1 88 HISTOLOGY 



more closely than the arteries. Valves are very numerous in lymphatic^ 

 vessels^ They are shown in section in Fig. 179. In the small vessels the 

 valves are described as folds of endothelium, such as would be produced 

 if the distal part of the vessel were pushed forward into the proximal part. 

 The vessels are often distended on the proximal side of the valve, produc- 

 ing bulbous enlargements, as shown in Fig. 178. Owing to the presence 

 of these valves, compression of tissue containing lymphatic vessels, or 

 the contraction of the muscles of the media, causes an onward flow of the 

 lymph. The nerves to lymphatic vessels are like those of the blood 

 vessels. Lymphatics are provided with vasa vasorum. As shown by 

 Evans (Amer. Journ. Anat., 1907, vol. 7, pp. 195-208) very small lymph- 

 atic vessels are accompanied by blood capillaries, and the larger lymphatics 

 are surrounded by a wide-meshed capillary network resting on the outer 

 side of the lymphatic media. (In the same volume of the Journal, pp. 

 389-407, Miller describes the network of blood capillaries around the 

 lymphatic vessels of the pleura.) 



BLOOD. 



Blood consists of round cells entirely separate from one another, float- 

 ing in an intercellular fluid, the plasma. The plasma also contains as a 

 regular and apparently important functional constituent, the blood plates 

 (or platelets), together with smaller granular bodies. Blood cells or cor- 

 puscles are of two sorts, (i) red corpuscles or erythrocytes, which become 

 charged with the chemical compound, hcemoglobin, and which lose their 

 nuclei as they become mature; and (2) white corpuscles or 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 superimposed layers of the haemoglobin-filled 

 red corpuscles. Thin films of blood, like the individual red corpuscles 

 seen fresh under the microscope, are yellowish green. Blood has a charac- 

 teristic 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. Development. The first cells in the embryonic 

 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 pro- 

 toplasmic reticulum; their relatively large nuclei contain a fine chromatin 

 network with several coarse chromatin masses. Haemoglobin later devel- 

 ops in their protoplasm, giving it a refractive homogeneous appearance. 

 Stained with orange G or eosin it is clear and brightly colored, generally 

 quite unlike any other portion of the specimen. Often the haemoglobin 

 has been more or less dissolved from the corpuscles, which then appear 

 granular or reticular. 



