198 THE CAT. [CHAP. vii. 



delicate, and readily allow their contents to exude. It is only thus 

 the unvascular tissues such as dentine arc nourished. Besides 

 the teeth, capillaries are ahsent from cartilage and from epidermal 

 and epithelial structures. 



The white corpuscles pass readily through the walls of the minute 

 capillaries, but the red corpuscles do so hut rarely. Both veins and 

 arteries merge insensibly into capillary vessels, and a constant stream 

 of blood passes from the latter to the former through them. 



The smaller capillaries consist of a delicate membrane lined with 

 endothelium ; the larger ones have also a layer of organic muscular 

 fibres. 



7. The LYMPHATICS are present nearly everywhere in the body. 

 They are smaller but more numerous than the veins, and anastomose 

 more frequently. In some situations, as in the brain, they surround 

 and enclose the blood-vessels, and they seem to take origin (as 

 will be more fully pointed out when they are described,) in the 

 mere interspaces between the elemental parts of different organs. 

 When the lymphatics have advanced somewhat from their origin 

 they possess three coats : namely, an inner one, of longitudinally 

 disposed fibres of elastic tissue, lined with endothelium ; a middle 

 one, of circular, organic, muscular fibres, mixed with elastic fibres ; 

 and an outer coat of connective tissue also with a few elastic 

 fibres. 



Like the blood-vessels, the lymphatics have vasa vasorum in their 

 walls. 



Valves, like those of the veins, exist in the lymphatics and 

 lacteals. They are distributed at shorter intervals, and their 

 structure is sometimes less regular. 



The lymphatics and lacteals in their course pass (as has been 

 already observed,) through certain structures called LYMPHATIC GLANDS. 

 These are rounded bodies, consisting mainly of a mass of lymph 

 corpuscles enclosed in a firm envelope and richly supplied with 

 blood-vessels and lymphatics. Each gland is coated externally by 

 connective tissue (which may contain muscular fibre cells,) and 

 which completely invests it, save where the vessels enter and leave 

 it. This fibrous coat sends in processes, called trabecula3, into 

 the substance of the gland, which substance the proper glandular 

 substance consists of a mass of lymph corpuscles, with connective 

 tissue. The outer part of this mass (the cortical substance,) is 

 generally enclosed in a number of chambers, alveoli, while the more 

 central part (the medullary substance) is enclosed like so many 

 cords between the meshes of the trabeculoB. In these chambers 

 thus containing masses of tymph corpuscles, a certain space is left, 

 called the lymph sinus or lymph- channel, which space is crossed 

 only by fibres of connective tissue, with their nuclei, and is traversed 

 by the lymph stream. The lymphatics which come to the gland 

 the afferent lymphatics lose all their coats as they enter, save the 

 epithelial lining, which is continued on over the trabeculae. Simi- 

 larly the lymphatics which leave the gland the efferent lymphatics 



