140 HISTOLOGY. 



the valve, as may be shown in injected specimens especially. One of these 

 swellings is shown in Fig. 163. The valves of a larger lymphatic vessel 

 appear in Fig. 164. 



In lymphatic vessels having a diameter of 0.2-0.8 mm. or more, 

 three layers may be distinguished very similar to those of thin walled veins. 

 The intima consists of endothelium and connective tissue containing deli- 

 cate elastic nets with longitudinal meshes. The media has circular smooth 

 muscle and but little elastic tissue. The externa has bundles of longitu- 

 dinal muscle fibers, and similarly arranged connective tissue. The nerve 

 supply is like that of the blood vessels. 



Although the present tendency, based upon the similar results of several 

 investigations, is to make a sharp distinction between tissue spaces and lym- 

 phatic vessels, it should be noted that these have long been regarded as in- 

 separable. Some authorities still consider that the lymphatic vessels open 

 freely at their distal ends and blend with connective tissue. Lymphatic ves- 

 sels have also been described as opening into the peritoneal cavity and other 

 parts of the coelom through definite mouths or stomata. The stomata are 

 thought to be artificial. The endothelium remains entirely separate from 

 mesothelium so far as is known. 



BLOOD. 



Blood consists of rounded cells entirely separate from one another 

 floating in an intercellular fluid, the plasma. The plasma also contains 

 fragments of cells called blood plates or platelets, together with smaller 

 granular bodies. The blood cells or corpuscles are of two sorts, (i) red 

 corpuscles (erythrocytes) which become charged with the chemical com- 

 pound, haemoglobin, and which lose their nuclei as they become mature; 

 and (2) white corpuscles (leucocytes) which are of several kinds, all of them 

 retaining their nuclei and containing no haemoglobin. The redness of 

 blood is not due to the plasma, but is an optical effect produced by super- 

 posed layers of the haemoglobin-filled red corpuscles. Thin films of 

 blood, like the individual red corpuscles as seen fresh under the micro- 

 scope, are yellowish green. Blood has a characteristic odor which has 

 been ascribed to volatile fatty acids; it has an oily feeling associated with 

 its viscosity, an alkaline reaction and a specific gravity said to average in 

 the adult from 1.050 to 1.060. 



Red corpuscles. The first cells in the blood are apparently all of one 

 sort, derived from the blood islands. They are large, round cells with 

 a delicate membrane and a pale granular protoplasmic reticulum; their 

 relatively large nuclei contain a chromatin network with several coarse 

 chromatin masses. Haemoglobin develops in their protoplasm giving it 



