J. Anat. (1977), 124, 3, pp. 643-649 643 With 7 figures Printed in Great Britain Development of ossification centres in the squamous portion of the occipital bone in man H. C. SRIVASTAVA Department of Anatomy, Medical College, Baroda, India. (Accepted 15 October 1976) INTRODUCTION The squamous portion of the occipital bone in man consists of an interparietal part which lies above the nuchal lines and ossifies in membrane, and a supraoccipital part which develops in cartilage and is situated between the nuchal lines and the posterior margin of the foramen magnum. The interparietal may exist as separate, symmetrical halves, or it may be in three pieces - or even four, in which case the upper two constitute the pre-interparietal (Brash, 1951). Misra (1960) has reported a skull with a separate interparietal bone. Kolte & Mysorekar (1966) have described a tripartite interparietal bone. Keith (1948) stated that a separate single interparietal bone in man is an extremely rare anomaly. No other anomalies of the interparietal, nor any anomaly of the supraoccipital, are mentioned in the literature. Analysis of the anomalies of the occipital bone reported in this paper has resolved differences between authors regarding ossification centres in the squamous portion, and also regarding the precise limits of the interparietal and supraoccipital. OBSERVATIONS A series of 620 human skulls was examined for anomalies in the squamous portion of the occipital, and 25 such were found. These are illustrated and described from the following representative specimens. Skull no. 1. A single separate interparietal is seen (Fig. 1). The suture between the interparietal and supraoccipital lies 2 cm above the external occipital protuberance and 0 4 cm above the superior nuchal line near the lambdoid suture. A sutural bone is present in the right lambdoid suture. Skull no. 2. A separate bone is present in the middle of the interparietal, leaving two nearly equal triangular areas on each side which are fused with the supra- occipital (Fig. 2). The sutures between this separate bone and the remaining bone extend from a point 2-3 cm above the external occipital protuberance in the midline to the lambdoid suture on each side. A line drawn in the central plane through this bone divides it into two more or less equal triangles. Skull no. 3. A large separate bone forms the left three fourths of the interparietal, with the right one fourth represented by a triangular area which is fused with the supraoccipital. The former is separated from the rest of the bone by a suture which begins like the interparietal-supraoccipital suture from the left lambdoid suture and extends to the midpoint 2-6 cm above the external occipital protuberance. From this point it runs obliquely upward and laterally through the interparietal region to join the right lambdoid suture near its middle. If from the above midpoint the left interparietal-supraoccipital suture is extended similarly on the right side, and 644 H. C. SRIVASTAVA Fig. 1. Complete separate interparietal bone. The superior and highest nuchal lines are below the interparietal-supraoccipital suture. Fig. 2. Shows a separate central piece in the interparietal region. The external occipital pro- tuberance and superior nuchal line are below it. the oblique suture is drawn to the left lambdoid suture, the interparietal part is divided into three parts - i.e. right and left triangular areas of similar shape and size and a central piece. The central piece resembles the separate bone described in skull no. 2. The superior and highest nuchal lines are below the interparietal- supraoccipital suture, which is 03 cm above the former line near the lambdoid suture. Small sutural bones are present in the lambdoid sutures. Skull no. 4. The interparietal is represented by three independent bones articu- lating with each other and with the supraoccipital by sutures. Right and left tri- angular plates are symmetrical mirror images (Fig. 4). The central piece resembles the separate bone seen in skull no. 2, and the right and left triangular plates corres- pond to the triangular areas drawn in skull no. 3. The superior and highest nuchal lines are below the three bones. The suture between these three separate bones and the supraoccipital part is 1 1 cm above the external occipital protuberance and 05 cm above the superior nuchal line near the lambdoid suture. Small sutural bones are seen in the lambdoid suture on both sides. Skull no. 5. The interparietal is a single bone. It is separated from the supra- occipital by a suture on the right side and by a suture and a gap on the left side (Fig. 5). The gap is 3-7 cm in length, its maximum width is 0O8 cm and it starts 05 cm to the left of the external occipital protuberance, ending 1-3 cm from the lambdoid suture. The superior nuchal