94 INTRODUCTION TO NEUROLOGY 



and whose axon passes onward through a sympathetic nerve to end in the 

 appropriate effector. The nerve-endings of this system are simple or 

 branched free terminals ending on the surface of the muscle-fiber (Fig. 37) ; 

 in the case of heart muscle the fibers usually have expanded tips (Fig. 38) . 



26. End-organs on Glands. The innervation of these organs is in most 

 respects similar to that of the involuntary muscles last described. A fine 

 plexus of unmyelinated fibers of sympathetic origin envelops the smaller 

 glands and pervades the larger ones; these are believed in some cases to be 

 the excito-glandular fibers. 



27. Special Visceral Motor End-organs. The nerves of these muscles 

 have no connection with the sympathetic nervous system. These effectors 

 are striated muscles which may act under the direct control of the will. 

 In their evolutionary origin they are derived from the muscles of the gills 

 of the lower vertebrates, and they are developed embryologically from the 

 ventral unsegmented mesoderm and not from the primitive mesodermal 

 segments which give rise to the somatic muscles. They are found only in 

 the head and neck and their nerve-endings are similar to those of the 

 striated muscles of the somatic series. 



Summary. We have seen that the chief function of the sense 

 organs is to lower the threshold of excitability of the body in 

 definite places to particular kinds of stimulation, and thus to 

 effect an analysis of the forces of nature so far as these concern 

 the welfare of the body. The nature of this analysis of the en- 

 vironing energy complex was illustrated by a review of the ways 

 in which the body may respond to different kinds of vibrations. 

 The senses, as this word is commonly used, were distinguished 

 by four criteria, termed briefly the psychological, physical, ana- 

 tomical, and physiological. Then followed a physiological classi- 

 fication of the receptors and effectors of the human body. 



LITERATURE 



BARKER, L. F. 1901. The Nervous System and Its Constituent Neurones, 

 New York. 



COGHILL, G. E. 1914. Correlated Anatomical and Physiological Studies 

 of the Growth of the Nervous System of Amphibia. I. The Afferent Sys- 

 tem of the Trunk of Amblystoma, Jour. Comp. Neur., vol. xxiv, pp. 161-233. 



VON FREY, M. 1897. Untersuchungen iiber die Sinnesfunctionen der 

 menschlichen Haut, Abhangl. kgl. sachs. Gesellsch., Bd. 40 (Math.-Phys. 

 Classe, Bd. 23). 



HEAD, H., RIVERS, W. H. R., and SHERREN, J. 1905. The Afferent 

 Nervous System from a New Aspect, Brain, vol. xxviii, pp. 99-115. 



HERRICK, C. JUDSON. 1903. On the Morphological and Physiological 

 Classification of the Cutaneous Sense Organs of Fishes, Amer. Naturalist, 

 vol. xxxvii, pp. 313-318. 



. 1908. On the Phylogenetic Differentiation of the Organs of Smell 

 and Taste, Jour. Comp. Neur., vol. xviii, pp. 157-166. 



. 1914. End-organs, Nervous, Wood's Reference Handbook of the 

 Medical Sciences, 3d ed., vol. iv, pp. 20-27, New York. 



