194 DISSECTION OF THE DOG 



the cranial bones, and that it is particularly adherent along the line 

 of sutures. Some difficulty will be experienced in the removal of the bony 

 tentorium, but this may be overcome by the gradual stripping of the 

 membrane from the bone. 



In the region of the foramen magnum the bone between the two occipital 

 condyles should be removed. As this means partial destruction of a 

 connection between the occipital bone and the atlas, note should be 

 taken of the presence of the dorsal atlanto-occipital membrane (membrana 

 atlanto-occipitalis dorsalis), a strong membrane passing from the edge 

 of the arch of the atlas to the margin of the foramen magnum. 



It will be noticed that the dura mater is firmly attached to the edge of 

 the foramen magnum. 



Dura mater encephali. — The brain is enclosed within three membranes, 

 or meninges, of which the outermost, and at the same time the thickest and 

 strongest, is the dura mater. The function of this membrane is a double one. 

 In the first place it affords protection to the brain, which it completely surrounds. 

 In addition it constitutes the periosteum of the inner surface of the cranial 

 bones, as evidenced by the appearance of the membrane when it is stripped 

 from the bones. The exposed surface of the dura in the specimen now being 

 examined presents a more or less ragged appearance, due to the presence of 

 stumps of vessels ruptured during separation of bone from membrane. 



Consequently, the dura mater within the cranium may be regarded as 

 composed of two layers : (1) an endocranial or periosteal layer, and (2) a 

 supporting layer applied to the brain and forming folds between certain parts 

 of this organ. In certain regions the two layers are separated by channels 

 containing venous blood, the blood-sinuses of the dura. One of these sinuses, 

 the sagittal sinus (sinus sagittalis), is easily recognised as it runs in a longi- 

 tudinal direction in the middle line. At the osseous tentorium the sagittal 

 sinus divides into right and left transverse sinuses (sinus trans versi) , which are 

 mainly contained in the temporal meatus, and consequently suffer considerable 

 injury during the removal of the bone. 



A much smaller sinus, the occipital (sinus occipitalis), sometimes difficult 

 of demonstration, lies in or about the middle line behind the tentorium, that 

 is to say, over the cerebellum. 



Dissection. — Raise the dura mater from the underlying brain 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 incisions, 

 and from about the middle of their length, make an incision on each 

 side as far as the cut edge of the cranial bones. 



*o v 



Cavtjm subdurale. — The subdural cavity — a space between the dura 

 and the arachnoid mater — is now opened. It will be observed that the 



