Development, &c., of Blood-vessels. By G. and F. E. Hoggan, 579 



as a liypothesis, but a hypothesis which seems to fit all the various 

 conditions. 



In some cases, as at d, Figs. 5 and 7, the terminal vessel- 

 forming cell is not continuous with a fibre, while in other cases, as 

 at a, Fig. 1, the ceil is distinctly continuous with or prolonged 

 into a fibre. This difierence may yet be found sufficient to decide 

 whether the cell came centrally from the vessel or peripherally to 

 it. The further growth in size and calibre of newly developed 

 capillaries into veins and arteries, as may easily be conceived, takes 

 place by the interposition of wandering cells between or upon the 

 already existing cells of the wall. >So much on the question of 

 development. 



Retrogression of Blood-vessels. 



While the process of formation of blood-vessels may be held to 

 follow the same course under all circumstances, retrogression may 

 take place from several causes and under difierent forms. These 

 forms may be classed under the two great heads of physiological 

 and pathological forms of retrogression ; but it is not our intention 

 to enter at present into the consideration of the changes which 

 may take place under the latter head, regarding which it may be 

 sufficient for us to say that, under pathological conditions, the 

 cellular elements of the vessel walls may undergo either degenera- 

 tion or malignant changes, which entirely alter their morphological 

 appearances and destroy their physiological properties. 



Confining ourselves, therefore, to physiological causes and forms 

 of retrogression, we shall direct special attention to changes which 

 result from, 1st, developmental, and 2nd, nutritive causes or 

 conditions. Ketrogression, as the result of insufficient nutrition, 

 can best be studied in connection with the great groups or tracts of 

 fat-cells to which innumerable blood-vessels are supplied within the 

 same serous membranes where we have already studied their 

 development. As the tracts of fat-cells disappear by physiological 

 absorption, either from want of food in a young and active animal 

 or through deficient power of assimilation of food in a very aged 

 animal, so likewise do the blood-vessels which supply them break 

 up and disappear when their presence there is no longer necessary. 

 In both these instances no disease is present, and the resulting 

 retrogression of blood-vessels -is therefore purely physiological, and 

 unconnected with any pathological condition. After the fat has Ijeen 

 absorbed i'rora all the cells, and these cells themselves are passing 

 away, we find notable changes taking place in the whole of the 

 blood-vessels passing to or snp|)lving a fat-tract. 



The changes taking place in the arteries are of two kinds. If 

 the artery is directed solely towards the fat-tract, we find 

 innumerable irregular constrictions of its lumen, the muscular coat 



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