769 



MEMBRANIPORA. 



MENDIP HILLS. 



770 



membrane, the pleura [PLEURA] which lines the walla of the chest, and 

 is then reflected on and covers the surface of the lung ; and thus there 

 is inclosed between the surface of that part which lines the chest and 

 of that which envelops the lung an extremely narrow space, a sac, 

 into which a very small quantity of fluid is secreted. During respi- 

 ration there is a constant friction between the lung and the walls of 

 the chest, which the fluid, by its lubrication of their surfaces, renders 

 easy. It is the general condition of serous membranes, that they exist, 

 with the single exception of the conjunctiva of the eye [EYE], wherever 

 there is friction between the surface of an organ and the cavity in 

 which it is contained. They are adapted for this condition by possess- 

 ing a remarkable smooth polished surface, covered by a very fine layer 

 of epithelium, through which then- moistening secretion can easily 

 pass. The basis of their structure is a fine and rather loose cellular 

 tissue, which by boiling is at once converted into gelatine. The 

 serous membranes in man are the arachnoid, which is found in the 

 cerebro-spinal cavity [BRAIN], lining the dura mater, and covering the 

 brain and spinal chord, and lining the ventricles ; the pluroe, lining the 

 chest and covering the lungs [RESPIRATION] ; the pericardial serous 

 membrane, similarly related to the heart and its investing sac 

 [HEART] ; the peritoneum, lining the abdominal walls and covering 

 the abdominal part of the digestive canal, the liver, spleen, part of 

 the pancreas, &c. [PEBITONEUM] ; and the tunica vaginalis, forming 

 the sac of the testis. 



The Synovial Membranes, by which joints are lined, and the heads 

 of bones which move on each other covered, may be regarded as a 

 modification of serous membranes, differing from them chiefly in the 

 character of their secretion and in some of their diseases. [ARTICU- 

 LATION.] 



A membrane very similar to the serous lines the whole vascular 

 system, "and form- the internal membrane of the arteries, veins, 

 lymphatics, and lacteals [ARTERY; VEINS; ABSORBENT SYSTEM], form- 

 ing a closed cavity with innumerable ramifications, and affording, with 

 its polished surface and fine epithelium, the least possible obstacle to 

 the movement of the circulating fluids. 



The Mucous Membranes, like the serous, are named from their 

 peculiar secretion. [Mucus.] While the serous membranes line all 

 those cavities whose surfaces are in contact with living parts, the 

 mucous membranes line those canals and cavities which in the adult 

 condition of man and the higher Vertebrata are exposed to the con- 

 tact of the air and other inorganic substances. The basis of these 

 membranes is a compact cellular tissue, which does not yield gelatine 

 in boiling, and whose areolae do not contain fat. [INTESTINES.] Their 

 epithelium is thicker than that which covers serous membranes, but 

 thinner than the epidermis covering the skin, to which they are in 

 many respsects similar. [SKIN.] In the parts where they line the 

 organs of sense the mucous membranes are generally beset with fine 

 nervous papillae ; hi other parts, numerous glands for peculiar secre- 

 tions open on their surface by orifices through which the membrane is 

 continued up the branches of the ducts into the very substance of the 

 gland. [GLAND.] Nearly all the tracts of mucous membrane in man 

 communicate with each other : they are, the nasal, which lines the 

 cavities of the nose [NOSE] ; the conjunctiva!, which covers the front 

 of the eye-ball and lines the eye-lids, and opens by the lachrymal duct 

 into the nose [EYE] ; the auditory, which lines the cavities of the 

 ear [EAR] and opens into the pharynx ; the digestive, including that 

 which lines the mouth, oesophagus, stomach, intestines, and the several 

 glands whose ducts open into this canal [STOMACH ; INTESTINES] ; the 

 respiratory, which lines the larynx, trachea, and bronchial tubes 

 [Luxos ; RESPIRATION] ; the urogenital ; and the mammary. [MAM- 

 MARY GLANDS.] 



The Fibrous Membranes are those which are chiefly formed of 

 tendinous tissue. They serve either to form strong cavities for the 

 protection of important parts, as the pericardium, the dura mater, the 

 fibrous capsules of joints, the sheaths of tendons, &c., or to envelop 

 and strengthen certain parts, as the periosteum, fascia;, &c. ; or they 

 are merely expanded tendons, as aponeurosea. They are tough and 

 inelastic membranes, composed of the shining dense wavy fibres which 

 constitute the usual structure of tendons, mixed with more or less of 

 a dense cellular gelatinous tissue. [AREOLAR TISSUE.] 

