THE STRUCTUKE OF THE HUMAN 1'LAC'KNTA. 53 



taining four or six blood disks abreast. It is also peculiar in 

 exhibiting sudden constrictions and dilatations, like an intes- 

 tine. 



These changes in form are most remarkable at the spots where 

 the vessel makes sudden turns, coils, or convolutions. Like a 

 capillary, however, this vessel may divide and again become 

 single, and may send off a division to a vessel of the same kind. 

 All such divisions and anastomosing vessels, however, preserve the 

 same mean diameter, and are in this respect distinguishable from 

 arterial and veinous branches. 



As regards the general arrangement of the vessels, it may be 

 observed, that 



1. One vessel may enter a villus, and returning 011 itself, leave 

 it again. 



2. Two vessels may enter a villus, may anastomose, and leave 

 it in one or two divisions. 



3. One, or more may enter, may each separate into two or 

 more divisions, which may reunite and leave the villus as they 

 entered. 



Many other modifications occur, but the general rule is, that 

 one vessel enters, and leaves the villus without dividing. 



As regards the particular arrangements of the vessels within 

 the villus, we recognize those leading varieties : 



1. The simple loop, a vessel turning closely on itself. 



2. The open loop, a vessel turning on itself, but leaving a space 

 within the loop. 



3. The wavy loop, resembling the first, except that the vessel 

 is wavy instead of being direct. 



4. The wavy open loops. 



5. The contorted loop, the contortion being generally at the 

 extremity or sling of the loop; the limbs of the loop being 

 straight or wavy as the case may be. 



6. The various modifications which arise from combinations of 

 the five foregoing forms, in single, double, triple, or quadruple or 

 anastomosing loops. The most common forms are the simple 

 and contorted loop. The simple loop, and the wavy loop, are 

 found in cylindrical villi. The open loop, and the wavy open 

 loop, occur in the flattened and heart-shaped villi. The contorted 



