NERVOUS SYSTEM. 243 



While the great nervous centres have thus arrived at their 

 maximum of development in mammalia, most of the cranial 

 nerves, like the spinal chord, when compared to them, are 

 proportionately small. The olfactory nerves and tubercles 

 are largest in the ruminantia and pachydernia (99. A. k } ) 

 smaller in the carnivora (99. B. i,) cheiroptera, and quadru- 

 mana, and are scarcely discoverable in several of the cetacea. 

 In the inferior orders and in the large eyed nocturnal qua- 

 drupeds the optic nerves are of greater size than in diurnal 

 and higher species, and in the blind subterranean moles not 

 only are the optic nerves extremely minute, but the motores 

 oculorum,. the trochleares, and the abducentes can scarcely 

 be detected. The optic nerves in this class unite before the 

 infundibulum, where they form a partial decussation of their 

 component fibres (99. B. #,). and the ophthalmic ganglion is 

 always perceptible in the orbit. The second and third 

 branches of the trigeminus have a great external distribution 

 in the long-muzzled, the proboscidian, and the large-lipped 

 quadrupeds, as the cetacea, ruminantia, pachyderma, and car- 

 nivora, and their internal dental distribution varies according 

 to the number and magnitude of the teeth and of the per- 

 forated fangs which they supply. Their development is also 

 influenced by the presence of horns on the frontal or nasal 

 bones, or of spines or bristles extending from the upper lip, 

 as in aquatic and terrestrial carnivora, or by other circum- 

 stances which influence the general form of the head or face ; 

 and the same causes influence the development of the facial 

 nerves and their branches, as in the inferior classes of verte- 

 brated animals. The bills of the ornithorhyncus, like those of 

 many aquatic birds, are supplied with large branches of the 

 superior and inferior maxillary nerves, and the pneumo-gas- 

 tric nerve in this animal is not united to the cervical portion 

 of the sympathetic, as it generally is in the neck of quadru- 

 peds. As the plan of development is most constant and ob- 

 vious in the great centres of animal and of vegetative life, 

 the spino-cerebral axis and the great sympathetic, it is chiefly 

 in those parts that we observe the highest condition arrived 

 at in the human body. The spinal chord of man (Fig. 100. 

 b 9 i,) is smaller compared with the cerebral mass contained 

 within the cranium than in other mammalia, and short from the 

 want of caudal prolongation of the trunk ; its posterior and 



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