Book - Contributions to Embryology Carnegie Institution No.30

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Wheeler T. Variability In The Spinal Column As Regards Defective Neural Arches (Rudimentary Spina Bifida) (1919) No.30 Contrib. Embryol., Carnegie Inst. Wash.

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This 1919 historic paper by Wheeler describes step abnormality of spina bifida. Published in the series Contributions to Embryology (Carnegie Institution of Washington).

See also by this author: Wheeler T. Study of a human spina bifida monster with encephaloceles and other abnormalities. (1918) Contrib. Embryol., Carnegie Inst. Wash., 22: .

Moder Notes: Axial Skeleton Development

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Variability In The Spinal Column As Regards Defective Neural Arches (Rudimentary Spina Bifida)

By Theodora Wheeler

Of the Department of Embryology, Carnegie InslUulion of Washington.

With eleven figures.


Studies of minor variations are continually being presented in biological fields. The reason for the persistent attention directed toward this line of study is not far to seek. To any observer the more or less frecjuent appearance of varied characteristics in form or function can not fail to suggest many perplexing questions as to the causes underlying such processes, and also as to their immediate and ultimate effects. As a consequence, some of the most valuable theories in the different branches of the natural sciences have been suggested, and are being worked out, by means of the evidence afforded by these variations. The theory of evolution, with all its ramifications, has been and is being developed along these lines. Since the beginning of the nineteenth century, especially, the attention centered about variations has been tremendous. Lamarck, Darwin, Mendel, and then Galton were really the first to get the upper hand in the study and treatment of variations, and more recently a rapidly enlarging group of workers in genetics have, by fresh methods, gleaned a most fruitful harvest in this field. The methods themselves have been most numerous and varied, and it is especially true of this subject that its history is to be found in the history of its methods. At first lax and slip-shod, they have gradually become, through the influence of the men just mentioned, thorough and accurate. Probably the chief ones in present use are the statistical and the experimental methods and their combinations. So, as time goes on, innumerable minutise and data are gathered on every side and made to serve their part in unraveling problems. A noteworthy instance of help afforded by such studies is the more fundamental idea which we now have of the biological conception of the normal or type. This very necessary standard, though even now far from finally understood, has developed in the minds of men from a very rigid concept into a far more plastic and adaptable principle, mainly through the insight gained by variation study.

Very often these studies may be applied with profit to specialized problems. An example of this is the frequent and rather interesting results obtained along morphological hnes in the higher animal forms in demonstrations of the persistence in postnatal life of earUer phases of development, which, as a rule, become changed or obliterated during the course of growth. The present investigation deals with the variabihty throughout the spinal column in respect to incomplete union of the posterior laminse of the vertebrae. Associated with this subject is the question of its relation to the pathological condition of spina bifida. The incomplete ossification of the adult vertebral spinous processes is also an example of a condition of delayed development which has been present throughout the spine at an earlier period.

The boiw closure of the posterior vertebral arches begins in the lumbar region, proceeding upward and downward from this point. In a model of the chondrocranium and cervical vertebrae of a 42 mm. embryo (No. 886, Carnegie Collection) made by Dr. C. C. Mackhn, the cartilaginous closure of the atlantal neural arches lags considerably behind that of the other cervical vertebrae, which evidence also substantiates this sequence in the cervical region. As is the case with other growth phenomena, there must be considerable variation in the time of completion of this process. According to Hennig (1880) the lumbar spinous processes are ossified by the third year. Macahster (1893) gives time of completion of the dorsal arch in the atlas as the fourth year, and Radlauer (1908) states that ossification of the sacral spinous processes, beginning with Si in the third year, is finally completed with S4 and Ss in the seventh year.

Incomplete closure of the sacrum has been the subject of numerous studies. In 1902 W. R. Smith noted the variation associated with left-sided sacraUzation of the fifth lumbar in a female Australian aboriginal sacrum, and more recently Radlauer (1908), Adolphi (1911), Frets (1914), and Wetzel (1915) have contributed considerable data on the subject. Radlauer studied 500 sacra, representing various races, from the University of Zurich and from several German collections. He found a completely open sacral canal in 5 per cent of the cases. He found also that closure of the hiatus sacralis occurred more frequently, (1) over the fourth and between the fourth and fifth sacral vertebra, 45.6 per cent; (2) over the third and between the third and fourth sacral vertebra, 27.4 per cent; and (3) over the fifth sacral vertebra, 14 per cent.

