296 



THE CIRCULATION. 



Fig. 99. 



until it is reduced to a single layer of circular, fusiform, unstriped, 

 muscular fibres, which in their turn disappear altogether, as the 

 artery merges at last in the capillaries ; leaving only, as we have 

 already mentioned, a simple, homogeneous, nucleated, tubular mem- 

 brane, which is continuous with the internal arterial tunic. 



The capillaries are further distinguished from b<>th arteries and 

 veins by their frequent inosculation. The arteries constantly 

 divide and subdivide, as they pass from within outward; while 

 the veins as constantly unite with each other to form larger and 

 less numerous branches and trunks, as they pass from the circum- 

 ference toward the centre. But the capillaries simply inosculate 

 with each other in every direction, in such a manner as to form an 

 interlacing network or plexus, the capillary plexus (Fig. 99), which 

 is exceedingly rich and abundant in some organs, less so in others. 

 The spaces included between the meshes of the capillary network 

 vary also, in shape as well as in size, in different parts of the body. 



In the muscular tissue they 

 form long parallelograms ; in 

 the areolar tissue, irregular 

 shapeless figures, correspond- 

 ing with the direction of the 

 fibrous bundles of which the 

 tissue is composed. In the 

 mucous membrane of the 

 large intestine, the capillaries 

 include hexagonal or nearly 

 circular spaces, inclosing the 

 orifices of the follicles. In 

 the papilla3 of the tongue and 

 of the skin, and in the tufts 

 of the placenta, they are 

 arranged in long spiral loops, 

 and in the adipose tissue in wide meshes, among which the fat 

 vesicles are entangled. 



The motion of the blood in the capillaries may be studied by 

 examining under the microscope any transparent tissue, of a 

 sufficient degree of vascularity. One of the most convenient parts 

 for this purpose is the web of the frog's foot. When properly 

 prepared and kept moistened by the occasional addition of water 

 to the integument, the circulation will go on in its vessels for an 

 indefinite length of time. The blood can be seen entering the 



CAPILLARY NETWORK from web of frog's foot. 



