220 



CAPILLARY BLOOD-VESSELS. 



rnious with the commencing twigs of the Venous system. The 

 communication is established by means of a set of extremely 

 minute vessels, which are termed Capillaries.* These capil- 

 laries form a network, which is to be found in almost every 



part of the body (fig. 121). 

 It is in them alone that the 

 blood ministers to the opera- 

 tions of nutrition and secre- 

 tion. Even the walls of the 

 larger blood-vessels are inca- 

 pable of directly imbibing 

 nourishment from the blood 

 which passes through them ; 

 but are supplied with minute 

 branches, which proceed from 

 neighbouring trunks, and form 

 a capillary network in their 

 substance. The diameter of 

 the capillaries must of course 

 bear a certain proportion to 



J . A , . . *, ,f , . , 



that 01 tllC DlOOd-dlSCS WIUCI! 



ho vp f n rqqq tVirmio-Ti fhpTn 

 naVG t0 P aSS mrou g n 



in Man they are Commonly 

 i_ j_ i O^AA.L'U t. 



irom about 1 - 2oOOth to 



1_1 600th of an inch in dia- 

 . 



meter. In the true capilla- 

 ries, it would seem that only one row or file of these particles 

 can pass at a time ; but we frequently see vessels passing 

 across from the arteries to the veins, which will admit 

 several rows. There seems, however, to be a considerable 

 difference in the diameter of the same capillary at different 

 times ; a change sometimes taking place from causes which 

 are not yet understood, f The rate at which the blood moves 



* From the Latin capilla, hair ; so named on account of their being, 

 like hairs, of very minute size. Their diameter is really, however, far 

 less than that of ordinary hairs. 



f The circulation of 'the blood in the Frog's foot, the tail of the 

 Tadpole, the gills of the larva of the Water-Newt, the yolk-bag of 

 embryo Fish, and other appropriate subjects for the observation, is one 

 of the most beautiful and interesting spectacles that the Microscope can 

 open to us. Details of the various modes of exhibiting it will be found 

 in the Author's treatise on " The Microscope and its Revelations," 

 Chap, xviii. 



Fig. 121. PORTION OF THK MEMBRANE 



BETWEEN THE TOES Of THE HIND 



FOOT OF THE FROG, more highly 

 magnified than in fig. 120, showing the 

 network of Capillaries that traverses it ; 

 a, small venous trunk; b b, branches 

 communicating with the capillaries ; 

 c intervening tissue covered with epi- 

 thehum cells. 



