514 DISSECTION OF THE HEAD. 



The BASILAR SINUS or PLEXUS is a venous network in the sub- 

 stance of the dura mater over the hasilar process of the occipital 

 bone, uniting the inferior petrosal sinuses. 



Arteries of MENINGEAL ARTERIES. These arteries supplying the cranium 



doramater an ^ jj^ dura mater come through the base of the skull ; they are 

 named from their situation in the three fossae, anterior, middle, 

 and posterior. 



The ANTERIOR MENLNGEAL are small branches of the anterior 

 ethmoidal artery, which enters the skull by the anterior internal 

 orbital canal. Its nieningeal branches are distributed to the dura 

 mater over and near the ethmoid bone. 



The MIDDLE MENINGEAL ARTERIES are three in number : two of 

 them, named large and small, are derived from the internal 

 maxillary trunk ; and the third is an onset of the ascending 

 pharyngeal artery. 



Large from a. The large menmgeal artery (often simply called the middle 

 menmgeal artery) from the internal maxillary appears through 

 the foramen spinosum of the sphenoid boue, and divides into two 

 principal branches. The larger of these passes to the deep groove 

 on the anterior inferior angle of the parietal bone, and ends in 

 ramifications which extend upwards to the top of the head and 

 forwards over the frontal bone. The posterior branch is dis- 

 tributed over the hinder part of the parietal and the upper part of 

 the occipital bones. Two rein* accompany this artery. 



- - As soon as the artery comes into the cranial cavity, it furnishes 



dun mater branches to the dura mater and to the ganglion of the fifth nerve. 

 One small offset, petrosal, enters the hiatus Fallopii, and supplies 



and bone. th e surrounding bone. One or two branches pass through the 

 sphenoidal fissure into the orbit, and anastomose with the ophthalmic 

 artery. 



Small ft. The small menmgeal branch is an offset of the large one 



outside the skull, and is transmitted through the foramen ovale to 

 the membrane lining the middle cranial fossa. 



One from c. Another meningeal branch from the ascending pharyngeal 



artery comes through the foramen lacerum (basis cranii). This is 

 seldom injected, and is not often visible. 



The POSTERIOR MENLNGEAL ARTERIES are small twigs of the 

 ascending pharyngeal which enter the skull by the anterior condylar 

 and jugular foramina, and supply the dura mater in that neighbour- 

 hood; also a branch of the vertebral artery is distributed over 

 the lower part of the occipital bone. The branch coming through 

 the jugular foramen is sometimes derived from the occipital artery. 



Nerresof MENLNGEAL NERVES. Offsets to the dura mater are derived 

 from the fifth, tenth and twelfth cranial nerves, and from the 

 sympathetic. 



CRANIAL NERVES (fig. 189, p. 515). As the cranial nerves pass 

 through their apertures in the base of the skull they are invested by 

 processes of the membranes of the brain, which are thus disposed : 

 those of the dura mater and pia mater are continued into the 

 sheath of the nerve ; while that of the arachnoid, except in the 



