524 THE VEINS OF THE HEAD AND NECK. 



excavated in the dura mater over the basilar process of the occipital bone, 

 opening into the inferior petrosal sinus on each side, and into the anterior spinal 

 veins below. 



Varieties of the sinuses. The disposition of the sinuses around the torrnUti' H 

 is subject to considerable variation. It occasionally happens that the lower part of the 

 superior longitudinal sinus is placed in the middle line, and then the straight sinus opens 

 into the front, while the lateral sinuses spring from the sides, of the torcular. so that a true 

 confluence of the sinuses is formed. In other instances the connecting 1 branch between the 

 torcular and the straight sinus is of large size, and conveys blood from the superior 

 longitudinal sinus, which then empties itself equally into both lateral sinuses. In either of 

 the foregoing- arrangements, the right and left lateral sinuses are of equal size, a condition, 

 however, which is of comparatively unfrequent occurrence. A great diminution, or even 

 absence, of one lateral sinus has been met with, the vessel of the opposite side being much 

 enlarged. 



The inferior longitudinal sinus is very inconstant, and according to some anatomists is only 

 exceptionally present. 



The occipital sinus is sometimes wanting ; or it may be of large size and form a groove 

 on the occipital bone, by the side of the foramen magnum, as it passes forwards to the jugular 

 foramen (fig. 404). 



The cavernous sinus is represented in the child by a venous plexus in the dura mater, 

 which, by enlargement and fusion of its channels, is converted into the sinus of the adult 

 with its characteristic structure. In old age the trabeculae diminish, and the cavity becomes 

 simpler. The basilar plc.ru s shows similar variations with age. (Langer, " Der Sinus 

 cavernosus der harten Hirnhaut," Wiener Sitzungsberichte, 1885.) 



The zplicno-parktal sinus is often wanting. On the other hand it may be of large size and 

 receive the termination of the middle cerebral vein. It sometimes joins the ophthalmic 

 vein at the sphenoidal fissure. 



An additional -pctro-squamom sinus is sometimes present, lying in a small groove along the 

 junction of the petrous and squamous portions of the temporal bone, arid opening behind into 

 the lateral sinus. In rare cases the petro-squamous sinus is found passing through an 

 aperture (foramen jugulare x/wv?/w) in the squamous part of the temporal bone, between the 

 orifice of the external auditory meatus and the glenoid cavity, and then joining the temporal 

 vein, thus resembling the arrangement in the dog and many other animals, in which a similar 

 vessel forms the principal outlet for the intracranial blood. In the human subject also, at an 

 early period of fcetal life, the lateral sinus is continued forwards in this course, and opens 

 into the primitive (afterwards the external) jugular vein, and the occurrence of a petro- 

 squamous sinus is due to the persistence of this channel, which usually becomes obliterated 

 after the development of the internal jugular vein. (On the varieties of the sinuses, see 

 J. F. Knott, Journ. Anat., xvi. 27 ; C. Labbe, Arch, de Physiol., 1883 ; G-. Sperino, op. cit.. 

 p. 521.) 



OPHTHALMIC VEINS. 



The ophthalmic veins are two in number, and collect the blood from the parts 

 within the orbit. They are of relatively large size, have a markedly flexuous course. 

 and their branches form numerous plexiform communications. 



The superior ophthalmic vein, much the larger, commences near the root of 

 the nose by a wide communication with the angular vein, and is also joined, usually 

 near its origin, by another communicating branch from the supraorbital vein. It 

 passes backwards in company with the ophthalmic artery, crossing the optic nerve 

 from within outwards, and gains the inner end of the sphenoidal fissure, where it 

 opens into the fore part of the cavernous sinus. It is joined in its course by anterior 

 awl posterior ethmmdal, muscular and lachrymal branches, and near its termination 

 by the central vein of the retina, all of which correspond generally to the arteries of 

 the same name. The veins from the eyeball are called anterior and posterior ciliary. 

 The anterior ciliary veins are small and accompany the corresponding arteries ; per- 

 forating the sclerotic coat close to the cornea, they join the muscular branches of the 

 ophthalmic veins. The posterior ciliary veins, of larger size, are four or five in number, 

 emerge from the eyeball about midway between the cornea and the entrance of the 

 optic nerve, and end partly in the superior, partly in the inferior ophthalmic vein. 



The inferior ophthalmic vein is formed by the union of the lower posterior 



