22 ELECTRO-PHYSIOLOGY CHAP. 



do not vanish on raising or lowering the objective. The number 

 of fibrils in the section diminishes towards the end of each cell 

 owing to their unequal length (Engelmann). The fibrils are so 

 highly refractive that it is sometimes difficult to recognise the 

 boundaries of adjacent cells, and a muscular layer of the kind we 

 are describing may even appear as a single, undifferentiated mass 

 of fibrils. 



Smooth muscle-fibres do not, for the most part, occur singly, 

 but arrange themselves into bundles, membranes, or bulky masses. 

 The finer and more delicate bundles of muscle-cells are often united 

 into a net with wide or narrow meshes, each adjacent process anasto- 

 mosing with its neighbour by branches. The bladder of amphibia 

 is a fine illustration of such a net with wide meshes. Among 

 Invertebrates a similar plexus of uninuclear muscle -cells is 

 found in the heart of molluscs, sucker of echinoderms, etc. 



The intestine of Vertebrates is the best example of the struc- 

 ture of a membrane composed of smooth muscle-cells. Here the 

 arrangement of fibre-cells in two layers, crossing at right angles, 

 which is characteristic of most hollow organs whose walls contain 

 smooth muscle-fibres, is very conspicuous. A similar arrangement, 

 i.e. a longitudinal and a circular muscle-layer, within which the 

 axis of the fibres stand vertically to each other, is found in 

 the ureter, and among Invertebrates in the cutaneous muscular 

 layer of worms. 



The anatomical relations of <(/<ii'ciif musclc-cdls with one anot//< r 

 is a point of great interest. A number of physiological facts 

 pointed to direct conductivity from cell to cell, in certain smooth 

 muscles, long before histology gave substantial support to the 

 conjecture. The idea of a direct communication by means of proto- 

 plasmic bridges between adjacent cell-elements (which is not at 

 all a new hypothesis) has recently obtained extensive confirmation 

 on the botanical side, and in animal histology such cell-bridges 

 have long been known in particular objects (bristle or prickle cells 

 of the epithelium). Analogous structures have recently been 

 described in smooth muscle-cells also. In the transverse section 

 of a crowded tract the single elements appear to be united 

 by a homogeneous cement substance, which gives a reaction 

 similar to that of the cement substance of endothelium. With 

 certain very preservative hardening methods, it becomes possible 

 to sec under the high power, that the single cells in the transverse 