Adolphi (1911), working in Dorpat with 292 skeletons (234 male and 58 female), found an open sacral canal in 3.4 per cent of the males and 1.7 per cent of the females — a total of 3.08 per cent. In 50.3 per cent (48.7 per cent of males and 56.9 per cent of females) there were 4 sacral vertebrae closed; in 24.3 per cent (25.2 per cent of males and 20.7 per cent of females) 3 vertebrae were closed; while in 12 per cent of both males and females 5 vertebrae were closed.* In 12.7 per cent there was some degree of opening of the first sacral arch.* In a group of 203 specimens (161 male and 42 female), in which there were 5 and 6 vertebrae with no transitionally formed ones, he found that the hiatus sacralis reached to the third vertebrae in 26.6 per cent (28 per cent of the males and 21.4 per cent of the females) ; to the fourth vertebra' in 51.2 per cent (48.4 per cent of males and 61.9 per cent of females); and to the fifth vertebrae in 12.3 per cent (13 per cent of males and 9.5 per cent of females).

Frets, in material drawn from the population of Amsterdam, in which no Jewish skeletons were included, found that out of 750 specimens the sacral canal was completely open in 15, or 2 per cent. He further found closure of the hiatus sacralis to be more frequent over S4. This occurred in the six-segmented sacra in 57.3 per cent of the specimens, and in the five-segmented sacra in 70 per cent. In a group of 265 sacra with five vertebrae there were 9 (3.4 per cent) wdth completely open canal, but none among 113 sacra in which the first coccygeal segment was fused with the sacrum, nor among 150 six-segmented sacra. Thus, in a total of 528 specimens representing these three types the open sacral canal exhibited this variation of 1.7 per cent.

'Adolphi's table 4. 'Adolphi's table 3, column 1. 'Adolphi's table 5, columns 2 and 6.

'Adolphi'8 table 3, columns 2 and S. 'Adolphi's table 4, column 2. 'Adolphi's table 5, columns 1 and 5.

Adolphi's table 3, columns 3 and 6. 'Adolphi's table 5, columns 7 and 9.

Frets's findings in regard to the hiatus sacralis and open sacral canal in this group of 528 cases are shown in table 1 (adapted from table iv in his article in the Morphologische Jahrbuch, 1914). Here a definite relation appears to exist between the number of segments in the sacrum and the region of closure of the hiatus. In sacra formed of 5 vertebrae it was open higher than in those formed of 6. In the six-segmented sacra the liiatus sacralis closed over the fifth vertebra in 34 per cent of the cases, as opposed to 5 per cent in those with 5 segments; while in the latter it closes over S3 in 20 per cent of the specimens, as opposed to 7.3 per cent in the six-segmented sacra.

Table 1. — Fodii of Ifialus canalis sacralis, 52S eases {adapted from Frets).

Location of hiatus canalis sacralis.

Sacra with 5 segments, 265 cases.

Sacra with 5 segments and fused H-ith Coi 1 13 cases.

Sacra with 6 segments, 150 cases.

No. of cases.

P. ct.

No. of cases.

P. ct.

No. of cases.

P. ct.


13 186 54


1 9



20 0.8 0.4 3.4

25 83 6

22 73 4.4






57 7.3 0.67



Posterior arch of Si reduced in varying amount







In 22 Australian sacra Wetzel (1915) found 2 with completely open canal. He also mentions a study of 257 specimens from the "Breslau Anatomy" (which I have not been able to locate) in which this variation was found in 3.5 per cent of the cases. He noted that the hiatus sacral is closed over Sj in 8 cases and between S4 and S5 in 8 cases, and that there were numerous instances where the arches of S1 either did not unite at all or united so low as to correspond to the junction of Si and S2.

