CHAPTER II THE PLAN OF THE HUMAN BODY
Tue human body is com- posed of a head, a trunk, and two pairs of limbs. It is sup- ported by a skeleton, the most important part of which is the spinal column, or backbone. In the head are eyes, ears, a nose, anda mouth. The body has in it two cavities, a dorsal or back cavity, and a ventral or front cavity. In these two cavities are found most of the organs of the body.
The Dorsal Cavity. In the head is a great cavity, and opening out of this cavity at the base is a long passage- way that runs through the spinal column from top to bottom. The cavity in the FiG.8, The cavities of the body. The head and the canal in the
dorsal cavity isin the head and the spinal backbone, taken together, are column, ‘The ventral cavity is in the front ’
ofthe trunk and is divided by thediaphragm the dorsal’ cavity. In this into an upper and a lower part. cavity lie the great centers of
the nervous system, the drain and the spinal cord.
1 The name dorsal comes from dor!sum, the Latin word for Sack. In the lower ‘animals it is easy to see that the cavity in the head and the canal in the spinal 16
THE PLAN OF THE HUMAN BODY 17
The Ventral Cavity. The ven- tral cavity is a great hollow in the front part of the trunk. Stretched across it is a thin sheet of muscle, called the dia- phragm, which divides it into an upper and a lower part. The upper part is the chest or thoracic cavity, It contains the heart and lungs and many of the great blood vessels. The lower part is the abdominal cavity. Inthe left side of this cavity, its outer end close up under the dia- phragm, lies the stomach. On the right side of the body and partly covering the stomach is the liver. The intestine is very long and is coiled again and again in the abdominal cav- ity, filling most of it. Attached to the back walls of the cavity are the two kidneys, which take waste matter out of the body. At the left end of the
FiG.9. Section of the body show- ing the positions of the organs in the cavities.
stomach is a dark red organ called the sp/een. Along the lower back part of the stomach is the pancreas, a very im- portant digestive organ, whose work we must take up in
another chapter.
Man’s Place in the Animal Kingdom. Man has a spinal
column are all part of one long cavity that runs along the back of the body and widens out at the head end to make room for the brain,
18 HUMAN PHYSIOLOGY
column, and therefore belongs among the ver- tebrates,1 or backboned animals. He has hair, and when young lives on milk, and therefore he belongs among the mammals, the highest class of the vertebrates. The five classes of ver- tebrates are the fishes, amphibians, reptiles, birds, and mammals, By studying Figure 11, you can learn some of the animals that belong in each class, and which | : animals are closely Fic. 10. The angen Sereatal savilys00n lated to man.
The Bodies of Verte-
brates Similar. A fish, a frog, a lizard, a bird, and a cat do
1 The vertebrates differ from worms, insects, and other lower animals in having backbones. The five classes of vertebrates have the following distinguishing characteristics :
Fishes live in the water and breathe by means of gills.
Amphibians have sticky skins, and both lungs and gills, Some salamanders have lungs and gills at the same time and can breathe either in the water or in the air, Other salamanders have gills at one time and lungs at another. Frogs and toads have gills in their early life and lungs in their later life.
Reptiles have scaly skins and breathe by means of lungs. Like the fishes and amphibians, reptiles are cold blooded.
Birds have wings and a body covering of feathers.
Mammals have the body partly or entirely covered with hair and feed their young with milk. The mammals and birds are warm blooded.
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‘The vertebrate animals, Man stands at the head of the mammals, the highest class of the vertebrates.
Fig, 11.
20 HUMAN PHYSIOLOGY
not seem much alike; and because man walks upright, the human body seems very different from the bodies of all these animals. Yet man and all other vertebrate animals are built on the same general plan.
