THE OPHTHALMIC ARTERY. 749 



of the orbit, along which it runs between the superior oblique and internal rectus 

 muscles to the inner angle, where it terminates by dividing into palpebral^ frontal, 

 and nasal branches. 



Branches. — (a) The arteria centralis retinae arises from the ophthalmic while that vessel 

 is still below the optic nerve. It runs forward along the under surface of the nerve to a point 

 about 15 mm. from the posterior surface of the eye, where it passes into the substance of the 

 nerve and continues its course forward in the centre of that structure. Arrived at the retina, 

 the artery divides into two main branches, one ascending and the other descending, and these, 

 branching repeatedly, form an arterial net- work upon the surface of the retina. The finer 

 branches of the net-work extend deeply into the substance of the retina, although none reach 

 the layer of visual cells. They pass over directly into the corresponding veins without making 

 connections with any of the other arteries supplied to the eyeball. Just after its entrance into 

 the eyeball, however, the main stem of the artery anastomoses with the short ciliary vessels. 



{(5) The ciliary arteries, which are distributed to the choroid coat, the ciliary processes, and 

 the iris, are somewhat variable in their number and origin. Two sets are distinguishable, and 

 are named from their relative position the posterior and anterior ciliary arteries. 



{aa) T\\Q posterior ciliary arteries (aa. ciliares posteriores) arise from the ophthalmic artery 

 as it crosses over the optic nerve, either as two trunks which pass forward, the one on the inner 

 and the other on the outer side of the optic nerve, or else as a variable number of small vessels. 

 Eventually the vessels break up into from ten to twenty branches, which surround the distal 

 portion of the optic nerve, and, piercing the sclerotic, are distributed to the choroid coat of the 

 eye. Two of the vessels, lying one on either side of the optic nerve, are usually stronger than 

 the others, pierce the sclerotic some distance nearer the equator of the eyeball, and are 

 termed the long posterior ciliary arteries (aa. ciliares posteriores longae). They pass forward 

 between the sclerotic and choroid coats, send branches to the ciliary muscle, and divide at the 

 peripheral border of the iris into two stems, which, passing around the iris, unite with their 

 fellows of the opposite side and with branches of the anterior ciliary arteries to form the 

 circulus arteriosus iridis, from which branches radiate to the iris and the ciliary processes. 



{bb) The anterior ciliary arteries (aa. ciliares anteriores) usually take their origin from the 

 muscular branches of the ophthalmic and accompany the tendons of the recti muscles (two 

 arteries being associated with each muscle, except in the case of the external rectus, where 

 there is only one) to the sclerotic, where they send off perforating branches which, after 

 piercing the sclerotic, unite with the long ciliaries to form the arterial circle of the iris. The 

 main stems are continued onward towards the margin of the cornea, where they divide and 

 anastomose to form a narrow net-work surrounding that portion of the eyeball and also give 

 branches to the conjunctiva. An anterior ciliary vessel is frequently contributed by the 

 lachrymal artery. 



[c) The lachrymal artery (a lacrimalis) arises from the ophthalmic as it passes upward over 

 the external surface of the optic nerve and passes forward and outward, in company with the 

 lachrymal nerve, along the upper border of the external rectus muscle. It traverses the substance 

 of the lachrymal gland, to which it gives branches, and terminates in small branches to the eye- 

 lids. In its course it gives off a number of small twigs to the external rectus muscle ; a menin- 

 geal branch, which passes back into the cranium through the sphenoidal fissure and anasto- 

 moses with the middle meningeal ; and a malar branch, which passes to the temporal fossa 

 through a small canal in the malar bone and anastomoses with the anterior deep temporal and 

 the transverse facial arteries. 



{d) The muscular branches ( rami musculares) are somewhat irregular in their number and 

 origin. Usually there are two principal stems and a variable number of small twigs, but occa- 

 sionally the two principal stems arise by a common trunk. When the two are distinct, the in- 

 ferior one arises close to the lachrymal, and is distributed to the inferior and internal recti and 

 the inferior oblique muscles ; while the superior, smaller and less constant, arises after the oph- 

 thalmic has crossed over the optic nerve, and is distributed to the superior and external muscles 

 of the orbit. In addition to branches to the muscles, these arteries also give origin to the 

 anterior ciliary arteries described above. 



{e) The supraorbital artery (a. supraorbitalis) arises as the ophthalmic passes over the optic 

 nerve. It is at first directed upward, and then passes forward between the periosteum of the 

 roof of the orbit and the levator palpebrae superioris, and, making its exit from the orbit through 

 the supraorbital notch or foramen, terminates in branches which ascend over the frontal bone 

 towards the vertex of the skull, supplying the integument and periosteum and anastomosing 

 with the superficial temporal artery. In its course through the orbit it gives off periosteal, 

 diploic, and muscular twigs, and, after its exit from the supraorbital notch, a palpebral branch 

 to the upper eyelid. 



if) The ethmoidal arteries are two in number, and arise from the ophthalmic as it passes 

 along the inner wall of the orbit. The posterior ethmoidal (a. ethmoidalis posterior) , which is the 