line is seen below the interparietal-supra- occipital suture on the right side. Examination of the internal aspect of the skull shows a well marked internal occipital protuberance, and the groove for the trans- verse sinus lies above the gap on the left side and at the interparietal-supraoccipital suture on the right side. The supraoccipital consists of central, right and left segments. The central segment is separated from the right and left portions by right and left vertical sutures which extend from the interparietal-supraoccipital suture to the posterior margin of the foramen magnum 1-5 cm on each side of the midline. The left vertical suture presents small gaps in its upper portion. Its lower portion is obliteratedjust above the foramen magnum, but its position is marked by a groove. The right vertical suture is distinct Ossification centres in the occipital bone 645 r....,,,h,^. on_ t: '_ .-:,. -2 ,. f g .# 9 E ses"Nt.i'.SsS.. _Ee'. _ ( ;) ; ,,s'>'\' st g \ss. a, \ ; : x ; x _Ss. -*r -sffi; * ' o_o '; ,;2 i I F..:. ,1-1 -1., Fig. 3. Three fourths of the interparietal forms a separate bone. The suture between this separate bone and the rest of the bone begins from the left lambdoid suture and extends to the midpoint above the external occipital protuberance. From this point it runs obliquely upwards and laterally through the interparietal region to the right lambdoid suture. Superior and highest nuchal lines are distinctly seen below the suture. Fig. 4. The interparietal bone is replaced by three independent bones. The right and left tri- angular plates appear as mirror images. Fig. 5. Shows a separate interparietal and a small pre-interparietal anterior to it. The inter- parietal-supraoccipital suture presents a big gap on the left side. The supraoccipital part shows right and left vertical sutures separating the right and left segments from the central segment. Fig. 6. Inferior view of the skull seen in Fig. 5 shows superior horizontal sutures at the level of the inferior nuchal line dividing the right and left segments into superior and inferior pieces, and inferior horizontal sutures behind the posterior condylar fossae. The lower part of the right vertical suture extends to the posterior margin of the foramen magnum. 646 3H. C.7 SRIVASTAVA~~~~~~~~~~~~8 'b.~~~~~~1 Fig. 7. Diagram showing the occipital bone of skull no. 5. 1, pre-interparietal; 2, interparietal; 3, left vertical suture; 4, left superior horizontal suture; 5, left inferior horizontal suture; 6, interparietal-supraoccipital suture; 7, superior nuchal line; 8, right vertical suture; 9, right superior horizontal suture; 10, inferior nuchal line; 11, right inferior horizontal suture. in its entire length and presents a small gap 1 8 cm above the posterior margin of the foramen magnum. The central segment forms more than half the posterior margin of the foramen magnum. The right and left segments articulate laterally with the inferior angle of the parietal bone and the mastoid part of the temporal bone, medially with the central segment, superiorly with the interparietal bone, and inferiorly with the condylar part of the occipital bone at the inferior horizontal sutures which begin near the lower ends of the vertical sutures and run laterally behind the posterior condylar fossae (Fig. 6). The inferior horizontal suture is marked on the left side by a gap in the middle and a groove on either side, while on the right side it is seen as a faint groove. The right and left segments are incompletely divided into upper and lower pieces by superior horizontal sutures situated at the level of the inferior nuchal lines (Fig. 7). These sutures are present medially near the vertical sutures, but are obliterated laterally. A small pre-interparietal bone, 1-3 cm long and 1-2 cm wide, is present anterior to the interparietal in the posterior part of the sagittal suture. From a study of 620 skulls a pre-interparietal was seen in 18 skulls, a complete separate interparietal in 3 skulls, a separate bone in the middle region of the inter- parietal part in 2 skulls, and the remaining anomalies reported each in 1 skull. No signs of injury or of any pathological condition were seen in the squamous portion of the occipital bone in any skull. Ossification centres in the occipital bone 647 DISCUSSION The usual textbook description of the ossification of the squamous portion of the occipital bone is that, above the highest nuchal line, it is ossified in membrane from two centres, one on either side of the midline. These form the interparietal part, and may remain separate throughout life. Below the highest nuchal line, the squamous part is ossified in cartilage from two centres which soon unite to form a single piece. An occasional centre appears in the posterior margin of the foramen magnum (Warwick & Williams, 1973).