CRANIUM. 



731 



of the olfactory nerves (foramina cribrosa). 

 These openings are variable in their number, 

 and differ from each other in their size and 

 modes of termination ; those nearest the crista 

 galli are the largest, and of them one or two 

 of the anterior ones are very considerable ; the 

 smallest are situated on the outer edge of the 

 cribriform plate, and both of these sets are 

 the orifices of canals which terminate, the 

 former about the root and upon the sides of 

 the septum, the latter on the outer wall of the 

 nose ; those which are intermediate and in the 

 centre of the channel, are complete foramina, 

 and open on the opposite surface of the plate. 

 Immediately in front of the inner set of fora- 

 mina, there is, between the crista galli and 

 cribriform plate, a fissure which gives passage 

 to the ethmoidal nerve and vessels. 



From the under surface of the cribriform 

 plate and at right angles with it, there descend, 

 on the median line, the nasal lamella, and, 

 on each side, a cellular mass which partly 

 forms the outer wall of the nostril and the inner 

 wall of the orbit. 



The nasal lamella, or vertical plate, forms 

 the upper portion of the septum narium ; it is 

 immediately underneath the crista galli, and 

 becomes gradually thinner as it descends; its 

 anterior border is rough, thicker above than 

 below, and articulates, first, with the nasal 

 process of the frontal bone, and, secondly, 

 with the nasal bones-themselves ; its posterior 

 border is also rough and is articulated to the 

 crest on the fore part of the sphenoid bone ; 

 its inferior border is, in its posterior half, 

 thin and inclined downwards and forwards to 

 be articulated to the vomer, and, in its anterior 

 half, somewhat thicker and rougher, and in- 

 clined downwards and backwards to be arti- 

 culated with the triangular cartilage of the 

 nose; its sides are plain, and exhibit sulci which 

 are continuous with the foramina that open on 

 its root. 



On each side of this lamella and between it 

 and the lateral masses there is a space which 

 is encroached upon in the middle more than it 

 is above or below, and a portion of the cribri- 

 form plate forms its roof. 



The lateral masses are delicate in their struc- 

 ture and complicated in their arrangement. 

 Each consists of a number of cells (cellule 

 ethmoidules), which are divided by a partition 

 into an anterior and a posterior set, with the 

 former of which the frontal sinus communi- 

 cates, and with the latter the sphenoidal. The 

 outer surface of each lateral mass is compact 

 and smooth, and constitutes the greater portion 

 of the inner wall of the orbit. This is the 

 orbitar process or os planum, which articulates 

 above with the frontal bone, below with the 

 superior maxillary and palate bones, behind 

 with the sphenoid, and in front with the la- 

 chrymal. On its upper border are seen the 

 two notches which assist the frontal in forming 

 the anterior and posterior orbital foramina. 

 Hie inner surface of this cellular mass, that 

 which looks towards the nasal lamella, is ren- 

 dered irregular by two curved processes (the 

 superior and middle turbinated processes), of 



which the upper one is smaller, delicate, re- 

 gular in its curve, and is seen only on the 

 posterior half of the wall ; the other is larger, 

 more spongy, and extends the entire length of 

 the wall. Both of them are convex on the 

 side next the cavity of the nostril, and concave 

 on that which looks towards the cells ; but the 

 inferior is also at its lower edge again curled 

 in such a manner as to offer a convexity on both 

 of its surfaces. Between the two turbinated pro- 

 cesses there is a triangular space (the superior 

 meatus) the apex of which is directed forwards, 

 and in which there is an opening commu- 

 nicating with the posterior ethmoidal cells. 

 Underneath the middle turbinated process, and 

 bounded by its concavity on the one hand and 

 the cells on the other, is the middle meatus ; 

 into which open the anterior ethmoidal cells, 

 and the tubular communication with the frontal 

 sinus, called infundibulum. 



The connexions of this bone are, behind to 

 the sphenoid ; in front to the frontal and nasal 

 bones; laterally by its upper borders to the 

 orbitar processes of the frontal, by its under 

 borders to the same-named processes of the 

 superior maxillary and palate bones, and by 

 its anterior border to the lachrymal ; by the 

 under edge of its middle vertical plate to the 

 vomer and triangular cartilage; and by the 

 anterior extremity of the outer surface of the 

 middle turbinated process to the inferior tur- 

 binated bone. 



The ethmoid is the most tardy in its deve- 

 lopment of all the bones of the cranium. The 

 lateral masses exhibit each of them an ossific 

 deposit about the middle period of intra- 

 uterine life, but neither the cells nor turbinated 

 processes are much developed at birth, at 

 which time also the central portion is carti- 

 laginous. The ossification of this part pro- 

 ceeds from above downwards, so that the 

 crista galli is completely formed while the 

 lower part of the nasal lamella is yet cartila- 

 ginous. During infancy the cribriform plate 

 becomes narrower, curved, and as it were 

 compressed; the nasal lamella advances for- 

 wards; and the spaces between the septum 

 and outer walls are considerably increased. 



The occipital bone (os occipitis; Germ. Occi- 

 pital-knochen, Hinterhaupts-knochen,) is situ- 

 ated behind the sphenoid, and forms the pos- 

 terior part of the base of the cranium and the 

 contiguous projection of the occiput. Its 

 figure is that of a lozenge with its anterior 

 angle truncated, and is so curved as to be 

 generally concave on one surface and convex 

 on the other. The inferior and anterior half 

 of it is situated between the two temporal 

 bones ; the superior and posterior half is be- 

 tween the posterior margins of the two pa- 

 rietal. 



At its anterior part it is pierced by a large 

 elliptical foramen (the foramen magnum), 

 through which there pass, from the skull, the 

 medulla spinalis and its membranes, the sinus 

 venosus and the spinal arteries ; and, into the 

 skull, the vertebral arteries, the posterior me- 

 ningeal arteries, and the nervus accessories. 



On the cerebral surface the internal crucial 



3 Q 2 



