26 An Introduction to Zoology. 



siderable minuteness^ until beueatli the microscope one is arrived at which 

 admits of no further subdivision, and which is therefore considered as the 

 primary muscular fibre, — not iiowever to be confounded with the still more 

 minute ultimate fibre before alluded to. The primary filament of muscle is 

 said to be about the diameter of a globule of the blood. 



The peculiar property of muscle, which fits it for its office, is contractibility, 

 by whicli property the lateral expansion of the muscle is caused, the two 

 extremities are approximated, and the parts with which they are connected 

 are tlirown into motion. 



Muscles terminate usually in inelastic strings or tendons, by means of 

 which their forces are exerted upon distant points, and the bulky substance 

 of the muscle permitted to occupy any position which may be most 

 suitable. It does not suit o\ir present limits to enter into the various forms 

 assumed by muscles, nor into the laws under which they act ; we may, 

 perhaps, recur to them at some more convenient period. 



Osseous matter is the next combination of which we have to speak. It 

 has been considered by many only as condensed membrane. Its appear- 

 ance as bone, crust, and shell, is familiar to every one. It is a hard un- 

 yielding substance, perJiaps the only one, strictly speaking, solid. The 

 calcareous case secreted by the zoophytes, the shells of tlie testaceous 

 mollusca, the crusts or armour of Crustacea, (lobster, &c.) and the phos- 

 phatic bones of the higher animals are all of this nature. Osseous matter 

 forms the basis of the frame, the points from and against which most of the 

 muscles act, the defence of the soft part, and the usual support of the 

 organs of locomation. In the lower animals this matter is exterior, in the 

 higher interior to the soft parts. 



The last and highest combination of which we shall speak, is the cerebral 

 substance. This is an ash grey, or white medullary substance, of but little 

 firmness, and forming the brain and nervous system. Its ultimate anatomi- 

 cal structure is involved in much obscurity, though it would appear from the 

 observations of Prochaska, the Wenzels, and M. Bauer, as cited by Bostock, 

 that this structure is globular. 



We have thus enumerated four principal combinations of the ultimate 

 anatomical element, by the modification of one or other of which every 

 solid structure in the body is supposed to be formed. We shall next say 

 a few words concerning the composition of the principal fluid, the blood. 



The Huids entering into the composition of the animal body, are very 

 numerous in kind, and constitute a very large proportion of it ; so that a 

 human body weighing eighty pounds, having being dried in an oven during 

 seventeen days, was found, upon being withdrawn, to weigh only twelve. 



In the lower animals there is no apparent distinction of the fluids, but as 

 we ascend in the scale, we find fluids of a totally distinct character con- 

 nected with the different functions of the body, and all, however dissimilar, 

 secreted from one parent fluid, the blood. 



