1 62 LOCOMOTION. [CHAP. vn. 



the skin may be dependent on a diffusion of this tissue, in small 

 quantities, throughout its areolar structure. The excretory ducts 

 of all the larger glands seem to possess a covering of fibres pertain- 

 ing to this variety : such is the case with the ductus choledochus in 

 birds, and probably in mammalia, and with the ureters and vasa 

 deferentia. The bronchial tubes may be here alluded to in their 

 capacity of an excretory apparatus, as furnishing the best marked 

 example of this arrangement. The trachealis muscle consists evi- 

 dently of the unstriped fibres, and the same may be traced down 

 the bronchial ramifications as far as the air-cells themselves, though 

 not into them. The distinctive characters of this form of muscle 

 may here be unequivocally discerned : and, if anatomists had been 

 better acquainted with them, there would not have been room for 

 those disputes regarding the muscularity of the bronchial tubes 

 which have so long attracted the interest of practical physicians. 

 Recently, indeed, there has been added to the satisfactory evidence 

 of anatomy the fact, that these fibres may be excited to contraction 

 by the galvanic stimulus.* In the case of other glands, it is still 

 unknown how far the muscular coat invests the ramifications of the 

 duct : it is most likely that it gradually ceases a short way within 

 the organ, and at least it seems clear that no portion of the secret- 

 ing membrane itself is ever invested by it. 



Distribution of the two kinds of fibre in the animal scale. 



The striped fibres have been found in all vertebrated animals, and in insects, 

 Crustacea, cirropods, and arachnida ; and future researches will probably shew 

 them to be even more extensively diffused. But, in the lower animals, we find 

 that the distinctive characters of the two varieties begin to merge together anc 

 be lost ; especially where the fibres are of diminutive size. The transver 

 stripes grow irregular, not parallel, interrupted ; a fibre will, perhaps, posses 

 them only near its centre, where its development is most advanced, and il 

 contractile energy greatest. Even the peculiarities of the unstriped fibre 

 sometimes no longer to be met with in parts which are undoubtedly muscular, 

 as the alimentary canal of small insects. It is evident that fibres of the usi 

 bulk would be greatly too large for the requirements of the case ; and they coi 

 sequently seemed to be reduced within limits which deprive them of the 

 anatomical characters by which alone we can elsewhere positively aver thei] 

 existence. It is possible that a tissue identical in nature and properties witl 

 that of striped muscle, may be the effective agent to which are due th< 

 wonderfully vivacious movements witnessed in the bodies of many of 

 minutest infusoria, where the best microscope can hardly discern even 

 organs put in motion. 



Dr. C. J. Williams, on Diseases of the Chest. Last edit. Appendix. 



