270 THE CAT. [CHAP. ix. 



4. Pathetic (or trochlear). 



5. Tri-geminal. 



6. Abducent ocular. 



7. Facial. 



8. Auditory. 



9. Glosso-pharyngeal. 



10. Pneumogastric. 



11. Spinal accessory. 



12. Hypo-glossal. 



Some anatomists reckon but nine cranial nerves ; for they count 

 the eighth as one with the seventh (calling the facial part its portio 

 dura, and its auditory part its portio motti-s), and reckon the 

 glosso-pharyngeal, pneumogastric, and spinal accessory (all taken 

 together), as their eighth. According to this latter system, the 

 hypoglossal nerve becomes the ninth. 



The cranial nerves generally are said to have two kinds of roots 

 or origins, one deep, the other superficial ; but these are but different 

 portions, or stages, of the same nervous cord. The superficial 

 origin of each nerve is the point where it is obviously attached to 

 the surface of the encephalon, while its deep (or real) origin in- 

 dicates the furthest point to which it has yet been traced backwards. 

 A nerve may be visibly attached to the encephalon by one or several 

 roots. Some of these nerves are what is called " sensory," and others 

 " motor," according as they minister to sensation or to motion. 



9. The FIIIST, or OLFACTORY nerves, are the numerous delicate 

 fibres which pass from the under surface of the olfactory lobes, 

 through the holes in the cribriform plate, down to the membrane 

 investing the nasal septum and ethmo-turbinals. What are, 

 however, often spoken of as the " olfactory nerves " are the olfactory 

 bulbs themselves, with the stalks, or peduncles, which connect them 

 with the under surface of the cerebrum. 



They are composed of grey matter mixed with white fibres, the 

 grey matter being especially abundant in the bulb. Each so-called 

 nerve has at least two roots : 



1. The external root is a broad band of white fibres, extending 

 outwards and backwards along the outer margin of the anterior 

 perforated space to the Sylvian fissure (Fig. 128). 



2. The inner root consists of a narrow band of white fibres, which 

 extends back along the inner side of the anterior perforated 

 space. 



10. The SECOND, or OPTIC, pair of nerves spring superficially 

 from the union of the optic tracts, in what is called the chiasma or 

 optic commissure (Fig. 128). 



Their deep origin may be traced back to the optic thalami, corpora 

 geniculata, and corpora quadrigemina. Fibres arising from these 

 parts converge on each side, and form the optic tract, which runs 

 obliquely across the lower surface of the crus ccrebri of the same 

 side, behind the anterior perforated space to the chiasma. Posteriorly, 

 it is more flattened ; anteriorly, it is more cylindrical. 



