THE PERINEURAL SPACES IN THE PIG EMBRYO. 97 



Any method of service in the adult which must have in consideration the physical 

 character of the skull as a closed box was here necessarily doomed to failure. 



Together with these technical failures to demonstrate a perivascular system, 

 it must be borne in mind that these are merely failures to demonstrate the existence 

 of the perivascular system in the pig embryo. The system will probably be demon- 

 strated as soon as a suitable technique is devised. The spaces are very likely present 

 soon after the capillary plexus invades the nervous system, but the observation in 

 many histological preparations of the spaces around the cerebral vessels must not 

 be considered as offering proof of their existence, because of the likelihood of 

 shrinkage influencing the picture. It is interesting, however, to note that elasticity 

 of the cerebral tissues seems greatest along the course of the blood-vessels, for here 

 the phenomenon of shrinkage is most frequently observed. The existence of the 

 perivascular and permeuronal spaces, probably of only capillary thickness, must 

 remain in the embryo as in the adult a subject of physiological demonstration; 

 histological evidence, except with proper physiological regard, is of no value. 



The early development and function of such a system as the perivascular and 

 perineuronal canals afford seems most likely from the standpoint of pure speculation. 

 It is not improbable that fluid is poured from this system into the embryonic sub- 

 arachnoid space at a period soon after the capillary plexus invades the cerebrum. 

 There is no evidence, however, from the observations recorded in foregoing para- 

 graphs, that adequate subarachnoid channels are afforded until the pig embryo 

 reaches a length of about 25 mm. The hypothesis of Essick^ 13 ) regarding the 

 damming of the perivascular fluid as the cause of the two cava corporis striati is of 

 extreme interest in this connection. It remains, however, for future work to afford 

 real evidence in regard to the embryonic perivascular system. 



XIII. THE PERINEURAL SPACES IN THE PIG EMBRYO. 



The question of the existence of potential or actively functional spaces around 

 the peripheral nerves is of great interest, partly because of the possible relation of 

 these spaces to the developing lymphatic system, and also on account of the anatom- 

 ical evidence of the possible existence of such spaces. 



It is realized that before much dependence can be placed on any theory regarding 

 these potential spaces around the cerebro-spmal nerves, the possibility of their being 

 purely artifacts must be dealt with. The methods of demonstration, in the adult, 

 in the hands of the earliest workers were such as to favor the production of artifacts. 

 As far as can be ascertained, CotugnoW, dealing with the nervus ischiadicus, was the 

 first to conceive of these possible spaces. His method of demonstration consisted 

 in filling the spinal subarachnoid space with mercury (in a cadaver placed in the erect 

 posture) . Globules of the mercury were subsequently found about the sciatic nerve 

 in what then became the perineural spaces. 



Modern anatomical interest in these spaces was aroused by the remarkable 

 injections of Key and Retzius^ 29 ). These investigators, by means of gelatin injec- 

 tions into the spinal subarachnoid space, were able to demonstrate perineural 



