240 THE ANATOMY OF THE HORSE. 



to the inner surface of the cranial bones, and forms for them an internal 

 periosteum. All over its outer surface it is connected by slender fibrous 

 processes and vessels to the bones ; but it is particularly adherent to 

 these along the lines of the sutures, and at the margins of foramina. 

 The meningeal vessels ramify on the outer surface of the membrane, 

 and leave their impressions on the inner surface of the cranial bones. 

 Sometimes the outer surface of the dura mater, on each side of the 

 middle line above, shows numbers of granular processes — the Pacchion- 

 ian bodies, which are developed from the subjacent arachnoid. Occa- 

 sionally they are large enough to cause the partial absorption of the 

 bones over them. The inner surface of the dura mater is smooth, in 

 virtue of an endothelial layer representing the parietal layer of the 

 arachnoid. This inner surface is closely applied to the brain contained 

 within the other two membranes ; and along certain lines it detaches 

 processes which pass inwards, and form partial partitions between the 

 different divisions of the encephalon. These processes are : the falx 

 cerebri and the tentorium cerebelli. 



The Falx Cerebri is a vertical, mesial, sickle-shaped process which dips 

 in between the two hemispheres of the cerebrum. The convex upper 

 edge of the process is attached to the cristagalli process, and to the inter- 

 frontal and interparietal sutures. The concave lower edge is thin and 

 lace-like, and rests free on the corpus callosum. The short posterior 

 edge, or base, is straight, and is attached to the intracranial projection 

 of the interparietal bone. 



The Tentorium Cerebelli is a vaulted partition extending transversely 

 between the cerebrum and the cerebellum. In outline it is crescentic, 

 having a superior convex, and an inferior concave, border. The former 

 is attached on the middle line to the intracranial projection of the inter- 

 parietal bone, and on each side of that its attachment descends obliquely 

 forwards and downwards along the crest formed by the parietal and 

 petrous temporal bones. The concave edge is free, and arches over the 

 crura cerebri. The anterior surface of the membrane is convex, and the 

 posterior ends of the cerebral hemispheres rest on it. The posterior 

 surface is concave, and is in contact with the cerebellum. 



The Sinuses of the Dura Mater — These are venous passages formed 

 by the splitting of the dura mater. They are as follows : — 



The Superior Longitudinal Sinus is of considerable size, and is found 

 in the falx cerebri at its attached or conVex edge. Beginning at the 

 crista galli process, it becomes larger as it passes backwards, and it 

 terminates at the intracranial projection of the interparietal bone.. 



The Inferior Longitudinal Sinus is small and inconstant. It 

 extends along the free or concave edge of the falx ; and, after receiving 

 the veins of Galen, it is continued backwards in the tentorium cerebelli, 

 terminating at the same point as the preceding sinus. 



