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Cauda equina as a root system to drain cerebrospinal fluid
Mammals have shortened spinal cords, ascensus medullaris (Nieuwenhuys, 1964), and their lumbar and sacral nerve roots continue within the vertebral canal as a bundle of nerves called cauda equina ( horse s tail ). Mammals are also warm blooded, and have a standard metabolic rate about an order of magnitude above the basal metabolic rate of cold blooded animals (Withers, 1992). The shortening of the spinal cord is often attributed no functional role, the lumbosacral part of the vertebral canal is claimed to be basically vestigial. That may be true, but, there could also be some other explanation. The cauda equina has some resemblance to the root system of a tree. Laurent Sakka citing a J.B. Brierley describes arachnoid villi in lumbosacral nerve roots that absorb cerebrospinal fluid and drains it into the lymphatic system. Arachnoid villi on lumbar and sacral nerve roots are also described by Weller, Kido and Welsch. The anatomical position of the cauda equina means that it would drain lymphatically directly into the cisterna chylii, and it is emptied using the respiratory pump, since it is in the abdominal coelom. Karl Bechter describes that "the total cerebrospinal fluid outflow volume at the lumbar site [along lumbar nerves] was remarkable .
The cauda equina could also drain CSF via venous system, also using respiratory pump as it drains into inferior vena cava in abdominal coelom. The vascularization of the cauda equina is often reported to be "hypovascular" (Parke, 1981, and people frequently citing Parke), but, the opposite is also reported (Kobayashi, 2000), claiming bias and false interpretations on those reporting hypovascularization.
References
Nieuwenhuys, R. (1964). [Progress in Brain Research] Organization of the Spinal Cord Volume 11 || Comparative Anatomy of the Spinal Cord. , (), 1 57. doi:10.1016/s0079-6123(08)64043-1
Withers, P. C. 1992. Comparative Animal Physiology, New York: Saunders.
L. Sakka; G. Coll; J. Chazal (2011). Anatomy and physiology of cerebrospinal fluid. , 128(6), 309 316. doi:10.1016/j.anorl.2011.03.002
Brierley JB, Field EJ, Yoffey JM. Passage of indian ink particles from the cranial subarachnoid space. J Anat 1949;83:77.
Weller, R. O., Djuanda, E., Yow, H.-Y., & Carare, R. O. (2008). Lymphatic drainage of the brain and the pathophysiology of neurological disease. Acta Neuropathologica, 117(1), 1 14. https://doi.org/10.1007/s00401-008-0457-0
Kido DK, Gomez DG, Pavese AMJ, Potts DG (1976) Human spinal arachnoid villi and granulations. Neuroradiology 11:221 228
Welch K, Pollay M (1963) The spinal arachnoid villi of the monkeys Cercopithecus aethiops sabaeus and Macaca irus. Anat Rec 145:43 48
Bechter, K., & Schmitz, B. (2014). Cerebrospinal fluid outflow along lumbar nerves and possible relevance for pain research: case report and review. Croatian medical journal, 55(4), 399 404. https://doi.org/10.3325/cmj.2014.55.399
Parke WW, Gammell K, Rothman RH. Arterial vascularization of the cauda equina.J Bone Joint Surg Am. 1981;63:53 62
Kobayashi S, Yoshizawa H, Nakai S. Experimental study on the dynamics of lumbosacral nerve root circulation.Spine (Phila Pa 1976) 2000;25:298 305