Like man, all the vertebrate animals have a head and trunk, with eyes, ears, and a mouth in the head. All of them have a dorsal and a ventral cavity in the body, with in general the same organs in these cavities as are found in man. All have skeletons resembling the human skeleton, not only in the spinal column, but also in many other parts, as we shall see later. All of them have two pairs of limbs corresponding to the arms and the legs of man. Ina fish, the limbs are the two pairs of fins found on the sides of the body. In the seal and the whale, they are paddles for swimming. In most other animals, they are the fore and hind legs, but in bats and birds the fore limbs are wings, and in man they are arms. Most snakes have lost their limbs, but in some of the great snakes the remains of little legs can be found, and in other reptiles the limbs are well developed.
How Man differs from Other Vertebrates. The brain of man is better developed than is the brain of any other animal, and in many ways the human body differs to a certain extent from the bodies of other vertebrates. But the great difference between man’s body and the bodies of other animals is that man is built to walk erect. Instead of carrying the head in front of the body and walking on all four limbs in the position that a fish is in when it swims or a cat is in when it walks, the human body stands on the hind limbs, with the body erect and the head above the body. When the body stands upright the fore limbs do not touch the ground. Therefore, in man the fore limbs are not fitted for walking, but are arms and have hands for grasping.
THE PLAN OF THE HUMAN BODY 2u
Summary. The human body is composed of a head, a trunk, and two pairs of limbs. It has in it a dorsal and a ven- tral cavity. The dorsal cavity contains the brain and the spinal cord; the ventral cavity is divided by the diaphragm into the thoracic cavity and the abdominal cavity. The tho- racic cavity contains the heart, the lungs, and many great blood vessels. The abdominal cavity contains the stomach, intestine, liver, spleen, pancreas, and kidneys.
Animals with backbones are called vertebrates. Vertebrates are divided into five classes, — fishes, amphibians, reptiles, birds, and mammals. Man has a backbone, and is a verte- brate. He has hair, and when young lives on milk; he is therefore a mammal.
The bodies of other vertebrates are built on the same plan as the human body. Every vertebrate has a head, trunk, eyes, ears,and a mouth. In the body are a dorsal cavity and aventral cavity. It has a skeleton with a backbone, and has two pairs of limbs corresponding to our arms and legs.
Man’s body differs from other vertebrate bodies chiefly in that it stands erect. The fore limbs do not touch the ground in walking, and are arms with hands.
QUESTIONS
Name the principal divisions of the human body. How is the body supported? What two cavities are in the body?
Where is the dorsal cavity? What does it contain? Into what two parts is the ventral cavity divided? What is the partition be. tween these parts called? Name the organs in the thoracic cavity ; in the abdominal cavity. Locate the stomach. Locate the liver.
Name the five classes of vertebrates. To which class does man belong? How does this class differ from other vertebrates?
22 HUMAN PHYSIOLOGY
Give four ways in which the bodies of all vertebrates are similar. Mention some different kinds of vertebrate limbs. How does man differ from other vertebrate animals?
What animals do you know that are not vertebrates? Can you name an animal that has no head? one with no mouth? one with no eyes? one with no ears? Name some animals that differ from verte- brates in the number of their limbs.
How does a fish differ from other vertebrates? What is the dif- ference between a shark and a true fish? How does an amphibian differ from other vertebrates? a reptile? What are the four reptile groups? What do you know about the reptiles that lived in former ages of the world?
Which is the lowest mammal shown in Figure 11? In what way is this mammal like birds and reptiles, and different from the mam- mals of other groups? How do marsupials differ from other mam- mals? In what country is the sloth found? Name some mammals that live in the water. Do these mammals have lungs or gills? On what do the ungulates (hoofed animals) feed? On what do the carnivora feed? the primates?
Can you name some relative of the seal? Where do most of the weasel family live and what is obtained from this group of animals? Name some members of the cat family not shown in Figure 11 ; of the dog family. What small North American animal is closely related to the bears? Name some rodents (gnawers) that are larger than any of the rodents shown in Figure 11. Draw in a larger form the ungulate branch of the vertebrate tree, putting on it all the hoofed animals that you know. Do you know any families of mam mals not shown in Figure 11?