At this point it may be well to call attention to the fact that in such reports rarely is any information given, or even an estimate made as to how much of the material presented was originally acquired because of the presence of such variations or anomaUes. The chances are that some of the material in almost any collection has been selected on this account, and therefore ratios based upon these cases are exaggerated. Very probably the rather wide range of variation of the open sacral canal (5.0 to 1.7 per cent) as presented by the three authors quoted above, Radlauer, Frets and Adolphi, is due as much to differences in collection as to true representative differences in populations or races.

In the present investigation 1,000 consecutive X-ray plates of the lumbar region in which adults (over 18 years of age) were studied at the X-ray department of the Johns Hopkins Hospital, as were also all the available cervical vertebra? in the Division of Physical Anthropology at the U. S. National Museum at Washington. From the latter source over 600 atlases and more than 3,000 other cervical vertebrae were examined. The kindness of Dr. Baetjer, of the Johns Hopkins X-ray department, and of Dr. Ales Hrdlicka, curator of the department of physical anthropology, National Museum, is acknowledged with pleasure.

As the osteological collection at the National Museum was studied mainly for cervical vertebra?, no thorough record of sacral conditions was attempted. In one group of 33 white skeletons, however, in which the sacrum was present with the rest of the spinal column, 2 cases of completely open sacral canal were noted. One of these was in a sacrum composed of 5 vertebrae; the other was also in a five-segmented sacrum, but in this case Ls was saeraUzed on the left side, and that vertebra showed also a bifid dorsal arch (fig. 5). In one sLx-segmented sacrum in this group the dorsal arches were united only over S3 and S4. In a similar group of 22 sacra from various American Indian tribes the condition of an entirely open sacral canal was noted twice.

Examination of the X-ray plates Table 2.— Types 0/ dejectwe sacrum.

showed 4 cases of completely open sacrum in white males and 4 in white females. These are given in table 2 and clinical aspects of the cases follow on page 102. Unfortunately, an accurate count was not kept of the entire number of plates in which the whole sacrum showed plainly, so that these cases could not be used in calculating percentages, and the groups of whites and Indians just noted are too small to be of statistical value. In the X-ray plates dorsal closure of the first sacral vertebra was found to be considerably defective in 78 cases of white males (14.57 per cent) and in 53 cases of white females (11.39 per cent). This ratio is in close accord with that of Adolplii (12 per cent), while that of P>ets is much higher (24.6 per cent). The possibility of difference of opinion on this point should, however, be taken into account. One has to choose a rather arbitrary standard of what constitutes a defective first sacral arch, since the transitions from a fully formed arch to an incomplete one are so very gradual. The author found the second as well as the first sacral arch to be defective in 4 cases (3 male and 1 female). Occurring with slightly less frequency than in the sacrum, complete union of the vertebral posterior laminic is found in the last lumbar vertebra in from 2 to 3 per cent of cases. In the 1,000 consecutive X-ray plates of the lumbar region of white adults that were studied, it was found 12 times among 535 males (2.22 per cent) and 11 times among 465 females (2.38 per cent), giving a total of 2.3 per cent. Table 3 shows its occurrence and association with other defective vertebra?. The designation, last lumbar, has been used instead of Ls or Le, as many of the X-ray plates did not show the exact number of lumbar vertebrae. The last lumbar was the only lumbar vertebra found to be so affected, except in the case of one female, where the fourth, as well as the fifth, and also the entire sacrum, showed the condition. (See case S. R.)

No. of cases.

.■\lone. Associated with Lfi defect.

Associated with L« and L5 defect.

Male.... Female. .





1 1






Table 3. — Occurrence of incomplete ossification of spinous process of last lumbar vertebra.





12 2.22

3 1 5


11 2.38

2 2 2


23 2.3

5 3


Cases with defective last lumbar spinous process:

No. of preceding cases where last lumbar defect was associated with defect in spinous process : R, .

