SKELETON. 375 



answer the purpose of a defence from external violence to the contained 

 parts. 



As the membranous skeleton is not stretchable, or has a sufficient 

 firmness in texture not to yield beyond a necessary extent to the natural 

 actions of the animal, and as, in such, the sections of motion are short, 

 a pretty regidar form is preserved ; for in every natural action there 

 is such a relationship depending upon it, that no distortion of parts 

 takes place, and only external violence deranges the form. A worm is 

 just as regularly a formed animal as any other, although it varies more 

 than those whose forms are more determined by [the harder nature of ] 

 the skeleton. 



The cartilaginous skeleton differs from the former in the nature of 

 its substance, as in the consistence whereby it retains its form ; but 

 there are considerable differences in the degree of consistence, and of 

 course in the power of retaining form. This substance is introduced in 

 various ways, but seldom alone, as we find in the membranous skeleton ; 

 in some animals there is more of it, and in some less, when the skeleton 

 approaches nearer-to the membranous. Cartilage is used as an external 

 covering, like a shell, as in those [Saljoa, Ascidia] which I have called 

 the ' soft-shelled animals 1 .' 



Cartilage is used in the body of some animals as a fixed point for the 

 muscles to act from. And as it is not so yielding as the membranous 

 skeleton, it is composed of parts which are united to each other, ad- 

 mitting of motion in those parts, and determining with more exactness 

 the places of motion ; although not perfectly, as it is elastic, yielding 

 and recovering without the aid of antagonizing muscles. These unions 

 mostly consist of membranes filling up the space between each cartilage ; 

 although in some the membrane makes a capsule. 



This mode of introduction of cartilage is principally in fishes ; and in 

 some parts of other animals, such as the cartilages of the ribs in man. 

 It gives more stability to the shape than membrane could, and admits 

 of more variety of shape in animals. 



Cartilage. — This is semitransparent ; of various consistence according 

 to its use ; and is commonly of a determined shape or outline, seldom 

 losing itself insensibly in the surrounding parts. Cartilages are of two 

 kinds respecting the power of being changed for bone ; one where it is 

 forming the skeleton of many animals only before birth ; the other where 



1 [They answer to the Mollusca tunicata of Lamarck and Cuvier. The external 

 skeleton, shown in the Hunt. Prep. Phys. Series, No. 76, is a dense gelatinous mem- 

 brane, containing ' cellulose : ' not true cartilage, but resembling it in physical 

 properties.] 



