THE NERVES. 381 



are destined for the organs of the senses entirely, the third and 

 seventh pass forth between the anterior and middle, the ninth and 

 twelfth between the middle and posterior cranial vertebrae. 



I. JW. olfactorius, the nerve of Smell. 



Origin : from the basis of the anterior cerebral lobe with three 

 white roots ; the external, longest, passes in a curve from the fossa 

 Sylvii inwards and forwards ; the middle, short, directly forwards, 

 arid the internal rather curved inwards, arises from the substantia 

 perforato, anterior. Each root consists of several smaller. They 

 are united into a three-sided nerve (tractus olfact.) which runs in a 

 sulcus on the inner side of the inferior surface of the anterior cere- 

 bral lobes, is soft, striped with gray, and surrounded by the pia 

 mater. It lies upon the cribriform plate, and terminates with the : 



Buttws cinereus, a club-shaped, flat enlargement, three lines 

 long, and usually of gray substance. This olfactory bulb is, in 

 young embryos and in the lower animals, hollow, and connected 

 with the lateral ventricles. From its inferior surface many deli- 

 cate nn. olfactorii, covered by dura mater , pass downwards through 

 the foramina of the cribriform plate in two rows. 



The external row (from twelve to sixteen nerves) spreads out 

 in a rete upon the mucous membrane of the two superior nasal 

 conchae. 



The internal row (from nine to twelve nerves) expands in tufts 

 upon the mucous membrane of the septum of the nasal cavity, in 

 the centre almost to its floor. 



631. II. IV. options, the nerve of Sight. 



Origin: from thalamus options, the posterior mass of which 

 passes into the optic tracts, corpp. geniculata and quadrigemina. 

 Course: 1. As tractus options (which see). 2. As chiasma n. 

 optic, (which see). 3. From the chiasma the n. options, even now 

 covered by the dura mater, passes forwards and outwards, through 

 foram. opticum into the orbit and to the globe of the eye, the coats 

 of which it perforates behind, rather internal to its axis, and ex- 

 pands as the retina (which see). In the foram. opticum it lies 

 above and internal to art. ophthalmica ; in the orbita, surrounded 

 by the four mm. recti, and much fat ; ganglion ophthalm., nervi 

 et vasa ciliares lie close to it. Its sheath (of the dura mater) 

 passes from the foramen opticum into the sclerotica. It nowhere 

 divides into filaments, but has in its centre a deficiency (porus) for 

 the vasa centralia iridis. 



