414 ANATOMICAL TBCSNOhOOY. 



§ 1080. Telm. — The pia bears to tlie brain which it covers the relation which the 

 tarred paper does to the house, while the paper covering the interior walls is represented 

 in the brain by the endyma. 



Where the proper nervous substance of the ccelian parietes is atrophied so that the pia 

 and the endyma ai'e nearly or quite in contact, the resultant is called a tela. 



With the cat, and probably with other animals, the telae sometimes seem to be not 

 altogether devoid oC nervous structure ; indeed, it would seem quite possible that between 

 a true membranous tela and a thin nervous lamina like the cephalic part of the valvula 

 there may be at least one intermediate condition. 



When the pia is removed, the telse, being connected therewith, are apt to be torn off ; 

 but the consequent exposure of the cavities is no more proof of the presence of a natural ori- 

 fice than is the fontanelle of a child, after the removal of the scalp and the dura, evidence 

 that the cranial cavity is naturally in free communication with the outside of the head. 



§ 1081. Plexuses. — Notwithstanding the more or less elaborate accounts in works upon 

 Descriptive Anatomy, and some recent efforts to elucidate their mode of development, 

 the precise structure and arrangement of the plexuses is far from well ascertained. Indeed, 

 it is probable that a plexus may be formed in two or more ways in different species or in 

 different parts of the same brain. 



As we understand the matter, a ccelian plexus is formed in one of two ways : — 



(A) Certain vessels of the pia are protruded entad of the proper nervous parietes so as 

 apparently to enter the ccelifB. 



(B) Certain parts of the pia containing vessels are carried as folds entad of the parietes 

 so as apparently to enter the ccelise (Fig. 121). 



In either case the endyma along the line of interruption of the proper nervous wall ia 

 reflected upon the intruded pia or vessels and covers them completely ; hence, while the 

 thinness of the epithelium permits osmosis to occur practically as if the vessels were free, 

 yet from a morphological point of view they are not free, but are excluded from the cavity 

 just as the kidney or the intestine is excluded from the abdomen by the visceral layer of 

 peritoneum (Fig. 78) ; in fact, the cases are strictly comparable. 



I 1083. The, alleged "Foramen of Mngendie." — Magendie described (A), under the 

 name "orifice des cavites encephaliques," an opening which he believed to exist in the 

 metatela near the caudal end of the metacoelia. Luschka figured it (A, Taf. Ill, Fig. 1), 

 but no other representation is known to us, although its existence is generally admitted 

 (as in Quain, A, II, 513 ; MihalUovics, A, 59). 



Todd, however (A, 641), believes such an orifice to be artificially produced ; the senior 

 author ( 14, 543, 555) could not find it in the cat, and its natural presence is emphatically 

 denied by Duval (1, 33). See, however, Westbrook {!). 



§ 1083. Complete Gircicmscripiion of the Cavities.— First in 1876 

 (Wilder, 4 ; 9, 136), and frequently since, we have made upon the 

 cat's Ibrain experiments (Iby inflation with air and by the injection 

 of alcohol, water and plaster), which failed to indicate the presence 

 of any natural communication (as by tlie rima or " fissure of Bi- 

 chat" or "great transverse fissure") between tlie coelise and the 

 exterior. 



Upon a point of general arrangement like this there is every 

 presumption in favor of uniformity throughout the vertebrate series ; 

 hence we may fairly regard the c(eUcB (excepting at an early stage 