Other cases where last lumbar spinous process showed irregular development

That the ratio obtained, representing the variation of bifid Lj, is a close approximation of its occurrence in any white population of mixed nationalities would seem to be a reasonable assumption. The X-ray plates constituted a consecutive series, the patients coming to the hospital for various causes, and in general the slightest irregularity of the last lumbar vertebral process shows very clearly in the plates. There are two modifications of the above statement, however, that must be taken into consideration as unknown factors, in spite of the fact that they probably represent only a small source of error and would tend to balance one another. On the one hand it must be admitted that a hospital population would be apt to include, somewhat in excess of an ordinary population, such cases of spina bifida as present clinical manifestations. On the other hand, a certain number of L5 with defective arch would not show on an X-ray plate, as the spht occasionallj' occurs close to the median line, but shadowed from the antero-posterior view by a bulky spinous process. In any event, since only 4 of the 23 cases where L5 was involved gave clinical symptoms (q. v. clinical), and as the additional group of L5 cases where irregularity of the spinous process was noted numbered only 7, the ratio of occurrence would not be greatly changed by these factors. Figures 1, 2, 3, and 4, taken from the X-ray plates, show the various types of incomplete closure encountered.

Figs 1 to 4 —Tracings taken from X-Ray plates, showing various types of incomplete dorsal closure of the fifth lumbar, vertebra. Fig. 1, J. H. H. No. 3659.5. Fig, 2, J. H. H. No. 45489. Fig. 3. J. H. H. No. 3.5876. Fig. 4. J. H. H. No. 43888.

A differentiation is to be made between the type of defective arch in which there is a medianly incomplete dorsal arch, as in these cases, and the condition studied b)' Manners-Smith and Hrdlicka in the last lumbar vertebra, where the whole vertebra was divided into two parts. In such instances the separation usually occurs bilaterally, between the junction of the superior articulating processes of the vertebra and laminic. Manners-Smith and Els attribute the condition chiefly to mechanical factors, such as strain from flexion, occurring suddenly or of long duration. Hrdhcka regards it as congenital. In osteological collections the posterior fragment is very easily lost, and thus a lumbar vertebra with a wide dorsal gap is encountered fairly frequently. A wide posterior gap, however, was not once observed throughout the 1,000 X-ray plates. Spina bifida of the other lumbar vertebra; does occur in varying degree, and records of such cases appear from time to time, usually in the clinical literature, but these are rather rare. A case of Voelcker (1903) is in point. Here, in a woman of 23, spina bifida of L2 is associated with a fatty and ligamentous tumor and various motor and sensory disturbances. George (1907) also gives a case of a child 2| years old, where L3 and L4, as well as the sacrum, arc cleft in the midline and a dislocation of the left femur is present.

As has been said before, considerable clinical interest is attached to certain phases of lack of posterior vertebral union; i. e., when it forms spina bifida occulta associated with neurological and other maldevelopments (Fuchs 1910, Krause 1911, Schulthess 1912, Els 1915, Findlay 1917). Among the maldevelopments, when the defect is in the lumbo-sacral region, may be mentioned trophic, sensory and motor disturbances of the lower extremities, bladder disturbances, uterine and rectal prolapse, club feet, congenitally dislocated hip, and pelvic and spinal malformations. Brickner (1918) has conveniently classified the abnormaUty into four divisions for clinical purposes: (1) Spina bifida occulta, with external signs and symptoms; (2) spina bifida occulta with external signs and no symptoms; (3) spina bifida occulta with no external signs but with symptoms; (4) spina bifida occulta with no external signs and no symptoms.

Of the 8 cases in tliis study in which the sacrum was found to be involved, 6 showed more or less marked clinical manifestations:

L.E., male, 27 years. Sacrum only bifid. Extreme pain i!i hack after working in stoojied position

Slight scoliosis to left. No other suggestive physical findings. A.M., male, 23 years. Sacrum only bifid. Bladder paresis, involuntary voiding as a child, later incomplete emptying of bladder. Depression and blind sinus over sacrum. Rt. foot supinated and varus position; large trophic ulcer under proximal enil of rt. .5th metatarsal. F.L., female, age 17 years, unmarried. Last hnnbar and sacrum bifid. Congenital dislocation of

right hip and well-marked congenital left-sided flat foot. M.R., female, age 55 years, nullipara. Sacrum only l)ifi(l. Pain in right leg and hip; pain in coccyx. S.R., female, age 35 years. Four healthy children. L4, Lj, and sacrum bifid. Hypertrichosis and nievus over sacral region. Marked lorilosis. Rt. foot showed mild hallux valgvis and

SchaefTer's non-defonniiig club foot. Congenital relaxation of bladder sphincter (w. plastic oijcration which did not ciu'e). E.S., female, age 20 years. i>ast lumljar bifid. Pain in lumbar region, began 1 year i>i'evious.

