124 Mammalia. Nervous System. 



bellum, called the ' Tentorium ;', and, exceptionally*, a third 

 process between the lobes of the Cerebellum termed the ' Falx 

 minor -J ossification is occasionally extended into both the 

 Tentorium and Falx (major). 



Nerves. Olfactory nerves are present in all saye one Order 

 of Mammals, and are sent off in greater number from the 

 Rhinencephalon than in lower Yertebrata : in one known 

 instance alone does the olfactory nerve quit the skull by a 

 single foramen {i.e. one from each Rhinencephalon), as in 

 Birds and Lizards. 



The Optic nerves, which rise chiefly from the ' nates,' de- 

 velope in almost all Placentalia an oblong nodule of grey 

 matter (corpus geniculatum of An thro.) prior to bending 

 round the outer and back part of the ' thalami,' from which 

 they receive accessory filaments. At their confluence beneath 

 the brain there is no trace of the laminated arrangement 

 commonly found in the 'chiasma' of Birds and E/cptiles : they 

 generally leave the skull each by a special ' foramen opticum'. 

 In those animals which have very small, rudimentary eyes, 

 the optic nerves are very delicate. 



The Sixth Nerve, besides supplying the ' rectus externus,' 

 is also distributed in most Mammals to an additional, ^ sus- 

 pensory', muscle of the eye, not found in Man, viz. the * re- 

 tractor oculi.' 



The Fifthy or Trigeminal Nerve, is frequently very much 

 developed, being commonly the largest of the Cerebral nerves. 

 The size of this nerve varies directly with the perfection of 

 sensitiveness ip. the parts to which it is distributed, not with 

 the proportion of the facial to the cranial part of the head. 

 A distinct gustatory nerve, communicating with a motory 

 'facial' nerve by a 'chorda t^^mpani' is an arrangement charac- 

 teristic of the Mammalian trigeminal nerve. 



* On account of the large vermiform lobe of the Cerebellum projecting beyond 

 its Hemispheres. 



