172 MANUAL OF ANIMAL BIOLOGY 



can be kept alive for the experimental work for some hours. The 

 muscle may next be attached to an immovable body at one end 

 which represents the origin, and to a movable body at the other 

 end, which represents the insertion. The nerve may now be 

 stimulated in various ways, as, for example, by contact, electrical 

 current, or chemical solutions. When the end of the nerve is 

 stimulated a nerve impulse passes through it and into the muscle 

 in the same way, apparently, as if it were normally situated in the 

 body and the muscle received a stimulus from the central nervous 

 system. Furthermore the result of this nerve impulse is the same, 

 for the muscle immediately contracts, thereby causing a move- 

 ment of the lever at its insertion. 



The above experiment demonstrates that living muscle tissue 

 will contract outside the body, and also that the nerve can be 

 artificially stimulated to bring about the muscular contraction. 

 The question as to what actually takes place under such circum- 

 stances in both the muscle and nerve tissue is largely unknown, 

 although it has been the subject of a great deal of experimental 

 work. 



3. Supporting and Connective Tissues 



These widely distributed tissues include a variety of different 

 types, all having as their chief functions the support and pro- 

 tection of the various organs. They are divided into two types, 

 the exoskeletal and the endoskeletal. 



The exoskeletal structures are external, and they develop 

 from both the inner and outer layers of the skin. In some Verte- 

 brate animals, as for example the Turtle, the exoskeleton forms a 

 protective covering over practically the entire body. In other 

 groups of Vertebrates, the exoskeletal structures, while not form- 

 ing a covering over the entire body, are present in the form of nails, 

 hair, feathers, etc. In the Frog, all such exoskeletal structures are 

 lacking in the skin. 



The endoskeletal structures are internal and comprise the 

 supporting and connective tissues, including bone which is regarded 

 as the highest development of the endoskeleton. In general, all of 

 this group of tissues contains a relatively large amount of ground 

 substance, or matrix, which is intercellular in position ; that is to 

 say, this material lies outside the cell walls, between the cells. In 

 some of the connective tissues the ground substance constitutes 