Lumbar lordosis increa.sed, and increased prominence of sacrum. Spine mobile. No tenderness.

Of the 15 cases (9 male and 6 female) in which the last lumbar vertebra only was involved, and the 5 cases (3 male and 2 female) in which the last lumbar and first sacral vertebrae showed defective ossification, only 1 case, and that in the former class, gave any clinical manifestations:

T.G., female, aged 36 years. Last lumbar bifid; Imnbo sacral strain; pain in sciatic nerve. Retroposition of uterus.

In one instance in the group where the last lumbar showed an irregularity there were clinical manifestations of associated malformations:

C.T., female, aged 33 years. Two cliildren, 14 and 12 years respectively. Lf.«<^ 'umbar irregular; ureteral stricture left, and sacroiliac strain left side. Retro-position of uterus.

In the dorsal region incomplete posterior vertebral closure is very rare and, Uke those cases of the upper lumbar vertebrae, are only occasionally reported. In approximate!}' 3,000 dorsal vertebrae seen at the National Museum the condition was not observed in a single instance. Joachlmsthal (1895) presents the case of a "Woman with the horse's mane." In this there were evident interruptions in the second to the fifth vertebral arches, with hair 27 cm. long over the site of the defect. In the cervical region, other than with the atlas, a bifid condition of the vertebral arches is likewise rare, only one case being found among 3,500 cervical vertebrae examined at the National Museum. This was in the bones of a ^Massachusetts Indian (No. 227471), in which the fifth cervical lacked dorsal laminae. Unfortunately C4 of the series was missing, nor were any other vertebrae present. These cervical vertebrae are shown in figure 6. One case was encountered when examining the X-ray plates; in this there were multiple spinal anomalies, and C7 showed incomplete dorsal union:

E.S., male, age 16 jts. In hospital for osteomyelitis of humerus. Right sided scoliosis in upper dorsal region. Body of D3 replaced by two triangular wedges on right side and one on left. C7 showed incomplete dorsal arch. Scoliosis with convexity to left in lumbar region.

Rauber (1907) cites a case in an adult male skeleton where Ce showed a bifid condition. The case described by Barclay-Smith (1910) may also be mentioned here:

Young female Egjiitian. Atlas synostosed to occiput. C2 and C3 fused together; C7 interrupted neural arch dorsi-median ; 2 laminae mutually independent, with spinous process subdivided; 8 cervical vertebroe and Cj showed a small, unilateral cervical rib. L3, 4. 5 and S] showed lateral interruptions to dorsal arch.

With the atlas the occurrence of a bifid condition of the vertebral arch once more appears as a fairly frequent variation. Here, as in the lumbar region, it is found in the vertebra which, in a freely articulating series, lies next to more rigid structures. Incomplete posterior arch of the atlas was observed in 11 out of 745 cases (1.47 per cent). Table 3 gives the incidence in the various groups studied. Two of these were included in a group of 50 cases of Swiss Alpine tjT)e, reported to me through the kindness of Dr. Adolf H. Schultz, and which belong to his private collection at Zurich. In addition to these 11 cases there are in Dr. Hrdlicka's exhibit cabinet at the National Museum 7 atlases showing incomplete posterior arch. The latter ha-\-e not been inchidod in the series, as they represent material wh ch has been selected on account of this variation, while the 11 cases cited above belong to routine acquisitions. In 2 of the Indian specimens there was absence of practically all arch formation (figs. 7 and 8), while in all of the remaining specimens the two lamina approximate within 0.5 to 4 mm. of each other.

Table 4.


Number of cervical vertebrae (excluding atlas).

Number of atlases.

Incidence bifid atlas.


Per cent.










