MAMMALIA. 
3 
these enigmatical animals is very considerable, and even in the highest of all, namely, the Ver- 
tebrates, there are one or two individuals whose position is but dubious. The best known of 
these creatures is the Amphioxus, a small, transparent fish, not uncommon on sandy coasts. 
In this curious animal the vertebral column is composed of, or rather represented by, a jelly- 
like cord, on which the divisions of the vertebrae are indicated by very slight markings. The 
spinal cord lies on the upper surface of this gelatinous substance, and there is no distinct brain, 
the nervous cord simply terminating in a rounded extremity. The blood is unlike that of the 
generality of Vertebrate animals, being transparent like water, instead of bearing the red hue 
that is so characteristic of their blood. Neither is there any separate heart, the circulation 
seeming to be effected by the contraction of the arteries. 
On account of these very great divergencies from the usual vertebrate characteristics, its 
claim to be numbered among the Vertebrates appears to be a very hopeless one. But the spinal 
cord is found to run along the back of the creature, and this one fact settles its position in the 
Animal Kingdom. 
It must be remembered that the Amphioxus is to be considered an exceptional being, and 
that when the anatomy of Vertebrate animals is described, the words “ with the exception of 
the Amphioxus” must be supplied by the reader. The character of the nerves, bones, blood, 
and other structures, will be shown, in the course of the work, in connection with the various 
animals of which they form a part. 
MAMMALIA. 
The Vertebrated animals fall naturally into four great classes, which are so clearly marked 
that, with the exception of a few singularly constructed creatures, such as the Lepidosiren, or 
Mud-fish of the Gambia, any vertebrate animal can be without difficulty referred to its proper 
class. These four classes are termed Mammals, Birds, Reptiles, and Fishes,— their prece- 
dence in order being determined by the greater or less development of their structure. 
Mammals, or Mammalia, as they are called more scientifically, comprise Man, the Monkey 
tribes, the Bats, the Bogs and Cats, all the hoofed animals, the Whales and their allies, and 
other animals, amounting in number to some two thousand species, the last on the list being 
the Sloth. The name by which they are distinguished is derived from the Latin word mamma , 
a breast, and is given to them because all the species belonging to this class are furnished with 
a set of organs, called the mammary glands, secreting the liquid known as milk, by which 
the young are nourished. 
The number of the mammae varies much, as does their position. Many animals that produce 
only one, or at the most two, young at the same birth, have but two mammae, such as the 
monkey, the elephant, and others ; while some, — such as the cat, the dog, and the swine, 
are furnished with a sufficient number of these organs to afford sustenance to their numerous 
progeny. Sometimes the mammae are placed on the breast, as in the monkey tribe ; some- 
times by the hind legs, as in the cow and the horse ; and sometimes, as in the swine, along 
the abdomen. 
The glands that supply the mammae with milk lie under the skin, and by the microscope are 
easily resolvable into their component parts. Great numbers of tiny cells, or cellules, as they 
are named, are grouped together in little masses, something like bunches of minute grapes, and 
by means of very small tubes pour their secretions into vessels of a larger size. As the various 
tube-branches join each other they become larger, until they unite in five or six principal vessels, 
which are so constructed as to be capable of enlargement according to the amount of liquid 
which they are called upon to hold. In some animals, such as the cow, these reservoirs are 
extremely large, being capable of containing at least a quart of milk. The reservoirs are much 
smaller towards the mamma itself, and serve as tubes for the conveyance of the milk into the 
mouth of the young. Of the milk itself we shall speak in another part of the work. 
The blood of the Vertebrate animals is of a light red color when freshly drawn from the 
arteries. This wondrous fluid, in which is hidden the life principle that animates the being, 
is of a most complex structure, as may be imagined when it is remembered that all the parts of 
