100 



NORMAL HISTOLOGY. 



tion of the walls of the medium- and large-sized arteries and veins. 

 These vessels arise some distance from the area which they supply, 

 frequently coming from a different branch or, as in the case of the 

 veins, from a neighboring arterial stem. 



The nerves of blood-vessels are mainly derived from the sym- 

 pathetic system, and hence are principally of the non-medullated 

 kind ; a few medullated fibres, however, are usually present. The 

 nerves accompanying the blood-vessel give off branches, which form 

 surrounding plexuses ; from these minute bundles pass, whose com- 

 ponent fibrillae are distributed to the media and the adventitia. The 

 capillaries are accompanied by correspondingly delicate fibres. 



Lymphatic clefts and vessels are found in the external coat of 

 the larger vessels. In many places, as in the nerve-centres, including 

 the organs of special sense, in the per^n^^m^ **** , the lymphatic 

 clefts~of the adventitia "unite to form a large ensheathing circular 

 sinus the perivascular lymph-space which separates a portion 

 of the adventitia from the remainder of the vessel 7 as a result ol tliis 

 arrangement, the blood-vessel seemingly lies within the lymph-space. 

 Perivascular lymphatics may be readily observed in the peritoneum 

 of the frog. 



THE HEART. 



FIG. 124. 



The heart-walls consist of three layers the endocardium, the 

 muscular layer, and the pericardium. 



The endocardium forms the serous lining 

 of all parts of the organ, becoming continuous 

 with the inner tunic of the blood-vessels at the 

 several cardiac orifices. The inner free sur- 

 face of the heart is covered with a single layer 

 of polyhedral nucleated endothelial cells. 

 These latter rest upon the substance proper 

 of the endocardium, a stratum composed of 

 fibrous connective tissue mingled with a felt- 

 work of elastic fibres ; the elastic net-works 

 are especially well developed in the auricles, 

 in certain parts of which the broad fibres join 

 to form fenestrated membranes. The outer 

 connective-tissue layer of the endocardium 

 is continuous with the perimysium of the 

 muscular tissue. 



The heart-valves are formed by duplica- 

 tures of the endocardium strengthened by 

 bands of fibrous tissue enclosing numerous 

 The endocardial layer of the auricular side of the 



NHP 



c 



Section of human heart show- 

 ing endocardium : a, endothe- 

 lium ; b, subendothelial con- 

 nective-tissue stroma in outer 

 layer (c), containing net-work 

 of elastic fibres (r) ; d, trans- 

 versely-cut bundles ; f, mus- 

 cular tissue. 



elastic fibres. 



