membranous sac的用法和样例：
例句
- At all leels, mean dural sac cross-sectional area in the supine position was significantly smaller than in other postures.
在不同脊椎高度，硬脊膜囊的平均横截面积在仰卧位是显著小于其他体位时。 - A meningomyelocele is a protrusion of the meningeal membranes through a defect in the vertebral column and has nerve elements within the sac.
在此种脊髓膜膨出中含有神经组织脊髓膜囊，由脊柱缺陷中膨出而成。
