306 PRACTICAL LESSONS IN SCIENCE. 



ive external organs, and make a special study of those organs. 

 Open the spinal canal and study the spinal cord; especially no- 

 tice the origin of the spinal nerves. Study the bones, ligaments, 

 muscles, tendons, joints, etc., in the legs, and in the spinal col- 

 umn. This study should be carried into detail as far as time 

 and circumstances will allow. If it is inconvenient to get animals 

 for dissection, material for an extended study may be obtained 

 at the slaughter-house or butcher shop. 



The Mammalia in general have a skeleton composed of com- 

 pact bones filled with marrow, and the vertebral column is gen- 

 erally divided into five well-marked regions the cervical, dorsal, 

 lumbar, sacral and caudal. There are two pairs of limbs with 

 the pectoral and pelvic arches well developed. The feet are es- 

 pecially interesting from the variety of forms under which the 

 same group of bones appears. The digestive organs are well de- 

 veloped in all, but they vary widely with the nature of the food 

 used by the animal. In nearly all cases the jaws are armed with 

 bony teeth, differing in form, number and arrangement with the 

 kind of food. The heart has four chambers and the circulation 

 is double. The lungs are capacious and free to move in a special 

 cavity, and the skin is usually covered with hair. The brain is 

 relatively large and usually covered with convolutions, and the 

 special senses are all highly developed. They easily lead the ani- 

 mal kingdom in matters of intelligence, and for this reason they 

 are more useful and companionable to man. 



There is considerable variation in the reproductive process, 

 but the young are always born alive, and need the mother's care 

 for considerable time afterward. 



The Monotremata. This order includes the ornithorhyncus, 

 or duck-bill, and the echidna, or porcupine ant-eater, both living 

 in Australia. They have short legs armed with strong claws 

 well adapted for burrowing. The urinary and reproductive or- 

 gans open into the intestine, as with birds. The jaws are elon- 

 gated into toothless beaks. The duck-bill is clothed with fur, 

 has a smooth brain, a flattened body and swimming feet. The 

 echidna is covered with spines, has a protrusible tongue and 



