GENERAL CUTANEOUS SUBDIVISION. 121 



to supply the corresponding area of skin. In higher forms the 

 differentiation of the gills and the development of an operculum 

 have led to changes in the general cutaneous components, such 

 that the VII and in some cases the IX nerve are without cutaneous 

 fibers. Then in the occipito-spinal region a variable number of 

 cutaneous nerves have been gathered into the single vagus root 

 and others have disappeared on account of the shifting of the 

 mesodermal organs and shortening of the cutaneous area. Within 

 the brain correlated changes have taken place. A general cuta- 

 neous center probably persists in the forebrain of selachians but 

 little is known of its structure or relations. In other vertebrates 

 the most cephalic cutaneous center is the tectum mesencephali. 

 The ophthalmicus profundus nerve once arose from this segment 

 of the brain and the trigeminus from the cerebellar segment. 

 Both of these regions still receive general cutaneous fibers, at least 

 in lower vertebrates, but both these and the tuberculum acusticum 

 have lost most of the fibers of this component which once entered 

 them. 



All these facts may be expressed or implied in a word by saying 

 that there has been a process of concentration of the tactile appa- 

 ratus of the head toward the caudal part of the cranial region. 

 This has been due in part to the usurpation of the cephalic part 

 of the brain by highly specialized somatic sensory organs, the 

 eyes and acustico-lateral system, and in part to some undefined 

 advantage that is probably gained by the concentration of a 

 system of nerves and centers instead of their being equally distrib- 

 uted segmentally. Such is the general cutaneous system; the 

 most primitive and the least specialized system of nerves and 

 centers, yet progressively more and more modified in its arrange- 

 ment, chiefly through the influence of more highly specialized 

 organs. 



DEMONSTRATION OR LABORATORY WORK. 



1. Review dissections of the dorsal spinal nerves and the trigeminus. 



2. Trace the position and relations of the dorsal tracts, acusticum, 

 and cerebellum in the brain of a large fish, bullfrog and a mammal, by 

 dissection. 



3. Trace the spinal V tract in Delafield haematoxylin or Weigert 



