114 MAMMALIA. 



suckle their young; and their blood-vessels contain 

 red blood. 



Blood. This blood consists of a yellowish liquid, in 

 which very numerous red blood-corpuscles or globules 

 (PL IX. fig. 1) are suspended, and to which the red 

 colour is owing. The blood-corpuscles are not glo- 

 bular, but discoidal, i. e. they are circular and flat- 

 tened, the sides being slightly sunk in. Their form 

 is best seen as they roll over on a slide, after the 

 application of a glass cover. The coloured corpuscles 

 are cells ; they appear yellowish red under the micro- 

 scope, the deep red colour of the blood depending 

 upon the large number of them seen at once and 

 crowded together. It need scarcely be stated that a 

 drop of blood may easily be obtained, by puncturing 

 the wrist with a clean needle. The blood is con- 

 tained in the blood-vessels. These consist of the 

 ar'teries, which convey the blood from the heart ; the 

 veins, which return it to the heart ; and a very fine 

 set of intermediate vessels, called the cap'illaries. If 

 a little water be added to a drop of blood on a slide, 

 colourless corpuscles, rather larger than the coloured 

 disks, will be seen scattered among the latter. These 

 are the colourless or lymph-corpuscles of the blood. 

 They are truly spherical, and granular on the surface. 



Bone. In examining a transverse section of a bone, 

 one or several very large cavities will be seen with 

 the naked eye in the centre of the section ; these 

 contain the marrow, or medulla. In tne long bones, 

 the medullary cavity is single, and runs longitudinally 

 down the bone ; whilst in the flat bones the cavities 

 are numerous, forming cancelli. Under the micro- 

 scope, thin transverse sections of bone exhibit oval or 

 rounded holes, or foramina (PL IX. fig. 15), which are 

 sections of canals conveying blood-vessels through the 

 bone; these are the Haver sian canals. Around the 

 sections of these canals are seen numerous concentric 

 rings, indicating layers or lamellae of bony matter. 



