336 



HISTOLOGY 



the invaginations. A peculiarity of this blood supply is the way that the 

 blood stream is divided. The dendritic method of division found in the 



circulatory supply of most 

 organs and tissues, where 

 the artery divides into 

 branches and subdivides un- 

 til the stream is running in 

 capillaries, is replaced by a 

 great mass of capillaries that 

 arise together on a few large 

 vessels that are found on the 

 inner surface of the bladder 

 and run in a mass with par- 

 allel courses to the secreting 

 epithelium. Here they enter, 

 and having entered, they be- 

 gin to separate from one 

 another. Toward the outer 

 surface of the mucosa, as we 

 shall call the layer of invagi- 

 nated epithelium together 

 with the connective and other 

 tissues that are involved with 



FIG. 299 Part of the distal region of the gas-secreting j t fa ca piH ar ies anastomose 

 gland of the cod, Gadus. bl c. , blood capillaries with 11 i i 



endothelial walls on which the proximal surfaces of and become Slightly larger. 



the gas cells rest; /., parts of two lumina on which They may be designated sinu- 



the distal ends of the gas cells border, x 1000. i i o T i 



soids here. So complicated 



have the histological relations become in this mucosa that the epithelial 

 nature of the secreting cells is doubtful without careful study. Many inner 

 cells are apparently devoid of a disto-proximal differentiation, owing to 

 the fact that the lumen of the gland is closed by the absence of any 

 secretion at the point where they are. In places where the comparatively 

 scarce lumina of secreting acini push their way down into the mass, the 

 relations of the cells are clearly seen, and in some cases they appear typi- 

 cally columnar, resting on the blood channels from which they draw their 

 materials with the even surface of their ends bounding the round, open 

 lumen. Such a lumen was probably full- of gas at the time of fixa- 

 tion. 



The epithelium directly on the primary surface of the gas gland is 

 slightly different from the cells bordering on the secondary lumina, and 

 probably have in addition to their duty of gas production the work of 

 secreting the peculiar layer that lines the entire inner surface of the swim- 

 bladder. The gas bubbles slowly force their .way through this layer, 



