THE AMERICAN 



MONTHLY 



MICROSCOPICAL JOURNAL 



Vol. V. 



Washington, D. C, April, 1884, 



No. 4. 



An Outline Study of Nervous De- 

 velopment. 



BY DR. CHAS. S. DOLLEY. 



It is both interesting and profitable 

 to the microscopist, in connection with 

 a study of structural development, to 

 pursue a parallel study of the develop- 

 ment of function. If, as Spencer puts 

 it, the life of an organism depends 

 upon a continual adjustment between 

 internal and external relations, any 

 change in environment necessitates a 

 corresponding change in habit ; and 

 new habits, when retained, give rise 

 to new tissue specializations. Our 

 understanding of the history of tissue- 

 development will be incomplete, if 

 derived solely from the study of struc- 

 ture. Histology, morphology, and 

 physiology should go hand in hand, 

 and the evolution of function be studied 

 simultaneously with the evolution of 

 form. It is the early stage of nervous 

 development which especially needs 

 elucidation. When once the evolu- 

 tion of nerve-fibres and cells and their 

 • arrangement into simple reflex mech- 

 anisms, is explained and understood, 

 the history of the development of com- 

 pound nei-vous systems may be readily 

 comprehended, and traced through 

 progressive series of centralizations. 



Life implies sensation ; general in- 

 nate sensibility is a necessary attribute 

 of all -living matter ; it exists independ- 

 ent of a nei-vous system or any trace 

 of nerve elements. 



It is this lowest form of sensibility 

 that indicates to the animated organ- 

 ism changes inherent or excited upon 

 it, without perception of the object 

 exciting ; it is necessary to the exist- 

 ence of all other particular sensations. 



and is the direct result of vital action. 

 From this ' obscure, original, innate 

 sensation' arises common sensation, 

 called cce7icesthesis. by which those 

 bodily conditions are revealed to the 

 organism which have their seat in the 

 vegetative life. These states are bodily 

 heaviness or buo3'ancy, atony or toni- 

 ety, hunger, thirst, sexual instincts, 

 etc. By the celebrated Sweed of 

 Upsal, sensibility was considered as 

 the characteristic attribute of animals, 

 and his successors have, in imitation 

 of him, seen in the existence of this 

 property the means of distinguishing 

 between the two kingdoms of living 

 nature, the proof of its duality. A 

 close comparative study of nervous 

 phenomena as exhibited by plants 

 and animals will, however, force us 

 to modify Linne's criterion of animal- 

 ity, and to concede to plants the prop- 

 erty of sensibility, in its general sense 

 of unconscious sentience or percep- 

 tion, and to find as peculiar to ani- 

 mals, and even then only to the more 

 highly differentiated forms, simply a 

 particular mode or manifestation of 

 sensibility known as reflection, atten- 

 tion, or thought. All phj^sic acts in- 

 volve nerve action, but the proposition 

 is not reciprocally true. As evidence 

 of this, and to prove the existence of 

 an innate corporeal sensibility in the 

 simplest forms of life both animal and 

 vegetal, as well as the fact that certain 

 functions generally supposed to be de- 

 pendent upon a nervous and muscular 

 system may be independent of any 

 specialized tissue, we will glance at 

 instances which have been noted by 

 some of the most trustworthy of scien- 

 tific observers. While the phenom- 

 ena described evince discrimination 



