GENERAL CONSIDERATIONS. 



605 



Fig. 364. 



The external tunic (tunica adventitia) is only a layer of connective tissue, with 

 some longitudinal recticulated elastic fibres in its deeper part. Though this 

 tunic is very thin, yet it is strong ; as a ligature tied tightly around an artery 

 will rupture the other tunics, but not this. 



The structure of the capillaries is not the same as that just described, but is 

 modified in proportion as they are fine. In the smallest capillaries, the walls are 

 formed by a thin amorphous membrane, in which (oblong) nuclei are somewhat 

 regularly disseminated ; in the finer arterioles, another layer external to this (the 

 contractile layer)^ and containing transverse nuclei, is 

 observed ; and in the larger arterioles — those imme- 

 diately succeeding the small arteries, these two nucleated 

 layers are enveloped by a thin tunic of connective tissue 

 {tunica adventitia). 



(The most minute capillaries are merely tubes formed 

 by a single layer of transparent, thin, nucleated, endo- 

 thelial cells, joined by their margins. When perfectly 

 fresh, the capillary does not show the edge of the cells, 

 owing to the uniform refractive property of the wall of 

 the tube. The nuclei show an internuclear plexus of 

 fibrils. The cells are united by cement substance, and 

 here and there minute dots or slits may be seen, which 

 have been supposed by some authorities to be openings 

 — stomata, or stigmata. The narrojvest meshes of capil- 

 laries occur in the lungs and liver, and the wider in 

 muscle, beneath serous membranes, and in the organs of 

 sense. The widest capillaries are found in the liver, and 

 the narrowest in the retina and muscle.) 



Vessels and nerves. — The arteries are provided with 

 vessels termed vasa vasorum, which are furnished either 

 by the arteries themselves, or by neighbouring vessels. 

 These vasa vasorum form a superficial network with 

 quadrilateral meshes, and a deep plexus, the principal 

 branches of which are helicoidal. The majority of 

 anatomists believe that this plexus does not extend be- 

 yond the external tunic. 



The lymphatic vessels maintain intimate relations, 

 in certain regions, with the capillaries. In the brain 

 and spleen there has been discovered, Around the arterial 

 capillaries, a vessel that completely envelops them, and 

 which has been named the lymphatic sheath. 



The nerves, designated vasa motors, accompany the 

 vessels and penetrate the muscular tunic, in which they 



are distributed. These vasomotor filaments join the branches of the capillaiy 

 plexuses, and form, at the points where they meet each other, ganglionic enlarge- 

 ments ; from these arise the fibres of Remak, the termination of which is 

 unknown. 



Anomalies in the Arteries. — In their arrangement, the arteries very often 



present anomalies which the surgeon should be guarded against. These usually 



are related to their number, their point of origin, and their volume. In a purely 



anatomical and physiological point of view, however, these anomalies are of no 



41 



TRANSITION OF A MINUTE 

 ARTERY OF THE BRAIN 

 INTO CAPILLARY VES- 

 SELS. 



1, Minute artery ; 2, tran- 

 sitional capillary; 3, 

 coarse capillary with 

 thick coat, represented 

 by a double contour line ; 

 4, fine capillary with 

 single contour. The 

 nuclei are seen widely 

 scattered in 4 and 3 ; 

 more closely congregated 

 in 2 ; and still more so 

 in 1, where they form an 

 epithelium. a, Trans- 

 verse elongated nuclei 

 of muscular cells, the 

 incipient muscular coat 

 of the artery. 