425 537

18 2

15 3





126 75 50


Negriti, P. I




1.43 0.79


Whites (miscellaneous)

2 3

4.0 4.41

Miscellaneous, wliite and negro, J. H. U. Department of Anatomy






  • 0ne case of incomplete posterior arch of Cj (fig. 6) .

Associated with this common type of incomplete arch there was a slight, though appreciable a.symmetry in a little less than one-fourth of the specimens observed (4 out of 13 unbroken atlases, exhibit specimens included). This consists apparently in a bending of one of the lateral masses forward or backward, and sometimes also outward, with subsequent anterior or posterior projection of the corresponding dorsal lamina of the atlas, so that the two laminae did not meet symmetrically. Figure 9 represents an atlas of this type in the exhibit cabinet at the National Museum, while a symmetrical atlas is shown in figure 10, representing the Egyptian specimen designated in table 4. That this asymmetry is associated with an asymmetrical position of the condyles of the occii)ut seems probable, as the latter condition is found in all types of crania. It was impossible to follow up this point.

In attempting to analyze the various factors that seem to influence the closure of the posterior vertebral lamina;, another group of cases, in which the atlas and occiput show physiological union, may be introduced. The condition has been frequently reported, and among 21 cases of physiological union found in the literature incomplete dorsal arch occurred 13 times, or 61.9 per cent, as shown in table 5.

In many of these cases a lateral twisting of the atlas on the occiput has been iHjted. At the National Museum there are 56 specimens of Dr. Hrdlicka's collection showing fu.sion between the atlas and occiput. Probably in 6 of these the fusion was the result of a pathological process, and in none of them was the atlantal ring incomplete. Seven other specimens were so damaged that it was impossible to determine whether or not the atlas showed a bifid condition. In 43 intact specimens there was physiological union of atlas and occiput and, among these, incomplete union of the dorsal laminse of the atlas was found in 25 cases (58 per cent), the bifid condition (fig. 11) ranging from 1 to 12 mm., with an average of 4.16 mm. There was one exception (an Eskimo specimen) in which the dorsal laminae of the atlas were entirely lacking. In every case of bifid atlas there was a marked asymmetrical arrangement in its fusion with the occiput, one side being more closely united than the other. In the 18 cases in which the dorsal atlantal arch was complete there was a much greater degree of symmetry in fusion of the atlas to occiput, and in 14 of these this symmetry was very marked.






1 3 15


1 2 9




Le Double



In summing up the various factors which may play an etiological role in these cases of minor incompletion of the posterior vertebral arch, we must first take into account the residual influence of early embryonic spina bifida. We know that localized delayed closures in the embryonic central nervous system occur, and it is very likely that with some of the less severe types of this condition a moderate degree of lagging is set up, resulting in the incoordination of the growing parts, and finally in the lack of bony union of the vertebral dorsal arches. One would suppose this factor to be at work to a greater degree in cases with associated neurological, meningeal, or skeletal malformations, or where a considerable length of spinal column is involved, than in cases where only one vertebra shows the condition. Here other factors may be called upon to explain it. Thus, in addition to the embryonic theory, which undoubtedly brings us closest to thinking of the cause in chemical terms, owing to the knowledge we have of experimental NaCl spina bifida, we must consider also the mechanical theories that appear to contain rational suggestions. However, it must be borne in mind that with any biological theory there is no such thing as real separation of the physical from the chemical — they are always in closest association. While possibly mechanical factors are present in this condition, great care should be exercised in placing confidence in any unproved statements which they may represent, for here, especially, mechanical explanations are rather specious and almost impossible to test directly. The mechanical factor upon which most stress has been heretofore laid is that of longitudinal flexion of the vertebral column, and the point has been emphasized that it is in the region of greatest flexion that the bifid condition most frequently arises. The element of lateral torsion has been considered as only a possible, minor factor. It is possible, however, that this may be of importance, especially where its action in individual vertebrae would seem to be combined with factors such as the following: (1) Position of the last vertebra of a free series, lying next to more rigid structures; i.e., atlas or last lumbar vertebra. (2) Asymmetry; i.e., atlas opposed to asymmetrical condyles of occiput, or asymmetrical fusion occurring between atlas and occiput.

