266 OF THE PRIMARY TISSUES OP THE HUMAN BODY. 



punctured; but this notion of its structure was purely hypothetical, neither 

 fibres nor cells having been made out by any anatomist. Attempts have been 

 recently made by Briicke and Hanover to show that this body has a laminated 

 structure ; but much of the evidence adduced by them has been shown by Mr. 

 Bowman 1 to be fallacious. Some such arrangement may be clearly made out, 

 however, in the vitreous body of the Fish's eye, and a definite fibrous texture, is 

 distinguishable in that of the Bird ; so that it is probable that some approach to 

 it exists in the vitreous body of Man. Whatever may prove to be its intimate 

 structure, the nutrition of this substance is certainly effected by the same means 

 with that of the cornea and crystalline ; namely, by imbibition from the vessels 

 distributed upon or near its envelop. The ciliary processes of the Choroid 

 membrane are almost entirely composed of large ampullated vessels, closely 

 resembling those of synovial membrane (Fig. 53); and the blood which they 

 contain is probably one of the chief sources of nutriment to the vitreous body. 

 The reproduction of the vitreous body, an escape of portions of which is not at 

 all an unfrequent occurrence during the performance of operations upon the 

 eyes, seems to take place with great rapidity and completeness. 



4. Of the Tissues consolidated by Earthy deposit ; Bones and Teeth. 



256. Both the Fibres and Cells of the Animal tissue may be consolidated by 

 earthy deposits ; these being chemically united with the gelatin of the Fibres ; 

 or secreted, either alone, or in combination with animal matter, into the cavities 

 of the Cells. An example of the formation of a skeleton by the consolidation 

 of fibresj is presented by the shell and other hard parts of the Echinodermata f 

 the intimate structure of which, as shown by the Microscope, strongly reminds 

 us of Areolar tissue that might have undergone the calcifying process. Again, 

 we have an example of the formation of a skeleton by the deposit of mineral 

 matter in the cavities of cells, in the shells of Mollusca ; 3 in many of which 

 (especially among the " bivalves") the cellular character is permanently shown, 

 a consistent membrane being left after the carbonate of lime that consolidated 

 the cells has been dissolved away by an acid an arrangement precisely similar 

 to that which is found in the Enamel of teeth ( 275), though the consolidating 

 material is different. In the skeletons of Invertebrated animals, which, with 

 few exceptions are dermal or tegumentary, there is no provision for any other 

 mode of increase than that which is effected by addition to the surface or edges 

 of the parts already formed ; and where, as in the Crustacea, such an addition 

 would not serve to maintain the form of the calcified envelop, and to preserve 

 its adaptation to the muscular apparatus which is attached to its interior, it is 

 periodically thrown off and renewed. And in the very few cases in which ab- 

 sorption takes place for the removal of parts that have become superfluous, this 

 absorption, like the previous deposition, is superficial only ; and is effected by 

 the mere contact of an absorbent surface, without any penetration of vessels 

 into the substance to be removed. The osseous skeleton of the Vertebrated 

 animal, on the other hand, is essentially formed by the consolidation of the 

 tissues immediately surrounding the nervous centres, and of outgrowths from 

 these '* it is invested by the muscular apparatus, which gives motion to its dif- 

 ferent parts ; and in order that it may keep pace with the progressive growth of 

 the organism in general, it must be made capable not merely of receiving addi- 

 tions to its surface, but also of having its interior gradually consolidated by new 



1 "Lectures on the Parts concerned in the Operations on the Eye," p. 94, et seq. ; also 

 the "Dubl. Quart. Journ. of Med. Sci.," Aug. 1848. 



2 See "Princ. of Phys., Gen. and Comp.," 3 195, Am. Ed. 



3 Op. cit., \ 197. 



4 See "Princ. of Phys., Gen. and Comp.," | 320, b, et seq., Am. Ed. 



