Recognition of variations in size, shape, and density of the normal thymus on computed tomographic (CT) scans is of paramount importance, lest it be misinterpreted as an abnormal mediastinal mass. Studying patients subsequently proved free of active chest disease, we analyzed 154 CT scans of the mediastinum, performed on a fourth-generation scanner, to determine the incidence of visualization and appearance of the normal thymus. The thymus was seen in 100% of patients under age 30, 73% of patients between 30 and 49 years, and in 17% of patients over 49 years of age. The thickness of the thymus showed a definite decrease in size with increasing age; although the width showed a similar general tendency, a wide variation was noted within each age group. In younger patients, the density of the thymus was similar to that of muscle; the attenuation values progressively decreased in older patients, finally approaching that of fat.

No abstract

A total of 1,239 skeletons from among Mongoloid, Caucasoid and Negroid population groups in North America was examined for variations in the number of presacral vertebrae. The overall incidence of variation was 11%; 6% with 23 and 5% with 25 presacral vertebrae. Differences in total variation among the three groups were not significant, but differences in the incidence of 23 and 25 presacral vertebrae among the groups were highly significant.Numerical vertebral variation occurred in 11% of both sexes, but with males having a higher frequency of 25 presacral vertebrae and females having a higher frequency of 23 presacral vertebrae. The incidence of the specific variation of 23 or 25 presacral vertebrae was not significantly different among the males of the three groups studied, whereas 23 presacral vertebrae were found significantly more often in the Negroid females. Numerical variation of vertebrae was not associated with age.The data strongly support the conclusion that the total frequency of variation in the number of presacral vertebrae is a specific characteristic of any particular population group and that there is a tendency in all population groups toward an increase in number in males and a decrease in number in females.

The weight of the skeleton is due, only in part, to the size or the volume enclosed by the surface of its individual bones. The size of the bones is determined by the many factors included in both heredity and environment. I t is generally believed that after growth is completed there is no si,&ficant change in the size of the normal skeleton, or of any bone comprising it, throughout the life span. However, bones are living organs and their maintenance is dependent on metabolic processes which lmay be affected by many factors, including stress, activity, growth and aging. Evidence of these effects is apparent in the internal structure of a bone through alteration in the amount and density of its deep, spongy substance (substawtia sportgiosa) and its superficial, compact substance (substawtia compacta).Koch ('17) considered the conformation of the intrinsic structures of the femur to be the result of adaptation to stress of pressure and function. A more realistic view has been presented by Murray ( '36) who recognized that the primary patterns of the skeleton are well defined early in life and that environmental conditions act as modifiers of the hereditary intrinsic patterns. The importance of functional stresses in maintaining the characteristic structures of bones is indicated by the work of Amprino ('38) who showed that, when normal movement of the limbs is inhibited, degenerative processes akin to osteoporosis set in. Sperling et al. ( ' 5 5 ) studied 'This investigation has been aided in part

On computed tomography, a mass-like density is often observed, just posterior to the ascending aorta, that occasionally has been mistaken for mediastinal lymph node enlargement. Cadaver studies confirmed this retroaortic structure to be an extension of the pericardial cavity, the superior sinus. Current anatomic texts sometimes depict this space without description. Anatomic studies revealed the presence of a superior sinus in all of the 28 cadavers studied. The mean cross-sectional diameter of the sinus in a coronal plane was 2.5 cm with a range from 1.2 to 3.8 cm. Retrospective review of 116 consecutive adult chest computed tomographic examinations disclosed its presence in 49%. In 15% of the total, the size of the sinus was sufficiently large potentially to simulate mediastinal lymphadenopathy. This normal variant has a characteristic location, shape, and attenuation value by CT that should allow recognition and prevent misinterpretation.