In several adult cases reported by Els a history of symptoms of spina bifida, appearing after more or less protracted trauma, is recorded. It is quite probable that in adults mechanical agents act more frequently as secondary than as primary etiololgical factors, producing a clinical spina bifida, as differentiated from a solely anatomical condition. In early childhood, however, the possibility of such factors being primary must be taken into account, as the vertebral dorsal arches are at that time incompletely ossified.

We are not as yet far enough advanced to determine definitely the correlation between time and etiology in such a variation as is here dealt with, but it is hoped that a small amount of speculation, in an effort to supply a preliminary orientation in regard to this factor, will be pardoned. It would seem that during prenatal hfe the organism is more sensitive to metabolic disturbances, and that these would have greater effect upon form than similar factors acting at a later time. On the other hand, the individual would appear to be slightly more subject to mechanical disturbances, secondary, perhaps, to metabohc disturbances, resulting in shght changes of normal growth sequences, during early postnatal than in either prenatal or adult life. So, while one can not make positive statements as to either the time or the character of the disturbances producing the different types of spina bifida, there is the possibility, when the defect is a limited one, that its presence may have been partially due to mechanical factors occurring during childhood. Where the process is more extensive, however, such a possibility disappears, and in these there must have been a more fundamental metabolic disturbance acting at an earlier date.

Before closing, mention should be made of the fact, brought out by Frets, that in sex-segmented sacra the percentage of lower closures (Si) of the liiatus of the sacral canal is greater (34 per cent) than in those with 5 segments (5 per cent) ; and that the percentage of reduction in the posterior arch of Si is somewhat ower (21.5 as opposed to 34.0) in 5-segmented than in 6-segmented sacra. Evidently the caudal extent of the sacrum is associated with a corresponding tendency towards posterior closure of the vertebral arches in this region — the longer the sacrum, the lower the closure of the hiatus.


The condition of incomplete closure of the vertebral posterior arches is present in the different regions of the spinal column in the following order of frequency:


Per cent.

(1) First sacral ; Adolphi 12 per cent, Frets 20

per cent, author 1.3.1 per cent.

(2) Eiiliri' sacrum:

500 292 750 S2S 257









(3) Last lumbar: author. 2.3 per cent in 1,000


(4) Atlas: author, 1.47 per cent in 745 cases.

Figs. 5 and 6. Two specimens of the atlas in which there is incomplete dorsal arch formation. Fig. 5, specimen Xo. 25S637 F2, U. S. National Museum. Fig. 6, specimen No. 262993, U. S. National Museum. Fig. 7. Specimen showing sym.netry of the dorsal laminse. Specimen No. 256457, U. S. National Museum. Fig. 8. Specimen showing asymmetry of the dorsal laminje. Specimen No. 271782, U. S. National Museum. Fig. 9. Posterior view of the cerv'ical vertebRe, of wliich the fifth lacks dorsal laminae. Specimen No. 227471, U. S. Nat. Mus. Fig. 10. Specimen showing open Siicral canal and partial sacralization of fifth lumbar vertebra, the dorsal arch of which is bifid. X 0.7. Specimen No. 17980506, U. S. National Museiuu. Fig. 11. Specimen showing fusion of atlas and occiput, the posterior arch of the atlas being incomplete.


Wheeler1920 fig05-11.jpg

Figures 5 to 11


Abolphi, H., 1911. Ueter den Bau des mpnschlichen Krpuzbeines, etc. Morph. Jahrb., Bd. 44, p. 101.

Barclay-Smith, E., 1910. Multiple anomaly in vertebral column. Jour. Anat. & Phys., vol. 4.5, p. 144-171.

Brickner, Walter M., 1918. Spina bifida occulta. Amer. Jour. Med. Sc, vol. 15.5, p. 473.

DwiGHT, Th., 1004. Diagnosis of anatomical anomalies. J. Med. Research, vol. 12, p. 17-39. 1909. Concomitant assimilation of the atlas and occiput. Anat. Rec, vol. 3, p. 321-333.

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