In studying the build of the various classes of vertebrate animals, and comparing them with man, one must be forcibly struck with the remarkable resemblance they all bear to each other. These animals, man included, are all built upon one general plan. The bony skeleton is simply modified in form to suit the requirements of the different animals. The essential part of the skeleton in all is the spine. It supports the head, carries the ribs and limbs, and contains the spinal cord-the great central nerve.
Our lessons have shown us how this column ofvertebrae is modified in the various animals, especially as regards the manner in which the bones are jointed. In man and the mammals generally, the body of each vertebra rests upon a smooth elastic pad of gristle, to provide for easy springy movement and to avoid friction.
In the snakes these elastic pads give place to ball-and- socket joints. Such joints are essential to these creatures, both for their peculiar locomotion, and for their mode of feeding, by swallowing their prey whole.
The vertebrae of the fish are joined by a double cup- joint, the little hollows thus formed between the bonesbeing filled with a thin fluid, to enable them to play freely, and so add to their flexibility as a whole.
Let us next consider the special development of the head. The head, you know, consists of two parts, the skull and the face. The skull contains the brain, and in man it is this part of the head which is most highly developed.
We have only to compare the head of a man with the head of any of the lower animals, and we shall see that in them it is the face and not the skull, which is specially developed.
From the brain, nerves pass out through special holes in the skull to the organs of sight, smell, hearing, and taste. Most of the lower animals are dependent entirely upon their sense organs, some for their daily existence, others for safety against their enemies. Hence it is that nature specially develops these organs to suit the life and habits of the individual.
The night-prowling flesh-eaters, which stalk their prey after dusk, develop large, powerful, piercing eyes for seeing in the dark. Timid and defenceless animals such as the deer and antelope, the ox and horse, the rabbit and hare, always on the alert against their blood-thirsty enemies, have large, erect, wide-open ears to give them timely notice of danger.
Some animals, again, rely to a large extent on their sense of smell. These are distinguished by their length of face, the nasal passages being specially developed to give greater scope for the nerves of smell.
We have already had occasion to notice the variety of development in the limbs of animals to suit their special style of locomotion.
Man has two pairs of limbs, but although the general plan of structure is the same as in other mammals, he is the one animal that uses only the lower limbs for the purposes of locomotion.
All the land mammals use both pairs of limbs, and even the apes and monkeys depend upon their fore limbs in their movements on the ground.
In birds and in the flying mammals the fore-limbs are built upon the same general plan as in the land mammals, except that the hand portion of the limb is lengthened out to form a wing for the purpose of locomotion through the air.
Among the reptiles, snakes have no limbs, but lizards and tortoises have the usual four limbs. In the latter the bones of the limbs are not immovably fixed like the rest of the skeleton, but are jointed and free to move.
The frog moves by successive leaps, and to provide for this peculiar mode of locomotion the hind limbs are lengthened very considerably. The feet of these limbs, too, are webbed to enable the creature to swim in the water.
Fishes require no limbs for their water locomotion. They propel themselves by the tail, and the fins serve to balance the body.
Lesson 51