SINUSES OF CRANIUM. 27 



a prolongation into the foramen caecum, as well as a series of tubes through 

 the apertures in the cribriform plate of the ethmoid bone. Through the 

 sphenoidal fissure it joins the periosteum of the orbit ; and through the 

 optic foramen a covering is continued on the optic nerve to the eyeball. 

 "Behind the sella Turcica, the dura mater adheres closely to the basilar 

 process of the occipital bone ; and it may be traced into the spinal canal 

 through the foramen magnum, to the margin of which it is very firmly 

 united. 



The tentorium cerebelli is the piece of the dura mater which is interposed 

 in a somewhat horizontal position between the small brain (cerebellum), 

 and the posterior part of the large brain (cerebrum). 



Its upper surface is raised along the middle, where it is joined by the 

 falx cerebri, and is hollowed laterally for the reception of the back part of 

 the cerebral hemispheres. Its under surface touches the little brain, and 

 is joined by the falx cerebelli. 



The anterior concave margin is free, except at the ends where it is fixed 

 by a narrow slip to each anterior clinoid process. The posterior or con- 

 vex part is connected to the following bones : occipital (transverse groove), 

 inferior angle of the parietal, petrous portion of the temporal (upper border), 

 and posterior clinoid process of the sphenoid. 



Along the centre of the tentorium is the straight sinus; and in the at- 

 tached edge are the lateral and the superior petrosal sinus on each side. 



Falx cerebri. The characters of this fold have been given in page 25. 



The Falx cerebelli has the same position below the tentorium as the falx 

 cerebri above that fold. It is much smaller than the falx of the cerebrum, 

 and will appear on detaching the tentorium. Triangular in form, this 

 fold is adherent to the middle ridge of the occipital bone below the pro- 

 tuberance, and projects between the hemispheres of the small brain. Its 

 base is directed to the tentorium ; and the apex reaches the foramen mag- 

 num, to each side of which it gives a small slip. In it is contained the 

 occipital sinus. 



The SINUSES are venous spaces between the layers of the dura mater, 

 into which blood is received. All the sinuses open either into a large 

 space named torcular Herophili, opposite the occipital protuberance; or 

 into the two cavernous sinuses on the sides of the body of the sphenoid 

 bone. 



A. The TORCULAR HEROPHILI (fig. 3, a) is placed in the tentorium, 

 opposite the centre of the occipital bone. It is of an irregular shape, and 

 numerous sinuses open into it, viz., the superior longitudinal above, and the 

 occipital below ; the straight in front, and the lateral sinus on each side. 



The superior longitudinal sinus has been already described (see p. 25). 



The inferior longitudinal sinus (fig. 3, c) resembles a small vein, and 

 is contained in the lower border of the falx cerebri at the posterior part. 

 This vein receives blood from the falx and the larger brain, and ends in 

 the straight sinus (d) at the edge of the tentorium. 



The straight sinus (fig. 3, o?) lies along the middle of the tentorium, 

 and seems to continue the preceding sinus to the common point of union. 

 Its form is triangular, like the superior longitudinal. Joining it are the 

 inferior longitudinal sinus, the veins of Galen from the interior of the 

 large brain, and some small veins from the upper part of the cerebellum. 



The occipital sinus (fig. 3, <?) is a small space in the falx cerebelli, 

 which reaches to the foramen magnum, and collects the blood from the 

 occipital fossae. This sinus may be double. 



