66 GENERAL ANATOMICAL CHARACTERS 



on one side, into which the splenic artery, a branch of the coeliac 

 axis of the abdominal aorta, enters, and from which the vein joining 

 the portal system emerges. The spleen varies much in size and form 

 in different mammals, being relatively very small in the Cetacea. 

 It is sometimes almost spherical, but more often flattened, oval, 

 triangular, or elongated, and occasionally, as in Monotremes and 

 most Marsupials, triradiate. The "suprarenal bodies" or "adrenals" 

 are two in number, each situated either in contact with, or at a 

 short distance in front of the anterior extremity of the kidney. 

 They are abundantly supplied with nerves, and are much larger re- 

 latively in early than in adult life. The "thyroid bodies," of which 

 there are generally two, though in Man and some other species 

 they are connected by an isthmus passing across the middle line, 

 are constant in mammals, though only met with in a rudimentary 

 condition, if at all, in other vertebrates. They are situated in the 

 neck, in contact with the sides of the anterior extremity of the 

 trachea. The " thymus " lies in the anterior part of the thorax, 

 between the sternum and the great vessels at the base of the heart, 

 and differs from the thyroid in being median and single, and having 

 a central cavity. It attains its greatest development during the 

 period in which the animal is nourished by its mother's milk, and 

 then it diminishes, and generally disappears before full growth is 

 attained. 



Nostrils. — Mammals breathe occasionally through the mouth, 

 but usually, and in many cases exclusively, through the nostrils or 

 tmres. These are apertures, always paired (except in the toothed 

 Cetacea, where they unite to form a single external opening), and 

 situated at the fore part of the face, generally at or beneath the 

 end of the muzzle, a median prominence above the mouth. This is 

 sometimes elongated to form a proboscis, to the extremity of which 

 the nostrils are carried, and which attains its maximum of develop- 

 ment in the Elephant. In the Cetacea the nostrils are situated at 

 a considerable distance behind the anterior end of the face, upon 

 the highest part of the head, and are called " blow-holes," from the 

 peculiar mode of respiration of those animals. The nostrils are 

 kept open by means of cartilages surrounding their aperture, 

 which many animals have the power of moving so as to cause 

 partial dilatation or contraction. In diving animals, as Seals and 

 Cetacea, they can be completely closed at will so as to prevent the 

 entrance of water when beneath the surface. The passage to which 

 the nostrils lead is in most mammals filled by a more or less 

 complex sieve -like apparatus, formed of the convoluted turbinal 

 bones and cartilages, over which a moist, vascular, ciliated mucous 

 membrane is spread, which intercepts particles of dust, and also 

 aids in warming the inspired air before it reaches the lungs. In 

 the Proboscidea, in which these functions are performed by 



