672 THE NERVOUS SYSTEM 



lobes of the brain gently raised. The following structures will then come into view, viz. the 

 olfactorj' bulbs, the second, third, and fourth nerves, the infundibuluui, and the internal carotid 

 arteries. The olfactory bulbs will come away with the brain, but the other structures willre(|uire 

 to be divided with scissors, as the nerves are frequently torn away from their coiniections by using 

 a scalpel for this jiurpose. The head should next be gently inclined towards the right side, and 

 the tentorium divided close to its attachment to the bone. The sixth, seventh, and eighth nerves 

 should be cut at the same time. This dissection should be repeated on the opposite side. The 

 head should then be tilted backwards, and the remaining cranial nerves, the vertebral arteries, 

 and the commencement of the spinal cord cut through. The latter should be divided as low down 

 as it can be reached with the scalpel. The brain can now be removed from the cranial cavity, the 

 veins of Galen being ruptured by this process. 



The DURA MATER is a tough fil)rous membrane of a blnish-Avhite colour pre- 

 senting externally a rough apj^earance, but internally smooth and shining. It 

 performs the double function of an endosteum to the interior of the skull and of 

 affording support and protection to the brain. In correspondence with this double 

 function it may be regarded as consisting of two layers, an outer or periosteal 

 lamina, and an inner or supporting lamina. These layers are inseparable for the 

 greater part of their extent, but along certain lines the inner lamina leaves the peri- 

 osteal lamina and forms shelf-like projections (of which the falx cerebri and the 

 tentorium cerebelli are examples) into the cranial cavity. Along the lines Avhere 

 these layers divaricate spaces occur which form cranial sinuses. A cranial sinus 

 may be defined as a space formed by the cleavage of the dura mater, lined by a 

 prolongation of the lining membrane of a vein and conveying venous blood. Some 

 of the sinuses are placed between the periosteal and supporting layers of the dura 

 mater — the lateral sinus is a good example of this class. Others (as the straight 

 sinus) are bounded wholly by the inner or supporting layer. 



The cranial sinuses form two principal systems, which, however, communicate with each 

 other. The following is a list of the sinuses which unite to form the larger of these two 

 systems : — superior longitudinal, inferior longitudinal, straight, occipital, superior petrosal. 

 These form a system which converges at the torcular Herophili, a dilated blood-space situated on 

 the right side (rarely on the left) of the internal occipital protuberance. The blood from the 

 torcular is drained away by the lateral sinuses (more particularly by the right sinus), which 

 also receive blood from the cavernous sinuses by the superior petrosal sinuses, and it leaves the 

 cranium through the posterior compartment of the jugular foramen. The straight sinus usually 

 passes into the left lateral sinus. The smaller sj^stem comprises the spheno-parietal, cavernous, 

 circular, transverse, and inferior petrosal sinuses. The blood from this system passes into the 

 internal jugular vein by the inferior petrosal sinus through the atjterior compartment of the 

 jugular foramen. 



Emissary veins. — These veins are possibly extracranial tributaries of the cranial sinuses under 

 normal conditions, but, in cases of engorgement of the sinuses, permit the flow of blood in the 

 opposite direction, and become, in accordance with their name, emissary. The mastoid emissary 

 is the most important of these veins ; it passes through the mastoid foramen to enter the lateral 

 sinus, and explains the value of applying leeches behind the ears in cases of cerebral congestion. 

 Three small emissaries enter the superior longitudinal sinus, one of these passes through the 

 foramen caecum : the others traverse the parietal foramina. An additional vein occasionally passes 

 to the torcular Herophili through a foramen in the occipital bone near the external occipital 

 jirotuberance. Emissary veins pass through the foramen ovale and foramen of Vesalius, and 

 place the cavernous sinus in communication with the pterj'goid venous plexus ; others traverse 

 the cartilage which occupies the foramen lacerinn medium. Small veins enter the lateral 

 sinuses through the anterior and posterior condyloid foramina. A minute venous plexus, which 

 accompanies the internal carotid artery through the carotid canal, establishes a communication 

 between the cavernous sinus and the internal jugular vein. (Eektorzik.) 



The dura mater of the brain is continuous through the foramen magnum with 

 the dura mater of the cord. It is also continuous, through the various foramina 

 in the cranium, with the external periosteum of the skull. This is most easily 

 observed at the sphenoidal fissure, but may be made out with ordinary care at any 

 of the other foramina. It sends a strong prolongation through the optic foramen 

 which splits into two layers, one of which forms the sheath of the optic nerve, and 

 the other is continued into the periosteum of the orbit. On account of the number 

 of foramina at the base of the skull, it follows that the dura mater is more firmly 

 connected to the bone here than at the roof and sides. In the region of the basilar 

 process of the occipital bone, the dura mater splits into sujtporting and periosteal 

 layers. The supporting layer passes through the foramen magnum to become 



