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 dis- 

 tinguishing characteristics 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 con- 

 siderable, 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 

 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 dis- 

 tinct 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, in- 

 stead 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 be- 

 ing, 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 precedence in order being determined by the greater or less develop- 

 ment of their structure. 



Mammals, 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 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 ; 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 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 



