HARD Wl CKE 'S S CI EN CE ■ G OS SI P. 



241 



THE HARD PARTS OF ANIMALS. 



By H. F. parsons. M.D. 



"HEM'ords "Hard Parts" 

 I use in their common 

 acceptation, without re- 

 gard to strict scientific 

 homologies. Thus the 

 satin-like skin of an in- 

 fant is homologous with 

 the scaly hide of the 

 crocodile, but for our 

 present purpose, the 

 former may be classed 

 with soft, the latter with hard parts. 



The uses of hard parts are numerous ; the chief 

 are : — 



1st. To protect soft tissues and important organs : 

 thus in many of the invertebrate animals, e.g. the 

 Sea-urchin, Oyster, and Crab, the soft parts are entirely 

 enclosed in a hard shell. Fishes and reptiles are 

 protected by a scaly armour, more or less dense, 

 sometimes, as in the Sturgeon and Crocodile, consist- 

 ing of strong bony plates. In the Turtles, the ex- 

 panded ribs and breast-bone blend with the homy 

 skin to form a carapace or shell, in which the soft 

 parts of the trunk are wholly enclosed. Even in the 

 higher vertebrates, as ourselves, in whom the hard 

 skeleton is entirely internal, we find the most impor- 

 tant vital organs, those which have been called the 

 tripod of life, the brain, heart, and lungs, placed 

 within the bony cases of the skull and thorax. The 

 extremities of the limbs which come in contact with 

 the ground are protected with pads and hoofs. 



2nd. To form a framework or skeleton for the sup- 

 port of the soft tissues. In vertebrates, the true 

 skeleton is internal, in many invertebrates external. In 

 sedentary compound animals, as corals, sponges, and 

 polyzoa, the skeleton serves both to connect the 

 different members of the community together, and to 

 attach the whole compound organism to tlie rock or 

 other substance on which it grows. 



3rd. As levers or passive instruments of motion, 



the active agents being the muscles. Each muscle is, 



as a rule, attached at either end, usually by means of 



a tendon, to some portion of the hard skeleton ; the 



No. 155. 



more fixed point of attachment, or the nearest to the 

 trunk, being termed the "origin"; the more mov- 

 able, or farthest, the " insertion." With few ex- 

 ceptions, the bones in our bodies form levers of the 

 3rd order ; i.e., the power — the muscle — is applied 

 between the joint or fulcrum and the weight. Levers 

 of this kind always act at a "mechanical disadvan- 

 tage"; i.e., a large power moving through a small 

 space is required in order to raise a small weight 

 through a large space. Nevertheless this form of 

 lever is for the purposes of the animal economy the 

 most useful that could be chosen, for the muscles con- 

 tract with enormous force, but through a limited 

 space (about ^ of their length), and it is plainly more 

 convenient for us to be able to move our limbs with 

 a moderate degree of force rapidly over a large area, 

 rather than with irresistible force through a small 

 range. In vertebrate animals, the muscles lie external 

 to the skeleton ; in articulate animals, as the Crab, 

 in which the skeleton is external, the muscles lie 

 inside it. In the Crab, the tendons are bony, and so 

 they are in birds, as any one will have observed who 

 has watched the cook drawing the sinews out of a 

 turkey's leg through the crack of the kitchen door. 



4th. For the seizing and mastication of food. Those 

 animals in which the food is ground small in a strong 

 muscular stomach or gizzard have frequently hard 

 plates or teeth to assist this process. In birds which 

 live on hard seeds, this object is effected by swallow- 

 ing small stones with the food, but some mollusks 

 and some insects, as the Cockroach, have teeth inside 

 the gizzard. About the last animal in which one 

 would expect to meet with a muscular gizzard fur- 

 nished with teeth, is the Flea, living, as it does, 

 wholly on liquid food , but this active little creature 

 is nevertheless so provided. We must infer that, 

 to an animal of that size a blood-coi-puscle is a tough 

 morsel, requiring careful mastication before it can 

 be digested. In most animals, however, the hard 

 organs of mastication are placed in the neighbour- 

 hood of the mouth. The simplest form of teeth is 

 the circle of booklets which surrounds the mouth of 

 some of the Entozoa, e.g., the tape-worm. In the 



M 



