4 ON THE NERVOUS STSTIM IV. 



A. Cranial Nerves : 



MAMMALS. FKOG. 



I. Olfactory I. Olfactory. 



II. Optic II. Optic. 



III. Motor communis ........ III. Motor communis. 



IV. Patheticus IV. Patheticus. 



V. Trigeminus \ 



VI. Abducens > are combined and form .... V. Trigeminus. 



VII. Facial ) 



VIII. Auditory VI. Auditory. 



IX. Glosso-pharyngeal ) 



X. Vagus > are combined and form . . . VII. Vagus. 



XI. Accessory ) 



XII. Hypoglossal. 



B. Spinal Nerves of the Frog. I. Hypoglossal. 



Brachial. 



II.) 

 II- 1 



Abdominal. 



Crural. 

 Coccygeal. 



From this table it will be seen that the hypoglossal nerve in the Frog occupies 

 a position somewhat anomalous, forming as it does the first pair of the spinal 

 series. If this were added to the cranial nerves, to which it belongs in nearly 

 all other Vertebrates, the whole number would be eight ; namely, three special 

 sense nerves, and two pairs (viz. the motor communis and patheticus) endowed 

 with common motor properties, and three (viz. the trigeminus, vagus, and hypo- 

 glossus) having both motor and sensitive filaments combined. To these 'last Sir 

 Charles Bell applies the term " spinal," and Muller that of " cranio-vertebral." 

 A larger number of cranial nerves has been described -by some anatomists in 

 closely allied Batrachians, but after repeated dissections, as will be seen in the 

 sequel, there seems satisfactory reason, as regards the species here under consid- 

 eration, for adhering to the enumeration given above. 



I. Olfactory Nerves. (Plate I. Fig. 1, i. and Plate II. Fig. 11, B.) These arise 

 from the anterior and under portion of the olfactory lobes, their union with which 

 can only be seen distinctly after the membranes have been wholly removed. On 

 the anterior extremity of the lobe they form a kind of cap, but beneath extend in 

 tfie form of a narrow band, situated near the outer edge, as far as the union of 

 the olfactory with the cerebral lobes, where they terminate in a rounded bulb. 

 From this band extending beneath the lobe, there is given off at its commence- 

 ment another series of fibres, which are directed inwards, separating from each 

 other and forming a brush. The trunk of the nerve is about two or three lines 

 in length, is directed forwards, and each nerve escapes through a separate opening 

 in the anomalous bone, described by Cuvier as the " os en ceinture" and regarded 



