MAMMALIA. 



warm blood, driven by a four- chambered heart through a double circalatory 

 system ; one portion of the blood being propelled through the lungs for the 

 purpose of re-oxygenation, while a second portion is sent through the body 

 by means of the general circulation. Whereas, however, in Mammals, the 

 aorta, or great vessel, by which the blood passes from the heart to the body, 

 crosses the left branch of the windpipe, in Birds it crosses the right. At no 

 stage of their existence do Mammals ever breathe by means of gills ; lungs, 

 even in the aquatic forms, being the sole respiratory agents. Neither do 

 they ever pass through any kind of metamorphosis, such as takes place in the 

 Amphibia. As a general rule, the young are born in a living, and frequently 

 in a more or less helpless condition ; but in the very lowest members of the 

 class the female lays eggs. 



With but very few exceptions, Mammals have seven vertebrae in the neck, 

 and thereby present a remarkable contrast to Birds and Reptiles, in which 

 the number is exceedingly variable. In all the terrestrial forms the two 

 pairs of limbs are fully developed ; but in the marine Cetaceans and Sirenians 

 the front pair are modified into paddles, and all external traces of the hinder 

 ones completely lost, while the body has assumed a fish-like form. In the 

 Bats alone are the fore limbs structurally modified to subserve the purpose 

 of true flight, like that of Birds. 



As in the following pages reference is often made to various bones of the 

 skeleton, this part of the subject cannot be passed over without brief mention. 

 The fully developed skeleton, as shown by the accompany- 

 ing diagram, comprises a skull (1), a jointed breast-bone, or 

 sternum (6), a still more numerously jointed backbone, the ribs (2), the limb- 

 bones, and the girdles by which they are suspended to the backbone and its 

 neighbourhood, and a pair of collar-bones, or clavicles (c). As already men- 

 tioned, the skull proper, or the skull without the lower jaw, is termed the 

 cranium ; this consisting of a number of (mostly paired) bones, articulated im- 

 movably together at their edges. The hinder portion of the cranium encloses 

 the brain ; while the front part, or facial region, contains the mouth and organs 

 of smell. Of especial importance are the bones forming the upper jaw, which 

 comprise in front a pair of premaxillse, containing the front or incisor teeth ; 

 behind which are the maxillae, containing the tusks and cheek-teeth. The lower 

 jaw has been already mentioned sufficiently. In the backbone the cervical, 

 or neck -vertebrae are recognised by having no ribs attached to them ; behind 

 these follow the dorsal vertebrae, each carrying a pair of movable ribs, of 

 which the more anterior are connected with the breast-bone ; and these in 

 turn are succeeded by the lumbar, or loin-vertebrae, to which no movable 

 ribs are articulated. Behind the lumbars comes the solid mass of several 

 united vertebrae, known as the sacrum, to which are firmly articulated the 

 haunch-bones, or ilia (), forming the upper part of the pelvis (3). Behind the 

 pelvis are the bones of the tail, or caudal vertebrae, which may be either very 

 few or very numerous. In the higher Mammals the shoulder-girdle proper 

 is formed only by the two blade-bones, or scapulas, which overlie the anterior 

 ribs without any attachment to the backbone, and have at their lower end a 

 shallow cavity for the reception of the head of the uppermost bone of the 

 arm or fore-lirnb. When fully developed, the collar-bones are articulated by 

 one extremity to the blade-bones, and by the other to the upper segment of 

 the breast-bone. Certain other bones entering into the composition of the 

 shoulder-girdle of the lowest representatives of the class, will be noticed 

 under that group. In the skeleton of the fore-limb, or arm, we have the upper 



