990 SPECIAL PHYSIOLOGY. 



In Amphibia, the skeleton is entirely osseous ; the bony tissue presents large 

 and wide lacunae, very complex and ramified canaliculi, concentric laminae, 

 and Haversian canals. In a few situations, the lacunae cross each other at 

 acute angles. 



In the Keptiles, and also in Amphibia and Fishes, the bones are solid, or 

 contain but a few recesses filled with fat. The Haversian canals in Keptilian 

 bone are small, the concentric laminae irregular and wavy, the lacunae of me- 

 dium size and shorter than in the Fish, and the canaliculi very fine. Some 

 lacunae cross at acute angles, as in crocodiles. 



In Birds, the lacunae are smaller than in Reptiles, but larger than in Mam- 

 malia. In the latter animals, the bony structure resembles that of Man. 



Muscular Tissue. 



The fibres of both the smooth and the striped varieties of this tissue have 

 been traced in their development from nucleated cells, derived immediately 

 from the embryonal cells. 



In the case of the smooth fibres, the nucleated cells, at first roundish, become 

 elongated and fusiform ; their cell-walls and their contents blend into one mass, 

 which assumes, by degrees, the sarcous character ; in the meantime, the nu- 

 cleus of each fusiform cell becomes much elongated. Many such fusiform cells 

 produce, by their cohesion, a smooth muscular band. 



The striped muscular fibres have been described, by some, as arising, each 

 from the coalescence of rows of nucleated cells. (Schwann.) But by other 

 and more recent authorities, they are regarded as being each developed by the 

 extreme growth of a single cell. (Kemak, Fox.) It has also been maintained 

 that they originate without the intervention of true cells, through the agency 

 of rows of nuclei, lying in a blastema, which afterwards gives rise to the fibre 

 by a series of changes occurring in it. (Savory.) These differences of opinion 

 are probably as much due to the different interpretation of the same appear- 

 ances by different observers, as to differences in the observations themselves. 

 They illustrate the difficulties of microscopic research. If the primitive ani- 

 mal cell which forms a muscular fibre be regarded as a gymnoplast, easily fused 

 with its neighbors, the discrepancy of opinion may, perhaps, be reconciled. 



Supposing rows of nucleated cells to coalesce to form a single fibre, it is be- 

 lieved that the coalescing parts of the cell-walls are absorbed, and that thus a 

 long tube is formed, which ultimately becomes the sarcolemma ; the contents 

 of the united cells, at first finely granular, are said to grow and become sar- 

 cous, their elements arranging themselves into linear and transverse series, 

 first on the outer surface next to the sarcolemma, and then more centrally, so 

 as to form the transversely marked fibrillae. In the meantime, as the cells 

 grow in length, the nuclei separate from each other, and become obscured, but 

 are never lost. If, however, only one long cell forms each fibre, the wall of 

 such an elongated cell is believed to constitute the sarcolemma, and the con- 

 tents, originally granular, are said to be gradually increased and differentiated 

 into the fibrillae, first becoming marked by longitudinal lines, and afterwards 

 by transverse striae. The nuclei multiply by successive subdivisions, and re- 

 main surrounded by granular matter. By many, it is thought that the orig- 

 inal cell-wall, or cell-walls, do not form the sarcolemma, but that this is the 

 result of a subsequent deposit of a homogeneous membrane around the nearly 

 perfectly formed bundle of fibrillae. 



Whatever their precise mode of origin may be, the muscular fibres seem, 

 when first recognizable, like very fine bands, sometimes not more than one- 

 tenth of the diameter of the fully formed fibre, and having bulging nuclei in 

 them at intervals. When composed of such fibres, the young muscles resem- 

 ble their tendons. As the fibres gradually increase in width, they assume the 

 adult characters, and become uniform in diameter, so that the nuclei are no 

 longer so easilv visible (Fig. 122, g). At birth, all the muscles are said already 

 to contain their full number of fibres, so that their future growth consists only 

 in an increase of length and width of the pre-existing fibres. At birth, the 

 fibres are about one-fifth of their ultimate dimensions. The fibrillae of each 

 fibre may during growth become a little wider ; but it is thought rattier that 



