HERRICK'S AN INTRODUCTION TO NEUROLOGY 429 



fibers having various or undertermined functions, he has used 

 the term " tractus " for those bundles, the connections of which 

 are known, giving in the second part of the name the origin and 

 termination of the constituent fibers, e. g., (1) Tractus spino- 

 tectal; (2) Tractus tecto-spinalis. It is not always easy to 

 use a nomenclature so defined in such a way as to avoid criti- 

 cism, but the introduction of the distinction which it allows is 

 a definite advance. 



In the presentation before us we find as distinctive features 

 the emphasis placed on the reflex circuit, as the unit of functional 

 response, and the application of the results of comparative 

 anatomy to the appearances found in the human nervous system. 

 Our author is peculiarly well qualified to write from this latter 

 standpoint and he has used his advantage with discretion. From 

 comparative anatomy come several general concepts which do 

 not always appear in the text books. One of these is the concept 

 of nerve components. The older descriptions assumed, for the 

 most part, a uniform function for each of the spinal and cranial 

 nerves. It has been shown, however, that bundles of fibers of 

 diverse function are to be found in most, if not all, of these 

 nerves; that in different vertebrates the same morphological 

 nerve may have a different set of components and also that com- 

 ponents of like functional value may connect with the central 

 system by way of several different nerves. These relations 

 serve to direct attention to the functional groups of fibers as 

 contrasted with the structural groups, and the change in view 

 has proved very helpful. Associated with these studies is the 

 concept of the gray substance of the axial nervous system as 

 comprising four functionally distinct cell columns in each late- 

 ral half. Enumerated from the dorsal to the ventral position, as 

 they appear in a cross section of the cord or medulla, these 

 cell columns are named somatic sensory, visceral sensory, visce- 

 ral motor and somatic motor. The consequences of this arrange- 

 ment are far reaching and are worthy of careful consideration 

 from the standpoint of the clinician. 



Comparative studies show also many remarkable variations 

 in the central cell groups, especially in those of the bulb. Among 

 the lower vertebrates, some forms depend for their principal 

 reactions largely on a single sense and it often follows that this 

 sense organ and its central connections are in such cases almost 



