HEAD AND NECK OF THE HORSE 139 



mater} This membrane has a double function. It affords protection 

 to the brain, which it completely surrounds, and it constitutes the 

 periosteum of the inner surface of the cranial bones. The latter 

 function is indicated by the character of the outer surface of the stripped 

 dura, which presents a more or less ragged appearance, due to the 

 presence of stumps of vessels that have been torn across during the 

 separation of the bone from the membrane. It is, therefore, customary 

 to regard the dura mater of the cranium as composed of two layers : 

 (1) an endocranial or periosteal layer; and (2) an inner layer applied to 

 the brain and forming supporting folds between certain parts of this 

 organ. In some regions the two layers are separated by venous 

 channels, the blood-sinuses of the dura mater. These will be examined 

 later, but the position of some of them should be noted at the present 

 time. One is easily recognised as it runs in a longitudinal direction in 

 the middle line. This is the dorsal sagittal sinus - (sinus sagittalis), 

 which passes backwards to near the osseous tentorium and there joins 

 the straight sinus (sinus rectus) at the confluence of the sinuses 

 (confluens sinuum). Right aud left transverse sinuses (sinus trans- 

 versi) are also visible now as they run in a transverse direction into the 

 temporal meatus on each side of the skull. From their association with 

 the cranial bones they have suffered injury during the removal of bone. 

 The relatively small and plexiform occipital sinuses (sinus occipitales) 

 lie over the cerebeUum behind the osseous tentorium. 



Dissection.— ^aisa the diua mater from the underlying brain with a 

 pair of forceps and make as long an incision as possible through it on each 

 side of and parallel to the sagittal and occipital sinuses.- At right angles 

 to the first incision.s, and from about the middle of their length, make an 

 incision on each side as far as the cut edge of the cranial bones. These 

 incisions open into the subdural cavity and make possible an examination 

 of the falx cerebri. 



The subdural cavity (cavum subdurale) is a potential space between 

 the dura and the arachnoid mater. The opposed surfaces of the two 

 membranes are smooth and glistening, and moistened by a small 

 quantity of serous fluid. 



The falx ^ cerebri is one of the folds of the supporting layer of the 

 dura mater. It lies in the median plane between the two hemispheres 

 of the cerebrum, and is in the form of a sickle with its point at the 

 crista galli of the ethmoid and its base at the osseous tentorium. The 

 thick convex border of the fold is attached to the middle line of the 



1 Durns [L.], hard, tough. Mater [L.], mother. 



- Sagitta [L.], an arrow (sagittal, in the line of an arrow shot from a bow). 

 Simis [L.], a curved surface, a bay, a gulf. 



2 Falx [L.], a sickle, a reaping-hook. 



