chap, iv.] SPINAL NERVES: APES. 139 



originate, allusion may therefore be made to some facts of normal 

 structure in differing forms in illustration of the nature of such 

 differences, and for comparison with the differences which are seen 

 to occur by Variation. The arrangement of the lumbo-sacral plexus 

 in the Primates well exemplifies some of these points. In Man, 

 Chimpanzee and Gorilla the 1st sacral vertebra is the 25th ; in 

 the Orang it is the 26th ; in the Baboons, e.g. Macacus inuus 

 (= Inuus pithecus Is. Geoff, the Barbary Ape) it is the 27th. 

 Now, as Rosenberg says, seeing that in Man the sacral plexus 

 receives one whole prae-sacral root, the XXVth, and part of the 

 XXIVth, it might be supposed that this plexus in the Orang 

 would receive two whole prse-sacral roots and part of a third, or 

 that in Macacus it would receive three prse-sacral roots and part 

 of a fourth. But, as a matter of fact, in each of these forms, 

 Chimpanzee, Orang and Macacus, according to Rosenberg, only 

 one whole prse-sacral root and part of the next above it enter 

 the sacral plexus, just as in Man, though the ordinal positions 

 of the nerve-roots are different. 



The Chimpanzee, however, which Rosenberg examined, was 

 the specimen described (No. 34), having the 25th as a trans- 

 itional lumbo-sacral vertebra, and rudimentary ribs on the 21st. 

 In this specimen the prse-sacral nerves received by the sacral 

 plexus were the XXVIth and part of the XXVth, thus bearing 

 the same ordinal relations to the sacrum that the nerves of the 

 lumbo-sacral cord do in the other forms and in Man, though each 

 is ordinally one lower in the whole series than it is in Man. The 

 same was true of the spinal roots composing the obturator and 

 crural. Rosenberg, E., Morph. Jahrb., I. 1876, pp. 148, 149 and 

 Tables, note 19. 



This case is interesting as an example of forward Homu'osis 

 in the vertebrae associated with forward Homceosis in the sacral 

 plexus. When compared with the following case of a Chimpanzee 1 

 having normal lumbo-sacral vertebrae, several discrepancies will 

 be seen beyond those which can be accounted for by the 

 single change of one in the ordinal position of the roots. No 

 doubt for the larger nerves Rosenberg's account is correct, but 

 as he states that the specimen was so badly preserved that 

 the nerves could not be satisfactorily traced, it is possible that 

 some of the branches may have been missed. However this may 

 be, the specimen dissected by Champneys had important features 

 of difference, notably that the sacral plexus received from the 

 XXIInd spinal, while the highest recorded as entering it in 

 Rosenberg's case was the XXVth, a greater difference than can 

 be accounted for on the simple hypothesis of a change of one 

 place throughout. Though, speaking generally, Rosenberg is 

 right in saying that the evidence of the normal condition in 

 Macacus and Orang as compared with each other and with Man 

 1 Champneys, F., Joum. Anat. Phys., Ser. 2, v. 1872, p. 17G. 



