2o6 Journal of Comparative Neurology and Psychology. 



less such variations of this nerve cannot be regarded as iierfedly 

 homologous with each other. 



(3) Perfect meristic or serial homology (homodynamy) requires 

 that the members of the series shall repeat the same pattern, both 

 in the components represented in the roots and in the peripheral 

 distribution. Typical illustrations of this serial homology are seen 

 in some of the segmental nerves of annelid worms and in some of 

 the spinal nerves of lower vertebrates. The human body probably 

 presents few such exact repetitions of a segmental pattern except 

 in some peripheral rami of a part of the spinal nerves. 



(•i) In general, homology requires that the nerves concerned 

 shall have similar segmental relations, similar components and simi- 

 lar distribution. 



(5) If, however, in the course of the phylogeuy a new component 

 is differentiated within a given segmental nerve from a more ancient 

 unspecialized element, this nerve does not thereby lose its homology 

 with the primordial unspecialized nerve ; for the genetic relation- 

 ship remaius unbroken. Accordingly, if it should prove (as seems 

 probable) that the gustatory component of the facial nerve was in 

 the early phylogeuy differentiated from the preexistent unspecial- 

 ized visceral system of that segment, no disturbance of the homolo- 

 gies would result. 



(6) Peripheral rami which are defined primarily with refer- 

 ence to other peripheral non-nciTOUS organs, like the sciatic nen'e, 

 may be regarded as homologous in different animals so long as 

 they possess the same functional components and maintain essen- 

 tially the same relations to the organs with reference to which they 

 are defined, even though the segmental relations of the roots and 

 plexuses from which they are derived may vary. 



(7) But if any peripheral nerve is a ramus by definition of 

 a definite segmental nerve (such as the r. hyoideus facialis), its 

 homology is not perfect unless in the types compared it is com- 

 posed wholly of fil)ors derived only from its own segment. Any 

 admixture of fibers from another segmental nerve to that extent 

 destroys the honiologj^, no matter how perfectly the mixed nerve 

 may follow the same course as the unmixed nerve. For instance, 



