242 PHYSIOLOGY AT THE FARM. 



of muscular parts that is, of parts in which the veins are sub- 

 jected to irregular pressure are those provided in particular 

 with valves. 



Capillaries. When the web of a frog's foot is inspected 

 through a microscope, the blood is seen passing rapidly along 

 the small arteries, and thence more slowly through a network 

 of finer channels by which it is conducted into the veins. These 

 small vessels, interposed between the finest branches of the 

 arteries and the commencing veins, are the capillary vessels. 

 The capillary vessels of a part most commonly assume the form 

 of a network, the branches of which, though not all absolutely 

 equal, are of tolerably uniform size, and do not divide into 

 smaller branches like arteries, or unite into larger ones like the 

 veins ; but the diameter of the tubes, as well as the shape and 

 size of the reticular meshes which they form, differ in differ- 

 ent textures. Their prevalent size in the human body may, 

 speaking generally, be stated at from l-3500th to l-2000th 

 of an inch, as measured when naturally filled with blood.* 



The capillaries have real coats, and are not mere channels 

 drilled in the tissue which they pervade. The number as well 

 as the structure of their coats differs according to the size of 

 the vessels. Capillaries of a diameter less than 1 -2400th of 

 an inch have but a single coat formed of simple homogeneous 

 transparent membrane, with nucleiforrn corpuscles attached at 

 intervals on the outer surface, or enclosed, as it were, in the sub- 

 stance of their long diameters parallel to the axis of the vessel.-f- 

 In vessels one or two degrees larger the structure is more com- 

 plex. " The corpuscles of the primitive simple membrane are 

 more numerous and more lengthened. An epithelium exists 

 on the inside of the primitive membrane, and on its outside is 

 added a layer containing nucleiform corpuscles elongated in a 

 direction across the diameter of the vessel. This layer corre- 



* Sharpey in 'Quain's Anatomy,' vol. i. p. 227. t Idem. 



