156 PEOFESSOR MARSHALL. 



between the third and fifth nerves ; and therefore, if the fourth is of 

 segmental nature, it must belong to one or other of these nerves. 



The following considerations seem to point very strongly to the 

 third and fourth nerves being connected together, and favour the view 

 that they are together equivalent to a segmental nerve. 



1 . The two nerves in question, the third and fourth, both arise 

 from the mid-brain or middle cerebral vesicle. Furthermore, they 

 are the only nerves that arise from this division of the brain, either 

 in the embryo or the adult. There are independent reasons for 

 thinking that these brain-vesicles have segmental value ; x and though 

 these reasons may not be considered conclusive on the point, they 

 nevertheless lend some support to the view that the two nerves arising 

 from one of these vesicles belong to the same segment. 



2. The third and fourth nerves, though arising separately from the 

 brain, may be connected together more or less intimately beyond their 

 roots of origin. This, for instance, is a marked feature both in Petro- 

 myzon and Lepidosteus, also in Salamandra and Triton, if Schwalbe is 

 correct in identifying the " nasal branch " of the fifth with which both 

 the third and fourth nerves are connected as the ramus ophthalmicus 

 'profundus. 



3. According to Meynert, 2 the third and fourth nerves arise in the 

 adult from a common nucleus. This has, however, been denied by 

 Forel, 3 though supported by other investigators, and probably requires 

 confirmation. 



4. The fourth, though chiefly known as a motor nerve, is really in 

 many animals a nerve of mixed function, giving off in Selachians and 

 Amphibians 4 sensory branches to the conjunctiva and skin of the upper 

 eyelid. This point is of importance, because if the third and fourth 

 are together equivalent to a segmental nerve, it would be only reason- 

 able to expect that certain of its fibres should be sensory ; and analogy 

 would certainly lead us to look for sensory branches in the portion 

 with the more dorsally situated root, i.e. the fourth nerve, which, 

 as we have just seen, does actually present such sensory fibres. 



1 Vide Foster and Balfour, Elements of Embryology, part i., p. 138 ; and Marshall, De- 

 velopment of Nerves in Birds, Journ. Anat. Phys., vol. xi., p. 510. 



2 Meynert, "The Brain of Mammals," Strieker's Histology, New Sydenham Society's 

 Translation, vol. ii., pp. 444, 445. 



3 Forel, Haubenregionen. 



* Schwalbe, Das Ganglion Oculomotorii, p. 14 ; Wiedersheim, Moi-phologische Studien, 

 p. 21 ; and in Ecker's Anatomie des Frosches, p. 24 ; also HoffmanD, Bronn's Thierreich, 

 Bd. vi., p. 203. 



