前后怎么理解？

The intercostal muscles are divided into the external group, fibres of which run in a caudal–ventral direction from their upper rib and are deficient anteriorly, and the less powerful internal group, which have fibres running caudal– dorsal from their upper rib and are deficient posteriorly. Internal intercostal muscles of the upper rib cage become thicker anteriorly, where they are known as the parasternal intercostal muscles.

肋间肌分为肋间外肌和肋间内肌，肋间外肌纤维从上肋骨向足-腹方向走行，前向分布不足；肋间内肌的纤维从上肋骨沿足-背方向走行，后向分布不足。上胸腔的肋间内肌向前变厚，称为胸骨旁肋间肌。

我不知道前后该怎么分，一般我的理解“腹侧属于前”、“背测属于后”，既然“肋间外肌纤维从上肋骨向足-腹方向走行”，又怎么“前向分布不够”？肋间内肌“上肋骨沿尾-背方向走行”，又怎么“后向分布不够”？