546 THE ABSORBENT VESSELS. 



the outer digital veins, and furnishes the radial and cephalic veins : the latter opens originally 

 into the primitive (external) jugular vein, and later forms a connection between the muscles 

 with the axillary vein. The occasional entrance of the cephalic vein into the external jugular, 

 as is common among the lower animals, or the occurrence of a jugulo-cephalic vein (p. 528), 

 which is normal in many apes, 1 is therefore a retention of the primitive ending. The two 

 trunks, basilic and cephalic, form connections at the front of the elbow, which are joined also 

 by the superficial and deep median veins, and through these a large part, or even the whole, 

 of the blood from the radial side of the limb may be conveyed to the basilic vein, the cephalic 

 vein then being correspondingly reduced or even disappearing. 



In the lower limb the postaxial marginal vein is also at first the main trunk, but when 

 the deep veins are developed it forms a connection with them at the knee, thus giving rise to 

 the external saphenous vein, which therefore corresponds to the posterior ulnar vein of the 

 forearm, and its femoral portion disappears more or less completely : the rare form of variety 

 in which the external saphenous vein is continued to the sciatic (p. 538) is probably a 

 persistence of the primitive trunk. Here also a secondary trunk, the internal saphenous, is 

 developed on the preaxial border of the limb, but its homology with the preaxial trunk of the 

 upper limb is doubtful. The internal saphenous vein opens into the femoral vein, which, in 

 association with the change that has taken place in the great arterial trunk, has become the 

 main vein of the. limb. (F. Hochstetter, " Ueber die Entwicklung der Extremitatsvenen bei 

 den Amnioten," Morph. Jahrb., xvii. 1891.) 



The deep veins, which as stated above are generally of secondary formation, are as a rule 

 companion vessels to the arteries, although there are many exceptions, for example most of 

 the larger veins of the brain and spinal cord. They differ from the arteries which they 

 accompany more or less closely in 1, being more numerous ; 2, having a greater capacity, the 

 calibre of the single vein or the sum of the venae comites always exceeding that of the 

 corresponding artery ; and 3, the greater number and size of their communications, which 

 often lead to the formation of considerable plexuses, especially in regions where there is much 

 movement between adjacent organs, or about parts which, although subject to rapid or con- 

 siderable alterations in form or bulk, are surrounded by resistent structures, such as arteries 

 in osseous canals, the viscera of the pelvis, the upper end of the pharynx, and the muscles in 

 the zygomatic fossa. In these situations the venous plexuses form a soft packing which can 

 readily adapt itself to the variations in shape and pressure of neighbouring parts. 



3. ABSORBENT VESSELS. 



The absorbent vessels are divisible physiologically into two sets : the lacteals, 

 which convey the chyle from the intestinal canal to the thoracic duct ; and the 

 lymphatics, which take up the lymph from all the other parts of the body, and 

 return it into the venous system. Anatomically considered, however, the lacteals 

 are not different from the lymphatics, and may be regarded as the absorbents of the 

 mucous membrane of the intestine. The larger lacteals and lymphatics are provided 

 with numerous valves, which give them, when distended, a somewhat moniliform 

 appearance ; and both are connected in their course with lacteal or lymphatic glands. 



The general anatomy of the absorbents being elsewhere detailed (Vol. I, 

 p. 376) only their course and position remain to be here described. They are 

 gathered into a right and a left trunk, which open into the angles of union of the 

 subclavian and internal jugular veins. The large vessel of the left side traversing 

 the thorax is named the thoracic duct : it receives not only the lymphatics of its own 

 side of the head and arm, and most of those of the trunk, but likewise the lymph- 

 atics of both lower limbs, and the whole of the lacteals. The short vessel of the 

 right side is named the right lymphatic duct, and receives the lymphatics only of that 

 side of the head and neck and upper part of the trunk, and of the right upper limb. 



(On the distribution of the lymphatic vessels and glands generally, see Mascagni, " Vaso- 

 rum lymphaticorum corporis humani historia et ichnographia," 1787 ; Cruikshank, " The 

 Anatomy of the Absorbing Vessels of the Human Body," 2nd ed., 1790 ; Teichmann. " Das 

 Saugadersystem vom anatomischen Standpunkte bearbeitet," 1861 ; and Sappey, " Descrip- 

 tion et iconographie des vaisseaux lymphatiques," 1874 85.) 



1 E. Ficalbi, " Di una particolare disposizione di alcuni vasi venosi del collo nelle scimmie," &c., 

 Atti d. Soc. Toscana d, Sci. Nat., iv; see also Biologisches Ceutralblatt, v, 1885. 



