The word "meridian" covers several different things in the classical literature, and the layers are not interchangeable. The Lingshu takes them one at a time — a chapter for the regular channels, one for the divergent channels, one for the sinew channels — and the Su Wen adds the skin regions.
The regular channels are the trunk. Twelve paired channels run lengthwise, each carries a fixed set of points, and each is attached to a Zang or Fu. This is the layer most readers mean by the word "meridian", and it is the layer that has been mapped most thoroughly against modern surface anatomy and against the WHO point standard.
The divergent channels branch off the regular channels, travel deeper into the body cavities and rejoin their paired channel. Their described job is to strengthen the link between the two channels of an exterior-interior pair.
The luo vessels are the crosswise branches that spread over the surface. The fifteen luo each has its own named luo point, and the superficial ones are visible — they are the ones bled in classical practice.
The sinew channels are the layer with no points and no organ attachments. They bind the joints and govern movement, and they are the layer most often compared to what modern anatomy calls myofascial chains. It is a comparison, not an identity.
The skin regions are the areas of skin allocated to each channel. They are the outermost layer, the route by which an exterior influence is said to enter, and the field in which surface techniques such as cutaneous needling are carried out. They are sometimes compared with dermatomes; the two do not divide the body the same way and should not be collapsed into each other.
None of these layers has been demonstrated as an anatomical structure, and this entry makes no such claim. The useful reading is functional: the layers describe different jobs. They also carry different risk — anything that breaks the skin is an invasive procedure whatever layer it is aimed at, and the deeper layers include the sites where pneumothorax and nerve injury have been reported.
「经络」这个词在古典文献里覆盖了好几种东西，各层之间不能互换。《灵枢》是逐层分论的——经脉一篇、经别一篇、经筋一篇，《素问》另有皮部之论。
经脉是主干：十二条对称循行，纵行走向，各有固定的穴位与所属的脏腑。这是读者说到「经络」时通常所指的那一层，也是与现代体表解剖、与 WHO 腧穴标准对应得最充分的一层。
经别从经脉分出，深入体腔，再与其相表里的经脉合流。文献给它的职责是加强表里两经之间的联系。
络脉是横行散布于浅表的分支。十五络各有自己的络穴，浮在浅层、肉眼可见的那些，正是古典实践里刺络放血所用之处。
经筋是唯一没有穴位、不与脏腑连属的一层。它联缀关节、主司运动，也是最常被拿来与现代解剖所说的肌筋膜链相比的一层——只是相比，并非等同。
皮部是按经脉划分到皮肤上的区域。它是最外一层，被认为是外邪由表入里的路径，也是皮肤针等浅表手法的施术范围。有人把它与皮节相比：两者划分身体的方式并不相同，不应互相折算。
这些层次没有任何一层被证明是解剖学上的实体结构，本条不作这种主张。更有用的读法是功能性的：各层描述的是不同的职责。它们的风险也不同——凡破皮者皆为侵入性操作，不论针对哪一层；而较深的层次里，包含那些有气胸与神经损伤记载的部位。
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