it is with great pleasure We acknowledge our indebtedness to Mrs L. G. Philippe for secretarial assistance, and to Mr Rudolph Skarda for his skill in the preparation of material
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Explanation of Plate I
fig. 1. Untouched photograph (infra-red, Wratten filter No. 25) of norms. lateralis of an ossifying human foetal skull, sitting height 80 mm. (13-5 weeks, Streeter’s tables).
fig. 2. Untouched photograph (infra-red, Wratten filter No. 25) of norma facialis of an ossifying human foetal skull, sitting height 126 mm. (16-5 weeks, Streeter’s tables).
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Cite this page: Hill, M.A. (2021, April 18) Embryology Paper - The ossification of the human frontal bone with special reference to its presumed pre- and post-frontal elements. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_ossification_of_the_human_frontal_bone_with_special_reference_to_its_presumed_pre-_and_post-frontal_elements
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