"medullary sheath" meaning in English

{
  "forms": [
    {
      "form": "medullary sheaths",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "medullary sheath (plural medullary sheaths)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with 1 entry",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "topical",
          "langcode": "en",
          "name": "Botany",
          "orig": "en:Botany",
          "parents": [
            "Biology",
            "Sciences",
            "All topics",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "A thin layer surrounding the pith."
      ],
      "id": "en-medullary_sheath-en-noun-HOz~IA0L",
      "links": [
        [
          "botany",
          "botany"
        ],
        [
          "pith",
          "pith"
        ]
      ],
      "raw_glosses": [
        "(botany) A thin layer surrounding the pith."
      ],
      "topics": [
        "biology",
        "botany",
        "natural-sciences"
      ]
    }
  ],
  "word": "medullary sheath"
}

{
  "forms": [
    {
      "form": "medullary sheaths",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "medullary sheath (plural medullary sheaths)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "Pages with 1 entry",
        "en:Botany"
      ],
      "glosses": [
        "A thin layer surrounding the pith."
      ],
      "links": [
        [
          "botany",
          "botany"
        ],
        [
          "pith",
          "pith"
        ]
      ],
      "raw_glosses": [
        "(botany) A thin layer surrounding the pith."
      ],
      "topics": [
        "biology",
        "botany",
        "natural-sciences"
      ]
    }
  ],
  "word": "medullary sheath"
}