THE OCCIPITAL BONE. 121 



The squamous or tabular part in shape somewhat resembles a Gothic 

 ; arch, and is curved from, side to side and from above downwards. It forms 

 il inferiorly a small portion of the middle of the posterior boundary of the foramen 

 < magnum, and unites, on each side of that, with the lateral parts of the bone. 

 I About the centre of the parietal surface of the squama there is a prominence 

 the external occipital protuberance, which varies considerably in its distinct- 



ness and projection, and serves for the attachment of the ligamenturn nuchae. 



From the protuberance, on each side, two lines curve towards the lateral angles 

 of the bone. These are known respectively as the linea nuchse suprema and linea 

 nuchae superior (highest and superior curved lines). To the upper of the two the 

 galea aponeurotica (O.T. epicranial aponeurosis") is attached, whilst the lower serves 



r for the origin of the trapezius and occipitalis muscles and the insertion of the sterno- 



) mastoid and splenius capitis muscles. The two lines together serve to divide the 



external surface of the squama occipitalis into an upper or occipital plane (planum 



occipitale), covered by the hairy scalp, and a lower or nuchal plane (planum nuchale), 



i serving for the' attachment of the fleshy muscles of the back of the neck. As a rule 



I the occipital part bulges backwards beyond the external occipital protuberance ; 



exceptionally, however, the latter process is the most outstanding part of the bone. 



The nuchal plane, irregular and rough, is divided into two halves by a 



median ridge the crista occipitalis externa (external occipital crest), which extends 

 I from the external occipital protuberance above to the posterior border of the foramen 



magnum below. Crossing the nuchal plane transversely, about its middle, is the 

 inferior nuchal line, which passes laterally and forwards on each side towards 



I the corresponding lateral margin of the bone. The areas thus marked out serve 



I for the attachment of the semispinalis capitis (O.T. complexus), obliquus capitis 

 superior, and rectus capitis posterior major and minor muscles. 



The cerebral surface of the squamous part, concave from side to side and from 

 above downwards, is subdivided into four fossae by a crucial arrangement of grooved 

 ridges called the eminentia cruciata. In the upper pair of fossae are lodged the 

 occipital lobes of the cerebrum, whilst the hemispheres of the cerebellum occupy 



j the lower pair. Near the centre of the eminence is the internal occipital 

 protuberance, an irregular elevation, the sides of which are variously channelled 



i according to the disposition of the grooves. Leading from this to the posterior 

 margin of the foramen magnum is a sharp and well-defined ridge, the internal oc- 

 cipital crest, which serves for the attachment of the falx cerebelli, a process of dura 

 mater which separates the two cerebellar hemispheres. Passing upwards from the 

 internal occipital protuberance there is usually a well-marked ridge, to one or other 

 side of which, more frequently the right (with the bone in the normal position and 

 viewed from behind), there is a well-defined groove, the sulcus sagittalis, the lateral lip 

 of which is generally less prominent. Placed in this groove is the superior sagittal 

 venous sinus, and attached to the lips is the falx cerebri. At right angles to the 

 foregoing, and at the level of the internal occipital protuberance, with which they 

 become confluent, are two transverse grooves, the sulci transversi. These grooves, 

 which have more or less prominent edges, lie between the upper and lower pairs of 

 fossse, and serve for the attachment of the tentorium cerebelli as well as the lodgment 

 of the transverse sinuses. Commonly the right transverse groove is confluent with 

 the groove to the right side of the median ridge, but exceptions to this rule are not 

 infrequent. The angle formed by the union of the venous sinuses lodged in these 

 grooves constitutes the confluens sinuum (O.T. torcular Herophili), which may 

 accordingly be placed to one or other side of the internal occipital protuberance, 

 more frequently the right ; in some cases, however, it may occupy a central position. 

 The superior angle, more or less sharp and pointed, is wedged in between the 

 two parietal bones, its position corresponding to the site of the posterior fontanelle. 

 Each lateral angle articulates with the posterior extremity of the mastoid portion 

 of the corresponding temporal bone. The superior borders, much serrated, articu- 

 late with the parietal bones, forming the lambdoid suture ; and the lateral borders, 

 extending from the lateral angles to the jugular process inferiorly, are connected 

 with the medial sides of the mastoid portions of the temporals. 



The lateral (or exoccipital) parts of the occipital bone are placed on 



