SO INTRODUCTION TO NEUROLOGY 



7. Organs of Vision. The stimulus is ethereal vibrations ranging be- 

 tween 400,000 billions and 800,000 billions per second. Here also there 

 are two forms: (1) brightness, stimulated by mixed ethereal vibrations; 

 (2) color, stimulated by simpler ethereal vibrations. (On the structure of 

 the eye and its connections see p. 204.) 



Fig. 29. Transverse section through the skin of the ear of a white 

 mouse. The dotted line marks the lower border of the epidermis: a, Hori- 

 zontal nerve-fibers; b, bifurcation of nerve-fibers; fn, cutaneous nerve- 

 fibers. (After Van Gehuchten, from Barker's Nervous System.) 



8. Organs of Smell. This sense has both exteroceptive and intero- 

 ceptive qualities, the latter being apparently the more primitive. (See 

 pp. 75, 91, and 215.) 



B.. THE PnopRiocEPTivK GROUP 



These sense organs are contained within the skeletal muscles, joints, etc., 

 and are stimulated by the normal functioning of these organs, thus report- 

 ing back to the central nervous system the exact state of contraction of the 

 muscle, flexion of the joint, and tension of the tendon. Cutaneous sensi- 

 bility may also participate in these reactions, which are generally uncon- 

 sciously performed. 



9. End-organs of Muscular Sensibility. The organ is a series of 

 nerve-endings among special groups of muscle-fibers known as muscle 

 spindles. These endings are usually spirally wound around their muscle- 

 fibers and are stimulated by the contraction of the muscle (Fig. 30). 



