CIRCULATION, ETC.] MAMMALIA 365 from the thorax and the left side of the head and left anterior extremity. Those from the right side of the head and right anterior limb usually enter by a small distinct trunk into the corresponding part of the right brachio- cephalic vein. The duct, and also the principal lymphatic vessels, are provided with valves. Lymphatic glands, rarely met with in the Sauropsida, are usually present in mammals, both in the general and in the lacteal system, the latter being called " mesenteric glands." They are round or oval masses, situated upon the course of the vessels, which break up in them and assume a plexiform arrangement, and then reunite as they emerge. No structures corresponding to the pulsating "lymphatic hearts " of the lower vertebrates have been met with in mammals. )uctless Associated with the vascular and lymphatic systems are
- lands. certain bodies, the functions of which are not properly
understood, and which are usually, on account of their general appearanee, grouped together under the name of "ductless glands." Of these the "spleen" is the largest, and single, always placed in mammals in relation to the fundus or left end of the stomach, to which it is attached by a fold of peritoneum. It is dark-coloured and spongy in substance, and has a depression on one side or " hilus," into which the splenic artery, a branch of the coeliac axis of the abdominal aorta, enters, and from which the vein, which, joins the portal system, emerges. It 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, tri- radiate. 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 relatively in early than in adult life. The " thyroid body," or rather bodies, for there are gene rally two, though in Man and some other species connected by an isthmus passing across the middle line, are constant in mammals, though only met with in a rudimentary con dition, if at all, in other vertebrates. They are situated in the neck, in contact with the sides of the anterior ex tremity of the trachea. The " thymus"- lies in the ante rior 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 of lactation, and then diminishes and generally disappears before full growth is attained. Nostrils. Respiratory Organs. Mammals breathe occasionally through the mouth, but usually, and in many cases exclusively, through the nostrils or nares. 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 development 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 these animals. The nostrils are kept open by means of cartilages which surround their aperture, and 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 com pletely 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, and 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 the walls of the long tubular proboscis, this apparatus is entirely wanting. The narial passages have the organ of smell situated in their upper part, and communicate posteriorly with the pharynx, and through the glottis with the " trachea" or windpipe, a Trachea, tube by which the air is conveyed to and from the lungs. The permanent patency of the trachea during the varied movements of the neck is provided for by its walls being stiffened by a series of cartilaginous rings or hoops, which in most mammals are incomplete behind. Having entered the thorax, the trachea bifurcates into the two bronchi, one of which enters, and, dividing dichotomously, ramifies through, each lung. In some of the Cetacea and Artio- dactyla a third bronchus is given off from the lower part of the trachea, above its bifurcation, and enters the right lung. The upper end of the trachea is modified into the organ Larynx, of voice or "larynx," the air passing through which to and from the lungs is made use of to set the edges of the " vocal cords," fibrous bands stretched one on each side of the tube, into vibration. The larynx is composed of several cartil ages, of which the "thyroid," the " cricoid," and the " arytenoid " are the principal, moved upon one another by muscles, and suspended from the hyoidean arch. By alteration of the relative position of these cartilages the cords can be tightened or relaxed, approximated or divari cated, as required to modulate the tone and volume of the voice. A median tongue-shaped fibro-cartilage at the top of the larynx, the "epiglottis," protects the "glottis," or aperture by which the larynx communicates with the pharynx, from the entry of particles of food during deglu tition. The form of the larynx and development of the vocal cords present many variations in different members of the class, the greatest modification from the ordinary type being met with in the Cetacea, where the arytenoid cartilages and epiglottis are united in a tubular manner, project into the nasal passage, and, being grasped by the muscular posterior margin of the palate, provide a direct channel of communication from the lungs to the external surface. An approach to this condition is met with in the Hippopotamus and some other Ungulates. Nearly all mammals have a voice, although sometimes it is only exer cised at seasons of sexual excitement. Some Marsupials and Edentates appear to be quite mute. In no mammal is there an inferior larynx, or "syrinx, as in birds. The thoracic cavity of mammals differs from that of the Dia- Sauropsida in being completely separated from the abdomen phragm. by a muscular partition, the " diaphragm," attached to the vertebral column, the ribs, and the sternum. This is much arched, with the convexity towards the thorax, so that when its fibres contract it is flattened and the cavity of the thorax increased, and when they are relaxed the cavity isdiminished. The lungs are suspended freely in the thorax, one on each Lungs, side of the heart, being attached only by the root, which consists of the bronchus or air-tube, and pulmonary arteries and veins by which the blood is passed backwards and forwards between the heart and the lungs. The remaining part of the surface of each lung is covered by serous mem brane, the " pleura," and, whatever the state of distension or contraction of the chest-wall, is accurately in contact with it. Inspiration is effected by the contraction of the diaphragm, and by the intercostal and other muscles ele vating or bringing forward the ribs, and thus throwing the sternum farther away from the vertebral column. As the
surface of the lung must follow the chest-wall, the organ