| myelin sheath | <anatomy, neurology> An insulating layer surrounding vertebrate peripheral neurons, that dramatically increases the speed of conduction. It is formed by specialised Schwann cells, that can wrap around neurons up to 50 times. The exposed areas are called nodes of Ranvier: they contain very high densities of sodium channels and action potentials jump from one node to the next, without involving the intermediate axon, a process known as saltatory conduction. (18 Nov 1997) |
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| posterior layer of rectus abdominis sheath | The portion of the sheath of the rectus abdominis muscle that lies posterior to the muscle covering only its upper two-thirds; it is formed by contributions from the aponeuroses of the internal oblique and transversus abdominis muscles; its free inferior margin forms the arcuate line; it is deficient below this, the posterior aspect of the muscle being covered only by transversalis fascia and peritoneum. Synonym: lamina posterior vaginae musculi recti abdominis. (05 Mar 2000) |
| crural sheath | The fascia enclosing the femoral vessels, formed by the transversalis fascia anteriorly and the iliac fascia posteriorly; two septa divide the sheath into three compartments, the lateral of which contains the femoral artery and the femoral branch of the genitofemoral nerve, the middle the femoral vein, and the medial is the femoral canal. Synonym: crural sheath, infundibuliform sheath. (05 Mar 2000) |
| cuticle of root sheath | cuticula vaginae folliculi pili |
| saber-sheath trachea | A type of tracheal collapse seen in chronic obstructive pulmonary disease in which there is an increase in the outer posterior tracheal dimension with side-to-side narrowing involving the lower 2/3 of the trachea. (05 Mar 2000) |
| Henle's sheath | <anatomy> The delicate bands of connective tissue among nerve fibres. Origin: NL, fr. Gr. Within + a sinew, nerve. Source: Websters Dictionary (01 Mar 1998) |
| prostatic sheath | Loose fibrous, partly vascular enclosure of the prostate and its dense (true) fibrous capsule; it is continuous inferiorly with the superior fascia of the urogenital diaphragm and posteriorly becomes part of the rectovesical septum; it contains the prostatic venous plexus. (05 Mar 2000) |
| Scarpa's sheath | H;one of the coverings of the spermatic cord, formed of delicate connective tissue and of muscular fibres derived from the internal oblique muscle (cremaster muscle). See: aponeurosis of internal abdominal oblique muscle. Synonym: fascia cremasterica, Cooper's fascia, Scarpa's sheath. (05 Mar 2000) |
| Hertwig's sheath | The merged outer and inner epithelial layers of the enamel organ which extends beyond the region of the anatomical crown and initiates formation of dentin in the root of a developing tooth; it atrophies as the root is formed, and any of the cells that persist are called Malassez' epithelial rests. (05 Mar 2000) |
| schwan's sheath | <anatomy> The neurilemma. Origin: So called from Theodor Schwann, a German anatomist of the 19th century. Source: Websters Dictionary (01 Mar 1998) |
| Huxley's sheath | A layer of cells interposed between Henle's layer and the cuticle of the inner root sheath of the hair follicle. Synonym: Huxley's membrane, Huxley's sheath. (05 Mar 2000) |
| sheath | A protective covering. The lower part of leaf enveloping stem or culm. A secreted, tubular structure formed around a chain of cells or around a bundle of filaments, cells within a sheath may or may not subsequently separate from the sheath. A layer of outer membrane covering the bacterial flagella. (09 Oct 1997) |
| sheath ligaments | See: fibrous digital sheaths of hand, fibrous digital sheaths of foot, fibrous tendon sheath. (05 Mar 2000) |
| sheath of eyeball | A condensation of connective tissue on the outer aspect of the sclera from which it is separated by a narrow cleftlike episcleral space; the sheath is attached to the sclera near the sclerocorneal junction and blends with the fascia of the extraocular muscles. Synonym: vagina bulbi, capsula bulbi, eye capsule, fascia bulbi, sheath of eyeball, Tenon's capsule, vagina oculi. (05 Mar 2000) |
| sheath of Key and Retzius | <anatomy> The delicate bands of connective tissue among nerve fibres. Origin: NL, fr. Gr. Within + a sinew, nerve. Source: Websters Dictionary (01 Mar 1998) |