376 ANATOMY. 



it is not, becoming the skeleton of some animals throughout life 1 , — that of 

 both kinds is a very uniform mass, breaking equally in every direction. 

 The one which is changed for bone is vascular, and when going to be 

 changed becomes more so, these vessels having now more to do 2 . 



The horny skeleton is truly animal, and is placed principally on the 

 outside of the animal, by which means it is kept dry, which renders it 

 stronger under the same quantity of matter. In the spade-tailed cuttle 

 [Loligo] there is a horny or tortoiseshell blade that runs through the 

 whole back 3 ; besides which there is a cartilage on the anterior end of 

 the back. In the common horny skeleton the muscles are placed on the 

 inside, by which means it becomes a compound part of the animal, serving 

 equally as skin and skeleton, and often other purposes. It is common 

 but not peculiar to the insect : it constitutes the scales [rings] or ex- 

 ternal covering of every flying insect, to which the muscles are attached ; 

 besides which it sends inward horny processes for the further attach- 

 ment of muscles ; just as bones send out processes for the same purpose. 

 I believe in the insect this substance is almost the sole, having very 

 little of the ligamentous, and, I believe, none of the cartilaginous 

 [substance combined with it]. We have no generic term for this sub- 

 stance 4 . I believe it is fibrous in all ; growing from an end bike hair, 

 or from the edge like scales, according to its form. But when it acts 

 simply as a cuticle, I believe it grows from a centre, or all round the 

 edge, as in the shell 5 of the tortoise or turtle. 



The bony skeleton belongs commonly to the higher orders of animals ; 

 although it is introduced into some of the lower, where firmness, 

 strength and determinate motion, with great variety, are wanted. As 

 it is internal, and of course always kept moist, it is a fitter substance 

 for those purposes than horn; for horn, when moist, is elastic and 

 yielding, and therefore when used is external. Bone is not the original 

 skeleton in any animal, but only of the adult ; for in the first formation 

 of any animal, which afterwards is to have bone, the skeleton is either 

 membrane or cartilage, which is changed for bone, but not into bone. 



The gradations of this change are beautiful. "When the animal has 

 no locomotion, nothing to support, no action of parts but such as im- 

 mediately concern life, we find no bone. But as the young animal advances 

 towards that period in which it is to take a scope of bodily action 



i [Hunt. Preps. Phys. Series, Nos. 78, 230—237.] 



2 [lb. Nos. 133 — 162. The above is an outline sketch of a histological chapter on 

 cartilage, which appears not to have been written.] 

 s [lb. No. 77.] 



* [It is now called ' chitine,' having a different composition from horn.] 

 5 [Tins is true 'horn.'] 



