64 CLINICAL APPLIED ANATOMY. 



spontaneously ; both these facts are explained by the protection 

 from external infection afforded by their position. The cysts 

 often communicate with the lateral ventricles, and serious loss 

 of cerebrospinal fluid may follow operations for their removal. 



Hydatid cysts in the spinal canal may be extradural or intra- 

 dural. The former may have extended from the vertebral bodies 

 or be primary, and in such cases should be very favourably 

 situated for operative interference. 



Hydatids of bones favour the cancellous tissue and medullary 

 cavities. In bone, possibly as the result of pressure, the cysts 

 usually, although not invariably, assume the exogenous form. 



As would be expected, by virtue of the blood-borne infection, 

 hydatid cysts are occasionally met with in situations other than 

 those described above. The orbit, pancreas, and the vermiform 

 appendix may be mentioned in this connection. 



Filariae. Embryo filar ial parasities are to be found in the 

 circulating blood, but the mature parasites of this group, so far 

 as they infect man, are found in the subcutaneous, interstitial, 

 subperitoneal or mesenteric connective tissue planes, with slight 

 variations according to species ; they also occur in the lymphatics. 

 Indications of the presence of the mature worm are forthcoming 

 when it is located in the subcutaneous or subconjunctival con- 

 nective tissue, or happens to block the thoracic duct or some of 

 the larger lymphatic trunks. The female of one species, the 

 guinea-worm, actually perforates the integument to discharge 

 her ova ; the perforation occurring at a spot in the foot or leg 

 likely to come into contact with water, since the young parasites 

 need an intermediate aquatic host. The embryos of other species 

 of filariae, circulating as they do in the blood stream, are able to 

 be extracted from it by certain blood-sucking insects. 



The Filaria Bancrofti, which is the parent form of the well- 

 known Filaria nocturna, particularly favours the lymphatics of 

 the trunk and extremities, and the complications induced by the 

 presence of this worm can be explained on anatomical grounds. 

 The occlusion of lymphatics, which is caused by the presence of 

 the parasite or the inflammation it has induced, is followed by 



