NEKVES OF MAMMALIA. 163 



niyelon are drawn down beyond the cranium. In the Vertebrates, 

 retaining the typical connections of the arch, the homologue of 

 the ' spinal accessory' retains its cranial place of origin, as well 

 as the connections with the ganglionic or sensory part of the 

 nerve. The next cranio-motory nerve, in advance, is that which 

 supplies the muscles of the parietal or third haemal, or hyoidean, 

 arch. Both ninth and spinal accessory have their ganglionic or 

 sensory complement in the ' vagus : ' and, with reference to the 

 place of origin of that nerve, it may be remembered that both 

 heart and breathing organs belong to the head in Fishes. 



The second, or frontal, or mandibular, haemal arch has its gan- 

 glionic nerves from the third division of the fifth, its non-ganglionic 

 by that part of the trigeminal supplemented by certain branches 

 of the ' facial.' The rest of the facial represents the motory por- 

 tion, as the first and second divisions of the ganglionic part of the 

 fifth are the sensory portions of the nerve of the nasal or maxillary 

 haemal arch and its clothing. The ' sixth,' ' fourth,' and ' third' are 

 parts of the cranial motory nerve-system applied to a special organ 

 of sense. 



The myelonal nerves indicate the segments of the axis enclosed 

 in their protecting vertebral rings : both segments and nerve- 

 pairs being called into being according to the requirements of 

 the trunk and limbs of the species. The head-segments and 

 trunk-segments directly succeed each other in Protopteri and 

 Teleostomi (vol. i. pp. 7, 14) ; but in Mammals, as in other air- 

 breathing Vertebrates, neck-segments and nerves are interposed ; 

 and, as the scapular appendage becomes developed into a jointed 

 limb, requiring a more backward position through its size, or one 

 of more freedom for the exercise of various movements, it attracts, 

 as it were, the requisite nerve-force from the successive points or 

 segments of the myelon, and chiefly from a post-cranial or cer- 

 vical portion. 



The development of nerves, as of vessels, is not primary and 

 independent, but secondary and subordinate to the parts needing 

 them. If the appendage of a haemal arch retain its archetypal 

 simplicity, as in Protopterus (vol. i. p. 163, fig. 101), one pair of 

 nerves serves it : if it grows to a maximum of size and number 

 of digital divisions, it may attract its nerve-supply from fifty 

 successive segments of the myelon (lit. pi. xi. Raia batis). In 

 Mammals eight or nine segments succeeding the encephalon 

 minister nervous power to the scapular arch and its appendage, 

 the latter chiefly drawing upon the last three, four, or five pairs, 

 which are proportionally large. 



M 2 



