STRUCTURE OF THE LACTEAL AND LYMPHATIC VESSELS. 4:3 T 



dentated, with 'their long diameter in the direction of the 

 axis of the vessels. 1 The vessels themselves are completely 

 closed ; and the openings which were described by the older 

 anatomists were either accidental or imaginary. 



In all but the capillary vessels, although the walls are ex- 

 cessively thin, three distinct coats can be distinguished. The 

 internal coat consists of a membrane lined with oblong 

 epithelial cells. These cells, however, do not form a con- 

 tinuous sheet, as in the capillaries. According to some anat- 

 omists, the membrane is composed of reticulated longitudi- 

 nal fibres ; but Sappey states that he has never been able to 

 distinguish any appearance of this kind. 2 This coat is some- 

 what elastic, though it readily gives way when the vessels 

 are forcibly distended. The middle coat is composed of 

 longitudinal fibres of the white fibrous tissue, with delicate 

 elastic fibres and unstriped muscular fibres arranged trans- 

 versely. The external coat is composed of the same struc- 

 tures as the middle coat ; but the fibres are arranged, for the 

 most part, longitudinally. In this coat, the muscular fibres 

 do not form a continuous sheet, but are collected into sepa- 

 rate fasciculi, which have a direction either longitudinal or 

 oblique. The fibres of connective tissue are very abundant, 

 and loosely unite the vessels to the surrounding parts. The 

 internal and the middle coat are closely adherent to each 

 other ; but the external coat may readily be separated from 

 the others. Blood-vessels have been found in the walls of 

 the lymphatics, but as yet, the presence of nerves has not 

 been demonstrated. 



The walls of the lymphatic vessels are very closely ad- 

 herent to the surrounding tissues ; so closely, indeed, that 

 even a small portion of a vessel is detached with great diffi- 

 culty, and the vessels, even those of large size, cannot be 

 followed out and isolated for any considerable distance. 



1 Op. cit. Journal de V Anatomic et de la Physiologic, Paris, 1866, tome ill, 

 p. 469. 



2 Op. cit., tome i., p. 625. 



