582 



THE CRANIAL NERVES. 



The pia mater and dura mater have both been detected about the seventh or 

 eighth week, at which period the tentorium cerebelli exists. At the third month 

 the falx cerebri, with the longitudinal and lateral sinuses, are perceptible; and the 

 choroid plexuses of both the lateral and fourth ventricles are distinguishable. No 

 trace of arachnoid, however, can be seen until the fifth month. 



II. THE CEREBRO-SPINAL NERVES. 



The nerves directly connected with the great cerebro-spinal centre 

 constitute a series of symmetrical pairs, the number of which has been 

 variously estimated from forty to forty- three. Of these nerves, when esti- 

 mated at the smaller number, nine issue from the cranium through different 

 foramina or apertures in its base, and are thence strictly named cranial. 

 The teuth nerve passes out between the occipital bone and the first vertebra, 

 and the remaining thirty nerves all issue below the corresponding vertebral 

 pieces of the spine. To the whole series of thirty-one nerves the name of 

 spinal is usually given. 



A. CRANIAL NERVES. 



The cranial nerve?, besides being named numerically, according to the 

 relative position of the apertures for their exit from the cranium, have 

 likewise been distinguished by other names, according to the place or 

 mode of their distribution, and according to their functions or other circum- 

 stances. 



The number of the cranial nerves has been variously stated as nine or as 

 twelve by different anatomists ; the difference consisting mainly in this, 

 that, under one system, the nerves which enter the internal auditory 

 meatus, and those which pass through the jugular foramen, are in each 

 case considered a single pair (seventh and eighth) divisible into parts ; 

 while under another system each of the nerves is held to constitute a 

 distinct pair. The following table presents a synoptical view of the cranial 

 nerves under these two modes of enumeration, as in the respective systems 

 of Willis and of Sommerring. 



Olfactory nerves. 



Optic. 



Oculo-motor. 



Pathetic or trochlear. 



Trifacial or trigeminal. 



Abducent ocular. 



Facial motor. 



Auditory. 



Glosso-pharyngeal. 



Pneumo-gastric. 



Spinal accessory. 



Hypoglossal or lingual motor. 



The arrangement of Sommerring is the preferable one, as being the 

 simplest and most natural ; for each of the parts included in the seventh 

 and eighth pairs of Willis is really a distinct nerve. But as the plan of 

 Willis is still in more general use, it will most conveniently be followed 

 here. The cranial nerves will therefore, when not otherwise designated, be 

 referred to as consisting of nine pairs. 



* Willis described the glosso-pharyngeal nerve as a branch of the vagus, and included 

 the suboccipital nerve as a tenth among the cranial nerves. 



