210 



DISSECTION OF THE DOG 



into a pyriform lobe. A small oral continuation of the lobe is separated from 

 the main mass by a deep transverse depression in which the middle cerebral 

 artery lies. Oral to the depression is the olfactory lobe (lobus olf actorius) , 

 consisting of the olfactory bulb (bulbus olfactorius) from which arises a flattened 

 white band, the olfactory tract (tractus olfactorius). At its posterior extremity 

 the tract divides into 7nedial and lateral striae ; the former disappearing into 

 the fissure between the two hemispheres ; the lateral coursing, as a white 

 band, across the surface of the oral prolongation of the pyriform lobe, and 

 finally disappearing in the prominent part of this lobe. Between the two 

 strise is a grey eminence, the olfactory tubercle (tuberculum olf actorium) , the 

 greater part of which is pitted with holes and thus constitutes the anterior 

 perforated svhstance (substantia perforata anterior). 



Nervi cbeebrales. — Twelve cerebral nerves arise from each half of the 

 brain. These are either named numerically or are given names signifying 

 their function or distribution : 



First cerebral nerve 



Second ,, 



Third 



Fourth 



Fifth 



Sixth 



Seventh , , 



Eighth 



Ninth 



Tenth 



Eleventh ,, 



Twelfth ,, 



Nervus olfactorius. 

 ,, opticus. 



ooulomotorius. 

 ,, trochlearis. 

 ,, trigeminus. 

 ,, abducens. 

 ,, facialis. 

 ,, acusticus. 

 , , glossopharyngeus . 

 ,. vagus. 



accessorius. 

 ,, hypoglossus. 



Each cerebral nerve is connected with cells in the interior of the brain, 

 from which it can be traced to some definite point on the exterior where 

 it has what is generally called its ' superficial origin.' The superficial origins 

 of the various nerves should now be determined. 



N. OLFACTOEiTJS. — Numerous small olfactory nerves leave the olfactory 

 bulb and immediately enter the foramina of the ethmoid bone. They are 

 generally left behind when the brain is removed. 



N. OPTICUS. — The optic nerve is a large round cord leaving the optic chiasma. 



N. octTLOMOTOEiTTS. — The oculomotor nerve has origin from the cerebral 

 peduncle about the middle of its length and towards its medial border. 



N. TEOCHLEAEis. — Since the origin of the trochlear nerve is from the 

 anterior medullary velum, it cannot be demonstrated at present. The nerve 

 itself, however, Avill be found curving round the lateral border of the cerebral 

 peduncle, in the interval between this and the cerebrum and cerebellum. The 

 trochlear is the smallest of the cerebral nerves. 



