Nose.—The sense of smell is imperfect in the infant, as may be inferred from the small capacity of the nasal fossæ, and the non-development of the ethmoid, sphenoid, frontal, and maxillary sinuses.
FETAL LUNGS.
The lungs, previously to the act of inspiration, are dense and solid in structure, and of a deep-red color. Their specific gravity is greater than water, in which they sink to the bottom; whereas lung which has respired will float upon that fluid. The specific gravity is, however, no test of the real weight of the lung, the respired lung being actually heavier than the fetal. Thus the weight of the fetal lung, at about the middle period of uterine life, is to the weight of the body as 1 to 60. But, after respiration, the relative weight of the lung to the entire body is as 1 to 30.
FETAL HEART.
The heart of the fetus is large in proportion to the size of the body; it is also developed very early, representing at first a simple vessel, and undergoing various degrees of complication until it arrives at the compound character which it presents after birth. The two ventricles form, at one period, a single cavity, which is afterward divided into two by the septum ventriculorum. The two auricles communicate up to the moment of birth, the septum being incomplete, and leaving a large opening between them, the foramen ovale (foramen of Botal).
The ductus arteriosus is another peculiarity of the fetus connected with the heart; it is a communication between the pulmonary artery and the aorta. It degenerates into a fibrous cord after birth, from the double cause of a diversion in the current of the blood toward the lungs, and from the pressure of the left bronchus, caused by its distention with air.
VISCERA OF THE ABDOMEN.
At an early period of uterine life, and sometimes at the period of birth, as Dr. Wilson observed in the imperfectly-developed fetus, two minute fibrous threads may be seen passing from the umbilicus to the mesentery. These are the remains of the omphalo-mesenteric vessels.
The omphalo-mesenteric are the first-developed vessels of the germ; they ramify upon the vesicula umbilicalis, or yolk-bag, and supply the newly-formed alimentary canal of the embryo. From them, as from a center, the general circulating system is produced. After the establishment of the placental circulation they cease to carry blood, and dwindle to the size of mere threads, which may be easily demonstrated in the early periods of uterine life; but are completely removed, excepting under peculiar circumstances, at a later period.
The stomach is of small size, and the great extremity but little developed. It is also more vertical in direction the earlier it is examined, a position that would seem due to the enormous magnitude of the liver, and particularly of its left lobe.