THE OCCIPITAL BONE 



51 



extends forward from the ventral margin of the foramen magnum. It is wide and 

 flattened behind, narrower and thicker in front. The ventral surface is rounded. 

 The cranial surface is concave and smooth; its posterior part supports the medulla 

 oblongata, and its anterior part has a shallow cavity on which the pons rests. The 

 lateral borders are thin and sharp, and form the medial margin of the foramen 

 lacerum. The anterior end has, in the young subject, a semicircular, flat, pitted 

 surface which is attached to the body of the sphenoid bone by a layer of cartilage; 

 in the adult there is complete fusion. On the ventral aspect of the junction are the 

 basilar tubercles (Tubercula basilaria) for the attachment of the ventral straight 

 muscles of the head. 



The squamous part (Squama occipitalis) ^ is the somewhat quadrilateral mass 



Fig. 29. — Occipital Bone of Colt; Front View. 

 1, Depreasion of squamoua part for cerebellum; 2, foramen magnum; 3, paramastoid process 



(2, squamous part; 6, lateral part; c, basilar part; d, junction with interparietal bone; 

 /, junction with petro-mastoid part of temporal bone. 



J^, condyloid fossa; 

 junction with parietal bone; 



situated dorsal to the lateral parts, from which it remains distinct till the second 

 year. The external siuface is crossed by a very prominent ridge, the nuchal crest 

 (Crista nuchalis) ; the middle part of this is thick, transverse in direction, and forms 

 the highest point of the skull when the head is in the ordinary position; laterally it 

 becomes thinner and runs downward and forward to join the temporal crest. ^ The 

 crest divides the surface into two very unequal parts ; the small dorsal area (Planum 

 parietale) presents a median ridge which is the posterior part of the external parietal 

 • crest (Crista sagittalis externa) ; the large area ventral to the crest (Planum nu- 

 chale) has a central eminence, the external occipital protuberance, on which the funic- 

 ular part of the ligamentum nuchse is attached. The internal surface is concave 



' Also known as the supraoccipital. 



2 The nuchal crest of this description is equivalent to the external occipital protuberance 

 and superior nuchal line of man; it has been commonly termed the occipital crest, but is not the 

 equivalent of that feature of the human skull. A curved hne a little lower down, which is con- 

 tinued on the paramastoid process, represents the inferior nuchal line of man. 



