DEVELOPMENT OF THE OCCIPITAL BONE. 



175 



the external occipital crest, '^ runs from the protuberance to the foramen magnum. 

 Above the middle of this crest the inferior czirved tine' leaves it to extend outward and 

 downward to the border of the bone. The inner part of this line is rough, the outer 

 indistinct. Below this line there is usually a depression on either side of the crest. 



The internal surface of the squamous portion is divided into four depressions 

 or fossae ; the upper two lodge the occipital lobes of the cerebrum and the lower two 

 the lateral lobes of the cerebellum. Below the middle is the internal occipital pro- 

 tuberance^^ approximately opposite to the outer. A ridge runs from the apex of the 

 bone to the protuberance, and is continued as the internal occipital crest'' to the 

 foramen magnum. Very often the second part of this ridge divides shortly after its 

 origin, so as to enclose a depression, the vermian fossa, so called because it is below 

 the middle lobe, or vermis, of the cerebellum. A ridge runs transversely from the 

 protuberance to the lateral angle of the bone. The superior vertical ridge may be 

 grooved for the superior longitudinal sinus and the transverse ridge for the lateral 

 sinus. More frequently the longitudinal sinus lies to one side of the vertical ridge 

 and is continued into one of the lateral ones, much larger than its fellow, and usually 

 the right, which lies above the 



Fig. 194. 



Superior median fissure 



superior oc- 

 cipital 



Squamous por- 

 tion 



—Fissure be- 

 tween upper 

 and lower 

 portions 



Supra-occipital 



Exoccipital 



transverse ridge, and shows in 

 the bone no communication with 

 the smaller, which lies in or above 

 the other ridge. There are many 

 variations in this arrangement, of 

 which the rarest is a symmetrical 

 course and division of the supe- 

 rior groove. A single or a bifur- 

 cated groove is sometimes found 

 on the internal crest. 



Development. — Four cen- 

 tres appear in the cartilage around 

 the foramen magnum about the 

 eighth week of foetal life : one for 

 the basilar, one for each exoccipi- 

 tal, and one (or more probably 

 a pair that speedily fuse) for the 

 lower part of the squamous \)OX- 

 tion, she supra-occipital. A week 

 or so later two nuclei appear in 

 the membrane above the latter, 

 from which a strip of bone de- 

 velops which soon joins it. From 

 this upper ossification, the supe- 

 rior occipital, is developed all the 

 upper part of the squamous por- 

 tion, including the external occipital protuberance and the superior curved line.° 

 Occasionally still another nucleus appears on each side, anterior and external to the 

 preceding, which probably accounts for certain separate ossifications often found in 

 the lambdoidal suture. The squamous part shows a median cleft above, which 

 quickly disappears, two lateral ones between the ossifications, which persist till 

 birth, and a notch at the posterior border of the foramen magnum. The squa- 

 mous portion joins the exoccipitals in the course of the second or third year. The 

 latter begin to unite with the basilar a year or so later. None of these sutures, es- 

 pecially the latter, is completely closed before the seventh year, or even later. 

 The front parts of the condyles are formed from the basilar, which joins the ex- 

 occipitals at the anterior condyloid foramina. Separate ossifications, large Wormian 

 bones, ^ are found in the suture between the squamous portion and the parietals. 

 Sometimes there is a large median triangular one which is interpreted as the result 

 of a want of union of the usual superior centre of the squamous portion, and said to 



* Consult Stieda : Anatomische Hefte, iv., 1892, and Debierre : Joum. de I'Anat. et de la 

 Phys., 1895. 



' Linia nuchae mediana. *L. ntichae inferior. 'Protnb. occip. interna. * Crista occipitalis interna. 'Ossa snturamm. 



Basi-occipital 



Posterior condyloid foramen 



Occipital bone at birth, from before. 