 MEMBRANIPORA. [PoLYZOA.] 



MEMECYLA'CE^E are a very small natural order of Polypetalous 

 Exogenous Plants, consisting of a few tropical species of little interest. 

 Most of them inhabit the East Indies, the Mauritius, and Madagascar. 

 They are in habit and foliage like Myrtacea, with which order they 

 agree in most respects ; but they have anthers which in form resemble 

 those of many Melaitomacea, and the leaves have no transparent dots. 

 From the latter order they are distinguished by their leaves not being 

 ribbed, and by their cotyledons being convolute. In the opinion of 

 Brown and Chamisso, it is rather as a section of Mdcutomacece than as 

 a peculiar order that Memecylacece are to be accounted. No useful 

 properties have been assigned to any of the species, except to Meme- 

 cylon edule, an East Indian plant, whose leaves, according to Roxburgh, 

 are an ingredient in the dyes of Coromandel, and whose ripe astringent 

 pulpy Mick berries are eaten by the natives. 



MKM1NNA, a genus of the sub-family Moschina (Gray). 

 MKNACANITE. [TITANIUM.] 

 MKN.-ETHIUS. [ 



HAT. HIST. DIV. VOL. HI. 



Memecylacea. 



1, Motiriria Guayanensii ; 2, a full-blown flower ; 3, a stamen, with the anther 

 opening by pores at the upper end ; 4, a transverse section of a ripe fruit. Copied 

 from a figure by Turpin. 



MENDIP HILLS, a long ridge of limestone extending from Wells 

 in Somersetshire to the Bristol Channel at Bleydon Hill and Brean 

 Down. Through its whole length it is what geologists term an anti- 

 clinal axis, the strata dipping to the north under the drainage of the 

 Avon and the Yeo, and to the south under the low plains watered by 

 the Axe and the Brue. This axis passes from Frome by the Beacon 

 Hill above Shepton Mallet, Masbury Castle, Nine-Barrow Hill, and 

 Black Down, to Bleydou Hill, Uphill, and Brean Down, from whence, 

 according to Buckland and Conybeare, it may be supposed to be 

 continued into the Steep Holm in the Bristol Channel. 



Along the line of the axis of Mendip old red-sandstone strata show 

 themselves for considerable lengths, and form the nucleus of this 

 miniature mountain range. They are exposed on the roads from Wells 

 to Chewtown Meudip, and to Harptree, in each case evidently lying 

 below the carboniferous limestone. Upon the slopes of this limestone, 

 both north and south, rest considerable stratified masses of what is 

 often justly termed magnesian conglomerate, and this is covered by 

 the general mass of red marls which fill so large a tract in the low 

 parts of Somersetshire. The limestone series is estimated by Buckland 

 ind Conybeare at from 500 to 700 yards thick. The axis of the 

 Mendip Hills runs irregularly east and west : the geological era of its 

 principal upward movement appears to be anterior to the red marls, 

 and probably to the red conglomerate; though near Wells and in other 

 parts the slope of the conglomerate beds proves a subsequent move- 

 :ni!iit . There is no better example known of the unconformity of strata 

 ;han that presented in Vallus Bottom, near Wells, by the junction of 

 ;he lower oolite formation and the mountain or carboniferous lime- 

 stone. Here the upturned and almost vertical strata of mountain- 

 imestone are found covered by horizontal strata of oolite, each of 

 ihese contrasted rocks containing the characteristic fossils which belong 

 ;o them elsewhere. What renders the case more curious is the fact 

 ;hat the level surface of the subjacent inclined beds of limestone is 

 not only worn smooth by littoral action below the oolite, but also 

 covered by attached oysters, and perforated by the lithophagous 

 shells of the oolitic sea into large and small holes now full of the oolite, 

 and partly retaining the boring shells not uncommon in that rock. 



o, the Oolite in level beds ; I, the Mountain Limestone in steeply-inclined beds. 



The most elevated point of the Mendip Hills is Masbury Castle, 

 about 999 feet above the sea-level. 



The features of these hills remind the observer of some parts of 



3D 



