866 



COMPARATIVE ANATOMY 



toward a food stimulus, and the glands which can secrete substances 

 that will repel an enemy serve such a purpose. 



In order, then, that an animal may profit by the various stimuli it 

 encounters, it must have (1), a sensory region or surface of some kind 

 which such stimuli may effect; (2), it must have an organ, such as mus- 

 cles or glands, which will permit a reaction to the stimuli, (3, it must 

 have a conducting mechanism by which the stimulus may be sent to the 

 reacting organs. 



The nerve cells become specialized in structure (or in their man- 

 ner of connection) in three different ways, namely: (1) they may de- 

 velop sensitivity and form organs of special sense. These nerve cells 

 then receive specific stimuli. (2) If the nerve cell develops conductivity, 

 it can transmit impulses, such as sensory, to the central nervous system, 

 or motor from the central nervous system. The conducting parts formed 

 by this second group of specialized neurons form nerve tracts. (3) The 

 third type of specialization of nerve cells is found in the central nervous 

 system itself. This type forms what are called correlation and associa- 

 tion fibers in the sensory field, and coordination fibers in the motor field. 

 In practically all parts of the skin, there are tiny nerve endings, 

 commonly called free nerve terminations, by which we recognize sub- 

 stances when we come in con- 

 tact with them. Then there are 

 certain parts of the tips of the 

 fingers where definite end or- 

 gans are found, and where the 

 sense of touch is quite highly 

 developed. The nerve endings 

 in such special tactile regions 

 are much more complicated 

 than in the simple free nerve 

 terminations. Figure 484 shows 

 some of the various types of 

 these tactile corpuscles. 



In our embryological study 

 we have already discussed the 

 lateral line organs (Figs. 340, 



B, Section "through the cochlea of a guinea pig. 479), which are in all probabil- 



a., ampullus ; etc., anterior canal; c., cochlea; cr., . , , , 



crus; de., endolymph duct; Is., spiral ligament; ity tactile, and probably 



nc. f cochlear nerve; r., Reissner's or vestibular 



membrane ; s., sacculus ; se., endolymph sac ; sg , 



spiral ganglion; sm,st,8v., scalae media (ductus 



cochlearis), tympani and vestibuli ; t, tunnel; u., 



utriculus; v vestibular nerve (From Kingsley, three great divisions of the ear, 



A after Streeter and B after Schneider.) 



namely, an external, internal, 



and middle ear. Of these three portions, the inner ear is the most primi- 

 tive. All lower vertebrates that develop a definite ear organ at all begin 

 by having simply an inner ear (Fig. 19). To this the next succeeding 



Fig. 486. 

 A, Labyrinth of human embryo, 30 mm. long. 



sound-perceiving organs. In 

 the higher vertebrates there are 



