92 



NINTH LECTURE. 



this, and consequently as the innermost layer of the figure, 

 we recognize at b a homogeneous, longitudinally striated, 



elastic membrane, the tu- 

 nica serosa of the older 

 anatomy. The same is 

 surrounded by a layer of 

 transversely running con- 

 tractile fibre cells at c. 

 The connective-tissue laver 

 d, with longitudinally ar- 

 ranged cells, forms the last. 

 Under certain circumstan- 

 ces, it may be very much 

 thicker than in our figure. 



Other small arterial 

 trunks show the muscular 

 layer to be constituted by 

 several layers of fibre cells, 



Fig. 89.— A small arterial trunk. At f>, the homo- lying Over One another, aS 

 geneous, nun-nucleated inner layer; c, the middle 



la\e-, consi>ting of contractile fibre cells ; d. thecon- \x\ Figr 88 d where tile 

 nsctive tissue external layer. ° ' 



adventitia is again formed 

 of reticular connective tissue. 



Larger trunks, finally, can no longer be surveyed in their 

 totality, under the microscope. One must, therefore, exam- 

 ine singly the separately prepared layers, or make longi- 

 tudinal and transverse sections through the hardened walls. 



The further transformations, from those immediately fol- 

 lowing up to the most remote of the largest blood-vessels, 

 consist in the following : The endothelial tube always 

 remains a single layer; the connective-tissue adventitia does 

 the same, but it increases in thickness; the connective-tissue 

 bundles become more distinct, and elastic fibre reticula are 

 more and more frequent, especially in the arteries. Both the 

 middle layers, the serosa and media, begin on the contrary 

 to become stratified ; each of them consists of an increasing 

 number of layers lying over each other. On this depends 

 the growing thickness of the vascular walls. The inner 



