Herrick, Subdivision of the Brain. 403 



be less than has sometimes been supposed.''^ Furthermore, as we 

 pass down to the lower vertebrates, the cochlear and the vestibular 

 systems not only become progressively less clearly separable from 

 each other, but they are also related to other still less highly dif- 

 ferentiated types of sensory systems, notably the organs of the 

 lateral line, and still lower in the series to the organs of ordinary 

 tactile sensation. In short, the acustencephalon in the lower mem- 

 bers of the phyletic series possesses no individuality, and in the 

 higher members where it is both structurally and functionally bet- 

 ter defined the system as a whole is not structurally sufficiently 

 separate from adjacent parts to justify giving it place in a scheme 

 of regional subdivision, however valuable it may be as a physio- 

 logical unit. 



This suggests a consideration of the relative merits of two mor- 

 phological standards which are not always easily reconciled, viz: 

 the functional system of neurones as contrasted with topographic 

 form relations of the nervous system. The real unit of the nervous 

 system is unquestionably the functional system of neurones and 

 the fruitfulness of this unit in morphology has been well illustrated 

 by the treatment adopted by Barker in his text-book on the Ner- 

 vous System (New York, 1901) and by the results attained by the 

 students of the so-called functional morphology, especially in 

 America. Any topographic subdivision of the brain will be fully 

 satisfactory as a basis for both morphological and physiological 

 work just in proportion as its divisions express the underlying 

 functional relations. But such a subdivision must take also into 

 account the phylogenetic history of these functional units and of 

 the brain as a whole, primary metamerism, the influence of vari- 

 ous mechanical factors which have afi^ected the differentiation of 

 the nerous system and also the practical consideration of anatom- 

 ical and didactic convenience. These factors are seldom in com- 

 plete accord. This indeed is what makes the problem of cerebral 

 regional anatomy so difficult. The relative weight to be given 

 to the different factors of the problem is a matter calling for the 

 exercise of the utmost skill and in the nature of the case the ques- 

 tion will be answered differently by speciahsts in different depart- 

 ments. The preceding discussion of the ophthalmencephalon 



* Cf. the recent results of C. Winkler. The central course of the nervus octavus and its influence on 

 motility. Verh. Kon. Akad. van Wettemchappen, Amsterdam (z Sec), Deel 14, no. i, 202 pp., 24 pi. 

 1907. 



