The spinal cord [medulla spinalis] is the lower (caudal) and most attenuated portion of the central nervous system. It is approximately cylindrical in form and terminates conically. Its average length in the adult is 45 cm. (18 in.) in the male and 42 cm. in the female. It weighs from 26 to 28 grams or about 2 per cent, of the entire cerebro-spinal axis.
After birth it grows more rapidly and for a longer period than the encephalon, increasing in weight more than sevenfold, while the brain increases less than half that amount. Its specific gravity is given as 1.038.
The urinary bladder is a sack-like dilatation of the urinary passages which serves as a collecting reservoir, the size and shape of which is dependent upon the degree of distention. Three chief portions may be recognized in it: the middle and larger portion of the bladder is the body; the upper portion, which is distinctly pointed, especially in the newborn, is the vertex; and the lowermost portion, directed toward the perineum, is called the fundus.
Change is a fundamental characteristic of all living things. The human body during its life cycle accordingly passes through various phases of form and structure. In the earliest embryonic phases of development, the changes are very rapid, decreasing in rapidity during the later fetal stages, but continuing at a diminishing rate throughout infancy, childhood and youth up to the adult. Following the acme of maturity, changes continue which lead gradually to senescence and final death of the body.
The peritoneum, as has been shown, is a serous membrane which lines the cavity of the abdomen from the diaphragm to the pelvic floor, and invests or covers to a varying extent the viscera which that cavity contains. Viewed in its very simplest condition, it may be regarded as a closed sac, the inner surface of which is smooth, while the outer surface is rough and is attached to the tissues which surround it.
The pulmonary artery [a. pulmonalis] passes from the right ventricle to the lungs. It differs from all other arteries in the body in that it contains venous blood. It arises as a short, thick trunk from the conus arteriosus of the right ventricle, and, after a course of about 5 cm. (2 in.) within the pericardium, divides into a right and a left branch. These branches pass to the, right and the left lung respectively.
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