730 



CRANIUM. 



The orbitar portion by its upper surface 

 supports the anterior lobes of the brain, and 

 its under surface forms the root of the orbits. 

 It is divided into two processes by a longitu- 

 dinal notch, which corresponds to the roof of 

 the nose. 



The orbitar process of either side is convex 

 in both directions on its upper surface, and 

 the mammillary eminences and digital- im- 

 pressions formed by the intergyral spaces and 

 convolutions of the brain are of a decided 

 character. On its under surface it is concave 

 and triangular, the base being directed for- 

 wards; at its anterior and outer part there is a 

 fossa (fossa lachry mails) for the lachrymal 

 gland, and which is overhung by the external 

 orbitar process ; at its anterior and inner part, 

 near to the internal orbitar process, and be- 

 tween it and the foramen supra-orbitarium, 

 there is a small pit (fossa trochlearis) to which 

 is fixed the cartilaginous pulley in which plays 

 the tendon of the superior oblique muscle of 

 the eye ; at the middle of its inner edge there 

 is a notch, which, applied to a similar notch of 

 the ethmoid bone, constitutes the foramen 

 orbitarium internvm anticum, through which 

 pass the ethmoidal twig of the ophthalmic 

 branch of the fifth pair of nerves, and the an- 

 terior ethmoidal branch of the ophthalmic 

 artery; and a little behind this there is another 

 notch, which by a like contrivance forms a hole 

 (the foramen orbitarium internum posticum ) 

 for the passage of the posterior ethmoidal 

 branch of the ophthalmic artery and corres- 

 ponding vein. 



The notch which is between the orbitar pro- 

 cesses is the hiatus ethmoidalis (incimra 

 ethmoidalis), and in the cranium it is filled up 

 by the cribriform plate of the ethmoid bone. 

 Its longitudinal is twice the length of its trans- 

 verse diameter ; anteriorly, it is bounded by 

 the notch which, in part, forms the foramen 

 ccecum and the posterior surface of the nasal 

 process ; posteriorly, it is open ; and its sides 

 are bounded by the commutual edges of the 

 orbitar processes, the tables of which are sepa- 

 rated in such a manner as to communicate with 

 the ethmoidal cells and close them at the 

 upper part, and at the anterior part of the 

 notch to communicate also with the frontal 

 sinuses. 



The frontal sinus is formed by the separation 

 of the two tables of which the bone is com- 

 posed, and by the absorption of the diploc ; 

 they are usually separated by a septum, and 

 they communicate on each side with the mid- 

 dle meatus of the nose in the manner indi- 

 cated above. 



The posterior and upper border of the bone 

 as far down as the posterior extremity of the 

 inferior margin of the fossa temporalis, is arti- 

 culated to the parietal bones ; and it will be 

 remarked that rather more than the middle 

 third of it advances upon and secures those 

 bones at the expense of their outer table, while 

 the inferior portions of it are in their turn 

 grasped by each parietal bone respectively, 

 the outer table of the latter advancing, at this 

 part, upon the inner table of the former. 



Behind the external angular process, be- 

 tween the temporal fossa on the one hand and 

 the orbitar process on the other, there is a 

 triangular rough surface which is implanted on 

 a similarly-disposed surface of the great wing 

 of the sphenoid bone. The posterior margin 

 of this surface is in apposition with the edge 

 of the thin extremity of the small wing of the 

 sphenoid, to which also is articulated the re- 

 maining portion of the posterior border of the 

 orbitar process ; but with this difference, that, 

 while in the former instance the edges are plain 

 and simply applied to each other, in the latter 

 the margins are denticulated, the sphenoid 

 overlapping the frontal so as to render the 

 roof of the orbit secure. 



Thus the frontal bone articulates by the pos- 

 terior borders of its two portions, with the 

 parietal and sphenoid ; by the inner edges of 

 its orbitar processes, with the ethmoid ; by its 

 nasal process, with the nasal ; by its internal 

 angular process, with the lachrymal ; by the 

 surface between the nasal and internal angular 

 processes, with the superior maxillary ; and by 

 its external angular process, with the malar 

 bones. 



This bone in the foetus, and for nearly two 

 years after birth, consists of two pieces, the 

 first deposit in each being at the prominence 

 already indicated. From this point the ossific 

 matter radiates, and approaching that from the 

 opposite side, the two combine so as to form 

 on the median line a suture which is speedily 

 effaced. Nevertheless it occasionally happens 

 that complete union does not take place, and 

 then the suture persists through life. 



The ethmoid bone (u6j".oEi^;, i?6//,o$, cribrum, 

 os ethmoideum; Germ.Ethmoidul-knoc/ien) com- 

 pletes that portion of the base of the cranium, 

 anterior to the sphenoid, which is not supplied 

 by the frontal. It is however devoted less to 

 the skuH than to the face, with many of the 

 bones of which it is connected ; and it con- 

 tributes greatly to form the nostrils and their 

 septum, as well as both of the orbits. 



As an element of the cranium it is very 

 simple, being merely a plate connecting the 

 two orbitar processes of the frontal bone, and 

 having on its median line a ridge, which joins 

 the frontal spine before, to the body of the 

 sphenoid bone behind. This plate is the cri- 

 briform plate or process; it is notched poste- 

 riorly where it receives the ethmoidal process 

 of the sphenoid bone, the apex of which pro- 

 cess is applied to the posterior extremity of the 

 central ridge. Advancing forwards, this ridge 

 quickly springs upwards as a pyramidal pro- 

 cess (the crista galli, or proccsxus cristatus}, 

 to which the falx cerebri is attached ; its pos- 

 terior edge is long and oblique, its anterior is 

 shorter, more vertical, and it terminates in- 

 feriorly in two slightly divergent plates, so as 

 to form by their articulation with the frontal 

 bone the foramen ccecum. On each side of the 

 crista galli, more especially towards the fore- 

 part, the cribriform plate is channelled for the 

 reception of the olfactory nerves, and each 

 channel is perforated by numerous foramina 

 for the transmission of the ramifications 



