CHAP. IV] THE GENERAL ANATOMY OF GORILLA 197 



distinguished consists in the presence of a contribution from the 

 fourth cervical nerve to the brachial plexus. To this plexus, six 

 segmental nerves contribute branches in the Hominidae, Simiidae 

 (except the Orang-utan), and most Cercopithecidae, while the 

 Platyrrhine apes and the Lemurs tend to possess a smaller repre- 

 sentation (viz. from five segments only, and of these the fifth 

 cervical is the most proximal in position). The Orang-utan in 

 this respect stands apart from all other Primates, since no less 

 than seven segments appear in its brachial plexus. 



In the next place, and returning to the special study of the 

 Gorilla, little direct evidence exists to shew that the great 

 plexuses are definitely prefixed as compared with the conditions 

 met with in Man. The evidence is in fact conflicting, and must 

 so remain until a larger number of dissections has been made. 

 Some suggestions of the prefixation of the cervical plexus in 

 the Gorilla are furnished by records given in the preceding list 

 (v. section [A] supra). In the brachial plexus, the median nerve 

 receives a contribution from the fourth cervical segmental nerve 

 and this record might also seem to point in the same direction. 

 Otherwise the available data fall within the limits of human 

 variation, and if the plexuses in Gorilla resemble the prefixed 

 human types, yet they are not more prefixed than these. 



Similar observations apply to the lumbar and sacral plexuses, 

 but here a particular point demands attention. Thus in the survey 

 of the lumbar plexus, if the nerve-roots contributing to such a 

 nerve as the N. obturatorius be considered, it will be found that 

 the segmental " thoraco-lumbar " roots of origin are alike in Man 

 and the Gorilla, those numbered fourteen, fifteen and sixteen. But 

 the Gorilla has commonly thirteen ribs, so that it is an open 

 question whether its fourteenth nerve is the equivalent with the 

 fourteenth in Man. If not, the obturator nerve in the Gorilla is 

 derived (inter alia) from a segment nearer the last rib-bearing 

 vertebra than is the case in Man, and it might be regarded in this 

 manner as being prefixed. The view here taken is that the verte- 

 brae are homologous " by numbers " regardless of the presence of 

 ribs, and consequently no essential difference obtains between the 

 Gorilla and Man in respect of the position of the lumbar plexus, 

 or indeed of the sacral plexus. 



