522 



SEROUS AND SYNOVIAL MEMBRANES. 



gular outline, from which cells may often be 

 seen projecting. The attrition which these 

 appearances would seem to denote appears to 

 be exerted upon the cells equally with the 

 interstitial substance of the cartilage, but is 

 more difficult to verify in the former tissue, 

 since such a cell that has suffered a partial 

 destruction of its form, has, at the same time, 

 lost a valuable optical means of detection. 

 Occasionally, however, as in/%. 402., on looking 



Fig. 402. 



Free surface of Articular Cartilage. From the elbow- 

 joint of an adult Cat. (Magnified 200 diameters.) 



directly at the free surface of the tissue, we 

 see a darkish nucleus, lying very superficially, 

 and surrounded by a clear space. In all pro- 

 bability, this was such a cell ground down to 

 a hemispherical cavity. More rarely, a profile 

 view of such a hemisphere is obtained. 



On examining similar specimens from ani- 

 mals of the same species at successively 

 younger ages, the intercellular substance 

 becomes gradually more scanty, and finally 

 altogether disappears, leaving the whole of 

 the surface occupied by a cell-growth, which 

 is a covering, but not an epithelium ; unless 

 we extend the application of this objectionable 

 word, and call the whole cartilage itself, what 

 indeed we might with perfect truth, " a 

 modified epithelium." 



The accuracy of this description of the 

 cartilage of very young animals is easily veri- 

 fied by a vertical section ; and, if it be made 

 sufficiently deep, it will include a portion of 

 another structure, and a different process, 

 with which it may be advantageous to com- 

 pare it. At the furthest extremity of such a 

 section, we see the ossification of temporary 

 cartilage actively going forward. First comes 

 the formation of cancelli, and the enclosure 

 of cells ; next, a little nearer the articular 

 surface, the greatly dilated cells are arranged 

 in closely-packed rows, the bottoms of which 

 rest in cups of bone, which will soon become 

 cancelli. Still approaching the articular sur- 

 face, we find the cartilage corpuscles smaller, 

 more refractile, and flatter ; but yet with a 

 distinctly linear arrangement. The loss of 

 this arrangement in rows seems to indicate 

 the limit of the ossifying cartilage and the 

 commencement of the articular lamina; and I 

 have often seen the distinction still further 

 marked out by a horizontal fissure in this 

 situation, the effect of accidental violence, 

 no doubt, but perhaps indicative of some 

 deficiency of cohesion dependent on structure. 



Immediately beyond this situation, the 

 cartilage cells are scattered irregularly but 

 closely through the transparent intercellular 

 substance. They are angular and refractile, 

 and they contain a large granular nucleus. 

 Many of them are elongated, and somewhat 

 spindle-shaped, while many more are tri- 

 angular ; and these two forms appear respec- 

 tively to precede and follow a fissiparous 

 multiplication of their numbers, the constancy 

 and accuracy of which would almost allow of 

 its being termed a bisection. The details of 

 this process have been already alluded to 

 in speaking of the subtendinous bursa-, and 

 are too well known to need any recapitulation 

 here. From hence onwards to the articular 

 surface, the cells become more numerous, 

 larger, and less angular in shape, until 

 finally, on the surface itself, the increase of 

 their number and size results in a continuous 

 layer. But the appearances of this multipli- 

 cation are not seen in the most superficial 

 stratum of all, although the prevalence of the 

 hemispherical outline still indicates the binary 

 nature of the fission ; whence it seems pro- 

 bable, that just upon the surface the increase 

 is one of bulk only. 



In fig. 403. is represented a vertical view of 

 the superficial and of a deeper layer, which 

 contrasts them in the particulars just men- 



Fig. 403. 



Articular Cartilage from a Kitten four days old. 



a, arrangement of cells on the free surface ; b, a 

 deeper stratum. (Magnified about 250 diameters.) 



tioned. The condition of these cartilages in 

 the adult fishes and reptiles closely resembles 

 this description of their appearance in the 

 young mammal, in the complete cellularity of 

 their surface. For the knowledge of this 

 fact, I am indebted to Mr. Quekett. 



SEROUS MEMBRANES. The serous mem- 

 branes, presenting a structure which offers a 

 close general parallel with that of the pre- 

 ceding tissues, are yet contrasted with them 

 in many important respects. The first and 

 most obvious distinction, and one on which 

 the other structural differences are to a great 

 extent based, is, that in place of their main- 

 taining a direct relation to the locomotive 

 apparatus, or being connected with the seg- 

 ments of the skeleton in the diarthrodial 

 joints, the organs to which they are more im- 

 mediately subservient are those concerned in 

 the organic or vegetative life. 



The serous membranes of the human body 

 are seven in number ; three bein.<j median 

 and sing'e, while two are double and lateral. 

 They are the arachnoid, pericardium, and 

 peritoneum, with the pleurae and tunicae 



