580 Transactions of the Society. 



at some parts having contracted so as to nearly obliterate that 

 lumen, leaving moniliform groups of dilatations enclosing numerous 

 blood-corpuscles along its course. Very often, however, the 

 afferent and efferent vessels passing to and from the capillary 

 plexus of a fat-tract are destitute of muscular elements in their 

 walls, except at the point where the afferent vessel begins as a 

 branch passing off at right angles from an artery of considerable 

 size. In such a case a coat of muscular fibres extends upon it 

 for a distance of five or six diameters from its point of junction 

 with the artery, so that when it is no longer necessary for a 

 nutrient current to pass towards the fat-tract, this sphincter-like 

 muscular coat contracts, and thus shuts off the blood current. 



The changes in the veins or vein-like afferent vessels are not 

 less strongly marked. These vessels contract their lumen by 

 causing the one layer of cells which form their wall to contract 

 laterally, and at the same time to become much thicker, so that 

 when we focus the Microscope upon the plane of the centre of the 

 vessel, we find the lumen obliterated, and the cells of the wall, 

 instead of having their nucleus standing in relief from the inner 

 surface of the vessel, now appear with a considerable thickness of 

 their protoplasm covering the nucleus on the internal as well as 

 upon the external surface of the vessel wall. This contraction 

 does not seem to be due to any nervous influence, but may in great 

 part be due to the pressure externally of the gelatinous matrix in 

 which the vessels lie embedded, and partly to their own proto- 

 plasmic contractile nature, these actions being permitted by the 

 absence of the distending fluid within them. 



It is, however, in the capillaries that the best marked changes 

 are to be observed. While the whole capillary plexus supplying 

 or ramifying in a tract of empty fat-cells contracts the lumen of 

 the vessels throughout, it is only the loops of capillaries forming 

 the outer border or edge of the plexus which first retrograde and 

 break up. In such cases we may observe constriction, or what 

 really ought in most instances to be called a withering, at one or 

 more points on the course of the loop, the capillary wall appearing 

 to become much thinner, as at h, h, Fig. 15, and losing the plump 

 cylindrical appearance seen in well nourished capillary walls. At 

 the same time the withered portion seems to lose its faculty of 

 being stained by certain staining agents which colour satisfactorily 

 those portions of the capillary intervening between the withered- 

 like constrictions. Finally, the capillary breaks at one or more 

 places, and the process of disintegration is carried on at the 

 extremities of the free ends, one cell after the other breakmg off, 

 as at a, Fig. 19, and appearing to move away, by means of long 

 delicate processes or branches, from the seat of its former functions 

 as an individual element in a capillary wall. 



