MAMMALIA. 3 
The position of the spinal cord is always along the back in every Vertebrate animal 
The insects, the lobster, and other invertebrate animals exhibit the principal nerve-cords 
running along the abdomen ; the position, therefore, of the chief nervous cord settles the 
division to which the animal belongs. This rule is of great importance in classification, 
because in every group of animals there are some in whom the distinguishing character- 
isties are so slight that they hardly afford a real criterion by which to judge. In the 
lower divisions the number of these enigmatical animals is very considerable, and even in 
the highest of all, namely, the Vertebrates, there are one or two individuals whose 
position is but dubious. The best known of these creatures is the Aimphioxus, 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 vertebrie are indicated by very shght markings. The spinal cord les on the upper 
surface of this gelatinous substance, and there is no distinet 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 ereat 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 
connexion 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, Birbs, REPTILES, 
and Fisies,—their precedence in order being determined by the greater or less develop- 
ment of their structure. 
Manuals, or Mammalia, as they are called more scientifically, comprise Man, the 
Monkey tribes, the Bats, the Dogs 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 nourish dl. 
The number of the mamme 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 
mammie, 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 mamme are placed on 
the breast, as in the monkey tribe ; sometimes 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 mamme with milk lie under the skin, and by the 
microscope are easily resoivable 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 
B2 
