346 PEOF. T. W. EEIDGE ON THE MOEPHOLOGT OF THE SKULL 



the present communication it is intended to refer only to those cranial nerves or their 

 branches which are related to cranial foramina, or to certain definite regions of the 

 skull. The nomenclature of the nerves will be that adopted by Pinkus in the case of 



Protopterus. 



The Olfactory Nerve [i.]. In passing from the olfactory lobes to the nasal sacs these 

 nerves traverse the olfactory passage on either side of the mesethmoid cartilage 

 (PI. XXIX. figs. 13, 14, and 15, i.), and reach their destination after passing forward 

 dorsad to the palatine symphysis. 



The Optic Nerve [ii.] This nerve leaves the cranial cavity through a linear cleft 

 between the ascending process of the palato-pterygoid and the anterior margin of the 

 lateral plate of the fronto-parietal, which is apparently a remnant of the much more 

 extensive anterolateral cranial vacuity of Protopterus, and in the latter Dipnoid also 

 transmits the Optic nerve (PI. XXVIII. figs. 1 and 4, ii.). 



With regard to the nerves supplying the muscles of the eye-ball, viz. the Motor 

 Oculi, the Patheticus, and the Abducens, the earlier writers, among whom may be 

 mentioned Owen, Hyrtl, and Humphry, failed to find any evidence of their existence 

 in the particular Dipnoi they examined. Wiedersheim [41] was apparently the first 

 to discover the Motor Oculi nerve in Protopterus, although at the time he was unable 

 to state with certainty whether it arose independently from the brain or was a branch 

 of the Trigeminus. More recently Pinkus (I. c.) proved that in the same Dipnoid all 

 three nerves are present, and not only determined their precise origin from the brain 

 but also their distribution in the eye-muscles. Similarly with the muscles themselves, 

 Owen [25] asserted that no such muscles were present in Protopterus, and hence the 

 absence of their nerves. Hyrtl (I. c.) described four recti in Lepidosiren, but failed to 

 find the two obliqui. Humphry [13] also found the four recti in Protopterus, but 

 no oblique muscles. Pinkus (I. c), on the contrary, found that the latter Dipnoid not 

 only possessed the usual recti, but superior and inferior oblique muscles in addition. 

 It is therefore extremely probable that all Dipnoi possess a complete series of eye- 

 muscles and also a Motor Oculi, a Patheticus, and an Abducens for their innervation, 

 as in the generality of Vertebrates. In the Paraguayan Lepidosiren there is no doubt 

 as to the existence of the usual six eye-muscles, but, with the exception of the Motor 

 Oculi, I was unable to detect any trace of their nerves. The analogy of Protopterus 

 suggests, however, that my failure to discover the Patheticus and Abducens was 

 probably due to the extreme tenuity and fineness of these nerves rather than to their 

 absence. Unfortunately, lack of the necessary additional material prevented me from 

 resorting to the only satisfactory method of determining with absolute certainty the 

 presence or absence of the nerves in question, viz. by sections of the head and brain. 



The Motor Oculi (iii.). — The third cranial nerve traverses one of the oblique fissures 

 in the ventral margin of the fronto-parietal (PI. XXVIII. fig. 4, iii.), about midway 

 between the foramen for the Optic nerve and that for the ophthalmic branch of the 



