THE LYMPHATIC SYSTEM. 



105 



Neural 

 al 



Aorta 



Mesocardium 



structural peculiarities of the cardiac fibres, as contrasted with those of 

 ordinary striated muscles, indicate a less complete differentiation in the heart- 

 muscle, this being particularly true of the Purkinje fibres. These charac- 

 teristics are probably correlated with the exceptional activity that the 

 heart-muscle is called upon to 

 endure, since, as seen in the 

 "red" muscles (page 59), a 

 lower degree of histologica) 

 differentiation favors pro- 

 longed exertion, although at 

 the expense of rapidity of 

 contraction. The valves are 

 formed from cushion-like 

 thickenings of the mesenchy- 

 ma. Those surrounding the 

 primary efferent vessel, the 

 truncus arteriosus, lead to 

 the subdivision of this tube 

 into the aorta and the pul- 

 monary artery and, likewise, 

 to the formation of the three 

 leaflets of the seinilunar 

 valves. In the case of the 

 auriculo-ventricular valves, 

 the septal leaflets are formed 

 from the endocardial cush- 

 ions, which appear on the 

 surfaces of the partition (sep- 

 tum intermedium) that divides the auricular canal, the channel connecting the 

 primary auricular and ventricular segments of the heart. The other leaflets 

 of these valves are derived from the walls of the auricular canal, a process of 

 undermining partially freeing portions of the innermost layer of the heart- 

 wall. These overhanging plates are connected, however, with the ven- 

 tricular myocardium by strands of tissue, the later papillary muscles. The 

 latter for a time are entirely muscular, but later the muscle-tissue disappears 

 near the valve-leaflet and the bands are converted into the fibrous strands, 

 the chordae fc tendineae. 



THE LYMPHATIC SYSTEM. 



The lymphatic or lymph-vascular system consists of an almost uni- 

 versally present system of channels, some of which are definite tubes, the 

 lymphatic vessels, and others uncertain and often illy defined clefts, the 

 lymph-spaces, between the bundles of connective tissue. The vessels con- 

 tain the lymph, a fluid usually colorless and containing numerous corpuscles, 

 the lymphocytes. Since the latter are familiar as one of the chief types of 

 colorless blood-cells, they are described in connection with the blood (page 

 96), in which they circulate. Although the lymph is ordinarily clear, that 

 within the lymphatics leading from the intestines appears, especially during 

 digestion, more or less milky, in consequence of the lymph-cells being 

 loaded with particles of fat which they have taken up from the intestinal 

 contents. For this reason these intestinal lymphatics are often termed 

 lacteals. The lymphatics resemble the veins, from which, indeed, they 



Myocardial 

 layer 



ricle 



FIG. 140. Transverse section of early rabbit embryo pass- 

 ing through young heart, showing venous segment behind 

 and arterial in front. X 75- 



