Skull and Hyoid 207 



by nuchal attachments, while the part above and behind this looks backwards and lies 

 under the scalp ; the latter is the (membranous) sbpraoccipital and is between the 

 parietals,* and the former is the (cartilaginous) post-occipital and abuts on the petro- 

 mastoid laterally. The borders of the squamous part show the distinction, for those 

 on the supraoccipital are strongly serrated, but those on the post-occipital are not 

 serrated at all. 



The cerebral surface (Fig. 170) has (a) The internal occipital protuberance, with the 

 internal occipital crest running down from it to the margin of the foramen magnum : 

 the crest is sometimes replaced by a groove, containing the occipital sinus, (b) Grooves 

 running out transversely for the lateral sinuses, (c) A groove ascending from the tube- 

 rosity for the superior longitudinal sinus, usually to the right of the middle line and 

 continuous with the right lateral groove : the left lateral groove appears to start at 

 the tuberosity, because the left sinus is generally continuous with the straight sinus, 

 which is in the tentorium cerebelli. The junction between left and right systems 

 is usually small and lies on the protuberance, but occasionally makes a groove on the 

 bone, as in that figured ; it is called the torcular Herophili. 



The lateral and vertical grooves have tentorium and falx respectively attached 

 to their margins, hence the cerebral hemispheres are in relation with the supraoccipital 

 and the cerebellum with the post-occipital. The falx cerebelli and occipital sinus 

 are on the internal crest. There are marks for cerebral gyri in the upper quadrants, 

 but the cerebellum and cisterna magna cause no marking on the bone, and ridges on 4 

 this part are due to posterior diploic veins running to the sigmoid sinus, where their '. f 

 terminal openings can be seen. 



The superficial convex surface of the tabular portion has (Fig. 171) its most promi- 

 nent r^art, the external occipital protuberance, a little below its centre. The superior 

 CHIT c (I lines run out from this to reach the mastoid process : below this, on the post- 

 occipital, are a vertical external occipital crest and a transverse inferior rcTirbm line : 

 between them are rough markings made by the thick areolar fasciae round the muscles 

 of the suboccipital region. 



The attachments (Fig. 171) are : 



To upper curved line : Trapezius, Splenius, Sternomastoid ; the last two 

 reaching mastoid. 



To lower curved line : Fascia separating posterior Recti from Complexus and 

 Superior Oblique. 



Between the lines : Complexus and Superior Oblique. 



To protuberance and median crest Ligam. nuchae. 



There may be a groove for the occipital artery outside the inferior curved line and 

 the area of Superior Oblique. 



The occipital artery reaches the base of the skull by crossing the carotid sheath in an upward, 

 outward, and backward direction from its origin from the external carotid on the pharyngeal wall. 

 In so doing, it lies on (see Fig. 22) the internal carotid, hypoglossal, internal jugular, and spinal 

 accessory, and reaches the outer edge of Rectus lateralis, along which it passes to the skull : it is 

 covered by the Digastric. The place where it first touches the skull, therefore, is between the 

 lateral Rectus, on the exoccipital, and the digastric groove on the petro-mastoid i.e., it is usually 

 to be found as a groove on or by the suture between these two parts of the skull. It now runs 

 upwards, inwards, and backwards, in more or less close association with the occipital (Fig. 171) 

 and round the outer side of the muscles forming the suboccipital triangle : it is covered at first 

 by the mastoid process and its attached muscles, but as it runs back it gets beyond the process 



* Hence frequently termed the " interparietal " part of the bone. 



