678 THE NERVOUS SYSTEM 



with the anatomy of the ventricles. The blood-vessels, which divide freely in tlie 

 subarachnoid tissue, sulidivide into the pia mater, forming by their inoscula- 

 tions fine networks from which innumerable minute vessels proceed to penetrate 

 the nervous substance. 



Lymphatics of the Brain and Spinal Cord 



The lymphatics of the brain and spinal cord are peculiar, inasmuch as thej^ open into the 

 subarachnoid space, and are oidy indirectly connected with the geneial lympliatic and venous 

 systems. The communications with the venous system are effected Ly the Pacchionian bodies. 

 The lymphatics of the peripheral nerves are in the form of tubular spaces jilaced between the 

 lamellae of the perineural sheaths. These tubular channels open into the subdural and subarach- 

 noid spaces. 



The subdural space is a very narrow inteival between the dura mater and arachnoid (page 

 675). It normally contains only sufficient fluid to moisten its surfaces. It is in communication 

 with the lymphatics of the neck and also of the back and loins. It also communicates with the 

 perineural spaces around the nerves, and with the lymph spaces which surround the olfactory, 

 optic, and auditory nerves. It sends prolongations around the Pacchionian bodies. It does not 

 communicate with the subarachnoid space. 



The subarachnoid space contains the greater part of the cerebro-spinal fluid, the fluid occupy- 

 ing the meshes of the subarachnoid tissue. The lymphatics of the brain and eoid and also the 

 perineural spaces of the nerves open into this space. It is also in communication with the ven- 

 tricular system of the brain by means of the foramen of INbijendie and the foramina of Key and 

 Retzius. Slit-like communications between the subarachnoid space and the descending horn of 

 the lateral ventricle have also been described (Merkel and Mierzejewsky). It also communicates 

 with the perilymph spaces of the internal ear and with the Ij-mphaties of the mucous membrane 

 of the nose. It sends prolongations around the optic and auditory nerves. 



The lymphatic vessels of the brain and cord surround the arteries, and are hence called peri- 

 vascular lymphatics. As each artery dips into the nervous substance it carries with it a tubular 

 process of the pia mater. This tubular jirocess is lined by endothelial cells and a .similar la.yer 

 covers the coat of the arterj'. In this manner the lymjih space is bounded. The perivascular 

 lymphatic follows the artery as far as its capillary ramifications. 



There is a Ij^mph-space between the two layers of the spinal pia mater. Lymph-spaces 

 between the outer and middle coats of the cerebral arteries ( Virchow-Robin space) and others 

 around the individual nerve-cells of the brain have been described, but these are very generally 

 believed to be artificial, being due to shrinking from the action of reagents. According to Ober- 

 steiner, however, the existence of the pericellular spaces 'is ]iroved by the ])reseuce within them 

 of lymphatic cells.' The so-called epicerebral and epispinal spaces, situated between the deep 

 surface of the pia mater and the nervous substance, are also artificial. 



Structure of a Pacchionian body. — A Pacchionian bodj' consists of (a) a central core of sub- 

 arachnoid tissue which is joined to the general subarachnoid tissue by a comparatively narrow 

 stalk. This is limited by (b) a laj^er of arachnoid ; around which is (c) a ])rolongation of the 

 subdural space. Bounding this space h{d) a very thin layer of membrane derived from the 

 inner layer of the dura mater. It has been shown exjierimentally that injections thrown into the 

 subarachnoid space permeate the Pacchionian bodies and pass into the venous sinuses into which 

 these bodies project. In this course the injection distends the subdural space of the Pacchionian 

 body, but does not enter the general subdural space. It should be mentioned, however, that 

 anatomical pores by which the injection could ]iass do not exist. It is probable from these 

 experiments that an outlet for the cerebro-spinal fluid is provided by the Pacchionian liodies. 



Cerebro-spinal fluid. — The cerebro-spinal fluid occujiies the subdural and subarachnoid 

 spaces of the brain and cord and also the ventricular cavities of the brain. The average quantity 

 is about two ounces (Landois and Stirling). Its sjiecifie gravity is about 1010. It is of a very 

 pale yellow colour, and presents many of the characters of ordinaiy lymjdi. but diff"ers from 

 IjMuph in not being coagulable, as it does not contain either fibrin f;ictors or fibrin ferment. It 

 contains a substance which acts on Fehling's solution like dextrose, but which is not a .sugar 

 (Foster). 



The cerebro-spinal fluid is derived in part from the lymi)]iatic vessels which ojien into the 

 subarachnoid and subdural si)aces, but is also believed to be secreted by the epithelial cells which 

 cover the choroid plexuses. These cells are cubical in form, and resemlde secreting cells; a 

 Jirocess of the choroid ])lcxus covered by these cells has been ajitly compared by Foster to 'an 

 everted alveolus of a secreting gland, with the epithelium outside and the blood-vessels within.' 



