342 



COORDINATION AND SENSATION 



Simple Forms of Sense Organs. The simplest form of 

 a sense organ (if such it may be called) is one found among 

 the various tissues. It consists of the terminal branches 

 of nerve fibers which spread over a small area of cells, as 

 a network or plexus. Such endings are numerous in the 

 skin and muscles. 



Next in order of complexity are the so-called end-bulbs. 

 These consist of rounded, or elongated, connective tissue 

 capsules, within which the nerve fibers terminate. On the 

 inside the fibers lose their sheaths and divide into branches, 

 which wind through the capsule. End-bulbs are abundant 

 in the lining membrane of the eye, 

 and are found also in the skin of the 

 lips and in the tissues around the 

 joints. 



Slightly more complex than the 

 end-bulbs are the touch corpuscles. 

 These are elongated bulb-like bodies, 

 having a length of about one three- 

 hundredth of an inch, and occupying 

 the papillae of the skin (Fig. 144). 

 They are composed mainly of connec- 

 tive tissue. Each corpuscle receives 

 the termination of one or more nerve 

 fibers. These, on entering, lose the 

 medullary sheath and separate into a number of branches 

 that penetrate the corpuscle in different directions. 



The largest of the simple forms of sense organs are 

 bodies visible to the naked eye and called, from their dis- 

 coverer Pacini, the Pacinian corpuscles. They lie along 

 the course of nerves in many parts of the body, and have 

 the general form of grains of wheat. (See Practical Work.) 

 The Pacinian corpuscles are composed of connective tissue 



FIG. 144. A touch 

 corpuscle highly magni- 

 fied. (See text.) 



