Besides the numerous glands situated within the intestine are two large ones, 'the liver and the ,pancreas, whose ducts open into the intes tines near the stomach. The liver is a large, flat gland, which may be divided nearly into aright and a left lobe as in man, the cetacea and ruminants, or may have these lobes again subdivided into two by a longitudinal fissure. Two smaller lobes, called the Spigelian lobe And the caudate lobe, are commonly added. Attached to the liver is the gall-bladder, which is, however, absent in the cetacea and some other orders.
The kidneys, whose function is to secrete urine, are situated in the upper part of the abdominal cavity near the vertebral column. They are two in number, oblong and usually simple, but in the cetacea, and also in bears and seals, are divided into separate lob ules. A duct or ureter leads from each kidney to the urinary bladder, from which in turn a common duct, called the urethra, leads to the exterior of the body. In the monotremes, how ever, the ureters do not enter the bladder, but into a common urnogenital passage or cloaca.
The lungs are situated in the tho rax, which is cut off below from the abdominal cavity by a muscular diaphragm whose action assists in the process of breathing.. The lungs consist of two spongy lobes, a right and a left, which are free below, but attached above to the two principal divisions of the windpipe. In the cetacea and sea-cows, the lobes are sim ple externally, but in other orders are more or less subdivided. A third median lobe, called the azygos lobe, is present in some groups. Air breathed in through the nostrils reaches the lungs through the trachea or windpipe, the upper end of which, the larynx, lies in the throat. Its orifice, the glottis, is protected by a eartilage, called the epiglottis, which prevents particles of food from entering the windpipe. The larynx is made up of cartilages, of which the largest are the thyroid, the crieoid and the arytenoid. Within the larynx are the vocal cords, two parallel elastic, fibrous bands, whose vibrations produce the voice. The lower end of the windpipe divides into two smaller tubes, or bronchii, each of which enters a lobe of the lungs and subdivides into numerous smaller branches. A third bronchius, which enters the right lung, occurs in some cetaceans and rumi nants.
Heart.— The heart in mammals is four chambered, consisting of two thin-walled auri cles and two ventricles, both with thick walls, but the right, which supplies only the lungs, thinner than the left. ;here is no direct com
munication between the left and right sides of the heart after birth. The valve between the right auricle and the right ventricle is tendi nous, except in the monotremes.
The aorta, or principal artery, bends toward the left immediately beyond its connection with the heart and gives off the innominate, left common carotid, and subclavian arteries, which, with their branches, supply the head and an terior limbs. The method of branching of these arteries from the aortic arch and from one another varies widely. Blood is carried from the alimentary canal to the liver by a single vein, except in Echidna, in which as in lower vertebrates the abdominal vein is present. The kidneys are supplied with blood only by the renal arteries.
Except for certain fossil forms, the brain of mammals is characterized by its relatively larger size as compared with that of lower vertebrates, and especially by the magni tude of the cerebral hemispheres and the per fection of the connections between them. In most mammals the surface of the brain is divided by numerous irregular fissures and con volutions. They are absent only in small bats, rodents and insectivores, and in Ornithorhyn chus. The largest forms in each order, gener ally speaking, exhibit the greatest complexity, and there has been a remarkable development in the class in this direction since Tertiary times. The cetacea have very large and com plex brains, though they are small relatively. when compared with the size of the body.
Sense The organs of sense, ex cept that of touch, are located in the head. The sense of touch is generally distributed over the skin, but is most acute in the snout, and in the extremities, except when used merely in locomotion. The wings of bats and the pre hensile tails of monkeys are also especially sensitive. Some burrowing mammals, such as the mole, have imperfect eyes, the optic nerve being more or less atrophied. The Indian river-dolphin, Platanista, is a blind form, having rudimentary eyes, without crystalline lenses. The mammalian ear is characterized, besides the chain of ossicles, already mentioned, by the complex cochlea, which is usually spirally con voluted. The tympanic membrane, or eardrum, seals the auditory chamber from without. In the majority of mammals the external orifice of the ear is surrounded by a fold of skin, called the pinna or external ear. These are absent in cetaceans, sea-cows, seals, etc., which live in the water, and also in some burrowing mammals.