ORBIT. 



783 



on the inner side a depression for the insertion 

 of a pulley, through which runs the tendon of 

 the superior oblique muscle. 



The inferior wall or floor is less extensive 

 than the roof, and is directed upwards, out- 

 wards, and forwards. It is formed chiefly by 

 the orbital plate of the superior maxillary bone, 

 in front of tins by the orbital process of the 

 malar, and at the posterior part to a slight ex- 

 tent by the orbital process of the palate bone. 

 It presents a suture marking the union of the 

 malar with the maxillary, and of the maxillary 

 with the palate bone ; about the middle of the 

 floor is the infra-orbital groove, which passes 

 forwards and becomes the infra-orbital canal. 



The external wall is directed inwards, for- 

 wards, and slightly upwards ; it is formed in 

 front by the orbital process of the malar, and 

 posteriorly by the orbital surface of the greater 

 wing of the sphenoid. It presents a vertical 

 suture at the junction of the malar with the 

 sphenoid bone, and the orifices of some small 

 canals which open externally in the temporal 

 fossa, and on the facial surface of the malar 

 bone ; some of these canals transmit filaments 

 of nerves from the lachrymal branch of the 

 ophthalmic, and from the superior maxillary 

 nerve. 



The internal wall is directed outwards, and 

 slightly forwards and upwards : it is formed 

 chiefly by the os planum of the ethmoid, in 

 front of this by the lachrymal, and behind by 

 the side of the body of the sphenoid. It pre- 

 sents a vertical suture between the lachrymal 

 bone and the ethmoid, and another between the 

 latter bone and the sphenoid ; and anteriorly a 

 vertical groove which lodges the lachrymal sac. 

 ANGLES. The superior and external angle 

 formed by the junction of the superior with 

 the external wall presents posteriorly the sphe- 

 noidal fissure, sometimes called foramen lace- 

 rum anterius; in front of this a horizontal 

 suture at the junction of the orbital plate of 

 the sphenoid with the orbital plate of the frontal 

 bone, and anterior to this the junction of the 

 malar with the frontal bone. 



The superior and internal angle formed by 

 the meeting of the superior and internal walls 

 presents a suture between the os planum of 

 the ethmoid and the orbital plate of the frontal ; 

 in this suture are two small holes, the anterior 

 and posterior internal orbital holes ; the ante- 

 rior transmits the nasal branch of the ophthal- 

 mic nerve, and the anterior ethmoidal arlery ; 

 the posterior gives passage to the posterior 

 ethmoidal artery ; in front of the last-mentioned 

 suture is another between the lachrymal and 

 frontal bones. 



The inferior and external angle, formed by 

 the meeting of the inferior and external walls, 

 presents posteriorly the spheno-maxillary fis- 

 sure, which is bounded externally by the orbital 

 plate of the sphenoid, internally by the orbital 

 plates of the superior maxillary and palate 

 bones, and in front usually by the orbital pro- 

 cess of the malar bone, but occasionally by the 

 junction of the orbital plates of the superior 

 maxillary and sphenoid bones at this point. 

 The inferior and internal angle formed by 



the meeting of the inferior and internal walls 

 presents a continuous horizontal suture, which 

 in front connects the maxillary bone with the 

 lachrymal, behind this the maxillary with the 

 ethmoid, and still more posteriorly the palate 

 bone with the ethmoid. 



The base or circumference is of an irregular 

 quadrilateral form with curved sides and 

 rounded angles; it inclines obliquely from 

 within outwards. It is formed above by the 

 supra-orbital arch of the frontal bone, on the 

 outer side by the external angular process of 

 the frontal and by part of the orbital border of 

 the malar ; below by the continuation of tlie 

 orbital border of the malar, and by the corres- 

 ponding orbital border of the superior maxillary 

 bone ; on the inner side it is completed by the 

 nasal process of the superior maxillary, and the 

 internal angular process of the frontal bone. At 

 the junction of the middle with the inner third 

 of the supra-orbital arch is the supra-orbital 

 notch or foramen, which transmits the frontal 

 nerve and vessels. There are three sutures in 

 the margin of the orbit, one between the fron- 

 tal and malar, a second between the frontal 

 and superior maxillary, and a third between 

 the malar and superior maxillary bones. At 

 the junction of the lower with the inner border 

 of the orbit is a small tubercle, the lachrymal 

 tubercle, which is sometimes pointed out as a 

 guide in the operation for fistula lachrymalis, 

 but it is seldom very prominent even in the 

 bare bone, and it could scarcely be detected 

 through the tumefaction consequent on ob- 

 struction of the lachrymal duct. The lachry- 

 mal groove is immediately behind the internal 

 margin of the orbit. 



In the apex of the orbit is the optic foramen 

 situated between the two roots of the lesser 

 wing of the sphenoid bone ; the direction of 

 the optic hole is backwards and inwards to- 

 wards the centre of the sella Turcica. The in- 

 ferior root of the lesser wing of the sphenoid, 

 which separates the optic hole from the sphe- 

 noidal fissure, presents anteriorly a small tuber- 

 cle which gives origin to the common tendon 

 of the internal, external, and inferior recti 

 muscles. 



Dissection of the orbit. Having removed 

 the skull-cap and brain, the roof of the orbit 

 may be taken away by two vertical cuts with 

 a saw, the inner cut extending from a point 

 just external to the internal angular process, 

 backwards along the roof to the optic foramen, 

 the outer cut extending from a point just in- 

 ternal to the external angular process, also back- 

 wards to the optic foramen. In making these 

 cuts care must be taken to avoid injuring in 

 front on the inner side the pulley and tendon 

 of the superior oblique muscle, on the outer 

 side the lachrymal gland with its vessels and 

 nerves, and posteriorly the optic nerve and 

 ophthalmic artery passing through the optic 

 hole. Having removed the bony part of the 

 roof the periosteum is exposed, and must be 

 examined before proceeding farther. 



The periosteum of the orbit appears to be a 

 continuation of the dura mater ; it passes in 

 through the optic hole, and through the sphe- 



