Occipital Bone 15 



its outer border, where it articulates with the frontal, are the foramina 

 for the anterior and posterior ethmoidal arteries, the nasal nerve 

 accompanying the anterior vessel. 



The lateral masses contain the anterior and posterior ethmoidal 

 sinuses, which open respectively into the middle and superior meatuses. 

 The superior and middle turbinated bones help to make up the mass. 

 They are covered with mucous membrane in which the olfactory 

 filaments are spread. The turbinated bones play the part of a re- 

 spirator, warming and moistening the inspired air, and filtering it 

 of solid particles. The outer wall of the lateral mass, the os planum, 

 forms part of the inner wall of the orbit. 



The occipital bone has its vertical part greatly strengthened by a 

 protuberance (to which the ligamentum nuchag is attached), by an in- 

 ternal and external median crest, and by two curved lines, or groins, 

 which arch laterally from the external crest. The trapezius and 

 occipito-frontalis arise from the superior curved line, and the sterno- 

 mastoid and splenitis are inserted into it. Between the lines the 

 complexus and superior oblique are inserted ; the ' straight ' muscles 

 of the back of the head are attached to the lower line. 



The condyles have their long diameter sloping forwards and 

 inwards, the movements at the occipito-atloid joints being only flexion 

 and extension. Between the condyles is the foramen magnum for the 

 transmission of the medulla oblongata and its membranes, the spinal 

 accessory nerves, and the vertebral arteries. At the front and outer 

 part of each condyle is the anterior condylar foramen for the trans- 

 mission of the hypoglossal nerve, and perhaps of a twig from the 

 ascending pharyngeal artery. 



A vein may enter the lateral sinus through the posterior condylar 

 foramen. In front of the foramen magnum is the basilar process, 

 with a spine for the attachment of the superior constrictor of the 

 pharynx. 



The cerebral surface shows the cruciform markings of the superior 

 longitudinal, the occipital, and the two lateral sinuses which meet at 

 the internal protuberance over which the torcular Herophili is placed. 

 The two superior fossae thus marked out are for the occipital lobes of 

 the cerebrum, the two inferior for the lobes of the cerebellum. The 

 lateral sinus, having grooved the vertical part of the occipital bone, 

 passes on to the posterior inferior angle of the parietal, the mastoid 

 portion of the temporal, and then to the jugular process of the occipital, 

 where it ends in the jugular fossa. The superior angle of the bone is 

 received between the parietal bones, and corresponds to the site of the 

 posterior fontanelle. 



The upper surface of the basilar process supports the medulla and 

 pons. Its border is grooved by the inferior petrosal sinus passing 

 backwards to the jugular foramen, which also transmits the blood of 

 the lateral sinus to the internal jugular vein, and the glosso-pharyngeal, 



