Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/388

Rh 366 MAMMALIA [NERVOUS SYSTEM. is itself expanded, and air rushes in through the trachea to fill all the minute cells in which the ultimate ramifica tions of the bronchi terminate. In ordinary expiration very little muscular power is expended, the elasticity of the lungs and surrounding parts being sufficient to cause a state of contraction and to drive out at least a portion of the air contained in the cells, when the muscular stimulus is withdrawn. The lungs are sometimes simple externally, as in the Sirervia (where they are greatly elongated) and the Cetacea, but they are more often divided by deep fissures into one or more lobes. The right lung is usually larger and more subdivided than the left. It often has a small distinct lobe behind, wanting on the left side, and hence called lobuhcs azygos. Air sacs. Most mammals have in connexion with the air passages certain diverticuli or pouches containing air, the use of which is not always easy to divine. The numerous air sinuses situated between the outer and inner tables of the bones of the head, which in Man are represented by the antrum of Highmore and the frontal and sphenoidal sinuses, and which attain their maximum of development in the Indian Elephant, are obviously for the mechanical pur pose of allowing expansion of the bone surface without increase of weight. They are connected with the nasal passages. The Eustachian tubes pass from the back of the pharynx into the cavity of the tympanum, into which and the mastoid cells they allow air to pass. In the Eqiddx, there are large post-pharyngeal air sacs in connexion with them. The Dolphins have an exceedingly complicated system of air sacs in connexion with the nasal passages just within the nostrils, and the Tapir and Horses have blind sacs in the same situation. In the males of some Seals (Cystojjhora and Macrorhimis) large pouches, which the animal can inflate with air, but which at other times are flaccid, and which are not developed in the young animal or the female, arise from the upper part of the nasal passages, and lie immediately under the skin of the face. These are very analogous, although not in the same situation, to the gular pouch of the male Bustard. The larynx has frequently membranous pouches in connexion with it, into which air passes. These may be lateral and opening just above the vocal cords, constituting the sacculi laryngis, found in a rudimentary state in Man, and attaining an enormous development, reaching to the shoulders and axillae, in some of the Anthropoid Apes ; or they may be median, opening in front either above or below the thyroid and cricoid car tilages, as in the Howling and other Monkeys, and also in the Whalebone Whales and Great Anteater. Kidneys. Urinary Organs. The kidneys of mammals are more compact and definite in form than in other vertebrates, being usually more or less oval, with an indent on the side turned towards the middle line from and into which the vessels and ducts pass. They are distinctly divided into a cortical secretory portion, composed mainly of convoluted tubes, and containing Malpighian bodies, and a medullary excreting portion, formed of straight tubes converging towards a papilla, embraced by the commencement of the ureter or duct of the organ. The kidneys of some mammals, as most Monkeys, Carnivores, Rodents, &c., are simple, with a single papilla into which all the renal tubuli enter. In others, as Man, there are many pyramids of the medullary portion, each with its papilla, opening into a division (calyx) of the upper end of the ureter. Such kidneys are, either in the embryonic condition only or throughout life, lobulated on the surface. In some cases, as in Bears, Seals, and especially the Cetacea, the lobulation is carried further, the whole organ being composed of a mass of renules, loosely united by connective tissue, and with separate ducts, which soon join to form the common ureter. In all mammals except the Monotremes the ureters termi nate by slit-like valvular openings in the urinary bladder. Bladder This receptacle when filled discharges its contents through the single median urethra, which in the male is almost invariably included in the penis, and in the females of some species of Rodents, Insectivores, and Lemurs has a similar relation to the clitoris. In the Monotremes, though the bladder is present, the ureters do not enter into it, but into the urogenital canal some distance below it, the orifice of the genital duct intervening. NERVOUS SYSTEM AND ORGANS OF SENSE. The brain of mammals shows a higher condition of Braiu. organization than that of other vertebrates. The cerebral hemispheres have a greater preponderance compared to other parts, especially to the so-called optic lobes, or corpora quadrigemina, which are completely concealed by them. The commissural system of the hemispheres is much more complete, both fornix and corpus callosum being present in some form; and, when the latter is rudimentary, as in Marsupials and Monotremes, its defi ciency is made up for by the great size of the anterior com missure. The lateral lobes of the cerebellum, wanting in lower vertebrates, are well developed and connected by a transverse commissure, the pons Varolii. The whole brain, owing especially to the size of the cerebral hemispheres, is considerably larger relatively to the bulk of the animal than in other classes, but it must be recollected that the size of its brain depends upon many circumstances besides the degree of intelligence which an animal pos sesses, although this is certainly one. Man s brain is many times larger than that of all other known mammals of equal bulk, and even three times as large as that of the most nearly allied Ape. Equal bulk of body is here mentioned, because, in drawing any conclusions from the size of the brain compared with that of the entire animal, it is always necessary to take into consideration the fact that in every natural group of closely allied animals the larger species have much smaller brains relatively to their general size than the smaller species, so that, in making any effective comparison among animals belonging to different groups, species of the same size must be selected. It may be true that the brain of a Mouse is, as compared with the size of its body, larger than that of a Man, but, if it were possible to reduce an animal having the general organization of a Man to the size of a Mouse, its brain would doubtless be very many times larger; and conversely, as shown by the rapid diminution of the relative size of the brain in all the large members of the Rodent order, a Mouse magnified to the size of a Man would, if the general rule were observed, have a brain exceedingly inferior in volume. Although the brain of the large species of Whales is, as commonly stated, the smallest in proportion to the bulk of the animal of any mammal, this does not invalidate the general proposition that the Ceiacea have very large brains compared with terrestrial mammals, as the Ungulata, or even the aquatic Sirenia, as may be proved by placing the brain of a Dolphin by the side of that of a Sheep or Pig, or a Manatee of equal general weight. It is only because the universally observed difference between the slower ratio of increase of the brain compared with that of the body becomes so enormous in these immense creatures that they are accredited with small brains. The presence or absence of &quot; sulci &quot; or fissures on the sur face of the hemisphere, dividing it into &quot;convolutions&quot; or &quot; gyri,&quot; and increasing the superficies of the cortical grey matter, as well as allowing the pia mater with its nutrient blood-vessels to penetrate into the cerebral substance, follow somewhat similar rules. They are related partly to the high or low condition of organization of the species, but also