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Show 40 MAMMALIA. cochlea, whi.c h t ermi. nat es by one canal in the vestibule, and b the other in the tympanum. . Y . . . bdivided into three portions; the ante- Their cramum IS su . b h . . d b the two frontal and ethmmdal ones, t e rior IS forme Y . . d th ethmoides and . ddle b the two ossa panetaha an e os . ' . tmhie posteYri. or by t h e os occi'p itis . Between. t he ossa parietaha, h . d 1. d the os occipitis, are Interposed the two the sp enm a IS an h f: temporal bones, part of which belong properl~ to t e ace. In the fretus, the occipital bone is divided In~ four p~rts: the sphen01' da 1 .m t o two halves ' which are aga. in subdivided into t h ree pa.i rs of lateral wings •' the temporal Into three, 1on e of which serves to complete the cranium, the seco~d to c ose the labyrinth of the ear, the third to for~ the parietes of ~e tympanum, & c. These bony portions, still more ·n udm erous m the earliest period of the fretal existence, are unite more or less promptly, according to the species, and the bones them· selves finally become consolidated in the adult. Their face consists of the two maxilla,:y ?ones, b~tween which pass the nostrils ; the two intermaxlllaries are situated before, and the two ossa palati behind them ; betwee? these descends the vomer, a bony process of the os ethmo~des; .at the entrance of the nasal canal are placed the ossa nasi; to Its external parietes adhere the inferior turbinated ?ones, ~he superior ones which occupy its upper and posterior portion belonging to the os ethmoides. The jugal or che~k bone unites the maxillary to the temporal bone on each side, and frequently to the os fro ntis ; finally, th~ os unguis, and pars plana of the ethmoid bone occupy the Internal angle of the orbit and sometimes a part of the cheek. In the embryo state'these bones also are much more subdivided. Their tongue is always fleshy, connected with a bone called the hyoides, which is composed of several pieces, and sus· pended from the cranium by ligaments. Their lungs, two in number, divided into lobes, an~ com· posed of an infinitude of cells, are always enclosed, Without any adhesion, in a cavity formed by the ribs and diaphragm and lined by the pleura; the organ of voice is always at the MAMMALIA. 41 upper extremity of the trachea; a fleshy curtain, called the velum palati, establishes a direct communication between their larynx and nasal canal. Their residence on the surface of the earth rendering them less exposed to the alternations of cold and heat, their tegument, the hair, is but moderately thick, and in such as inhabit warm climates, even that is rare. The Cetacea, which live exclusively in water, are the only ones that are altogether deprived of it. The abdominal cavity is lined with a membrane called the peritoneum, and the intestinal canal is suspended to a fo~d o~ it called the mesentery, which contains numerous conglobate glands in which the Iacteals ramify: another production of the peritoneum, styled the epiploon, hangs in front of and under the intestines. The urine which is retained for a time in the bladder finds an exit in both sexes, with very few exceptions, by orifices in the organs of generation. In all the Mammalia, generation is essentially viviparous; that is, the fretus, directly after conception, descends into the uterus enveloped in its membranes, the exterior of which is called chorion and the interior amnios; it fixes itself to the parietes of this cavity by one plexus, or more of vessels called the placenta, which establishes a communication between it and the mother, by which it receives its nourishment, and most probably its oxygenation, notwithstanding which, the fretus of the Mammalia, at an early period, has a vesicle analogous to that which contains the yolk in the Ovipara, receiving in like manner vessels from the mesentery. It has also another external bladder named the allantoid, which communicates with the urinary one by a canal called the urachus. Conception always requires an effectual coitus, in which the semen masculinum is thrown into the uterus of the female. The young are nourished for some time after birth by a :fluid (milk) peculiar to animals of this class, which is produced by the mammre at the time of parturition, and continues to be so as long as is necessary. It is from the mamrrnE that VoL. I.-.-F |
