RELATIONS OF THE C(ELI^. 413 



each Other only through the portcn and aula. In addition to the line representing the pia 

 in direct contact with the masses, the araclmoidea {arch.) is represented by the line 

 bridging the interval between them. 



§ 1075. Relations of the Coeliae.— These are most clearly indicated in Fig 110, repre- 

 senting a horizontal section of the typical brain. Tlie coeliffi form two series, caudal and 

 cephalic. The former are mesal or azygous, the latter are lateral or paired. The arrange- 

 ment may be roughly compared to a two-tined fork, the handle representing the mesal 

 series and the prongs the two lateral extensions. 



A more accurate analogy is with the apartments of a house. A narrow passage (the 

 eanalis centralis of the myelon) opens into a wider apartment, or rather a suite of apart- 

 ments but slightly distinguished from each other. From the farther (cephalic) end (aula) a 

 passage {porta) upon either side opens into a wing or lateral extension, each containing 

 two apartments, the second of which is closed at the farther end. 



§ 1076. Comparison of the Brain with a House. — Let us imagine that a house con- 

 sisting of a series of apartments in the order represented in Fig. 110 is completely envel- 

 oped by a continuous layer of tarred paper, and that its rooms are lined throughout 

 with waU paper, the ceilings and floors being covered with the same. 



Now it is conceivable that (1) the proper wooden wall of any apartment might be so 

 reduced in thickness at any point as to hardly merit the name ; (3) it might be omitted 

 altogether along a given line, leaving only the two layers of paper ; (3) a fold of the cntal 

 or lining paper might hang within the apartment ; (4) between the two layers of the fold 

 might he interposed a fold of the ectal or covering paper ; (5) instead of a complete fold 

 of the ectal paper there might be supported in the fold of the ental some looped strings or 

 fringes connected primarily with the ectal layer. 



It is also evident that (1) while the fold of ental paper is really projected into the apart- 

 ment, (2) the fold of ectal paper, or the strings or fringes of that paper, are covered by the 

 ental paper, and are therefore not really within the apartment ; (3) any force applied from 

 within or without will he likely to rupture the wall along the line of interruption of the 

 proper wooden wall, corresponding with the line of reflection of the ental paper therefrom 

 to form the fold. 



§ 1077. Arachnoidea. — After the removal of the cranium and the dura wMch lines it, 

 the brain of the frog, cat, man, and presumably of all Vertebrates, is found to be covered 

 by two membranes. Of these, the ectal is the more delicate, and is known as the arach- 

 noid. It was tbrmerly described as presenting two layers, a mseeral next to the brain and 

 a panetoZ lining the dura ; according to Quaiu (A, II, 573), there is insufficient evidence of 

 the existence of the latter, and it is not represented herein. 



The arachnoid passes from lobe to lobe and from fold to fold across intervening spaces 

 or fissures, or dips but slightly therein. 



§ 1078. Pia. — This is in direct contact with the brain, follows closely the contour of 

 the lobes and folds, is pigmented in the frog and some other animals, and supports Uoud 

 vessels which send branches into the substance of the brain. 



The pia is represented in all parts of Fig. Ill and 113 ; the arachnoid only in Fig. 

 113, H. 



§ 1079. Ehidyma.—Ks intimated by Todd (A, 684), Duval {2, 164), Wyman (34, 15), 

 Balfour (A, II, 364), and Quain (A, It, 540), and confirmed by our observations, all parts 

 of the true cavities of the vertebrate brain are lined by a smooth epithelium called epen- 

 dyma or endyma, the shorter name being preferable. This is akin to a serous mem- 

 brane, and secretes a watery liquid which may (as in hydrocephalus) be produced in large 

 amount. 



