6 COMPARATIVE ANATOMY 



ourselves, through the means of a special living mechanism, 

 and each such mechanism is called, in technical language, an 

 organ. An organ is not, as a rule, a simple homogeneous 

 substance of like material throughout, but is a composite 

 structure, formed of special kinds of living material, and each 

 kind of living material entering into the composition of an 

 organ is called a tissue. 



Since an animal is locomotory, or at least endowed with the 

 power of movement, we may expect to find a special set of 

 motile organs or tissues, and we find them in muscular tissue. 

 The substance commonly known as flesh is muscular tissue. 

 It exhibits in an eminent degree the phenomenon of con- 

 tractility by which is meant the power of decreasing in one 

 dimension whilst it increases in another. There must be no 

 confusion between the vital muscular contraction and the 

 physical phenomenon known by the same name. Put the 

 poker in the fire, and it will expand as it becomes heated ; 

 remove it and it will contract as it cools. In expansion and in 

 contraction its bulk increases or diminishes in every dimension, 

 and that to so slight a degree that special means are required 

 to measure the change. But nobody ever heard of a poker 

 suddenly, as the result of a shock or blow maybe, diminishing 

 in length to a very appreciable amount, and at the same time 

 swelling up in the middle, so that its bulk remained the same, 

 but its shape altered. This, however, is what a muscle does, 

 and everybody can observe this property in the biceps muscle 

 of his own arm. 



Muscles contract with considerable force, and, by pulling or 

 pushing on or against some fixed object, effect movements of 

 the animal's body. In many animals they are attached to a 

 system of levers, which form a part of what is known as the 

 skeleton of the animal. In ourselves, and in animals allied to 

 us, these levers are internal, and clothed by muscle ; but in 

 many other animals such as crabs and lobsters, spiders and 

 insects, the hard movable parts are outside and the muscles 

 inside. The skeleton, when present, is a very important part 

 of the animal economy, and besides affording a point cFappui 

 for the muscles, it gives support to the whole body and pro- 

 tects some of the delicate organs from injury. Skeleton and 

 muscles taken together form far the greater part of the bulk 

 of the higher animals. 



