. Quain's elements of anatomy . omposed almostentirely of a plexus of minute arteries, which are derived in the one case directlyfrom the carotid, in the other from the middle sacral. The plexiform vessels areinvested by one or more layers of granular polygonal cells, apparently like thosefound in the interstitial tissue of some other organs (testis, ovary, thyroid, supra-renal bodies), and probably modifications of the more widely distributed plasma-cells of the connective tissue. The whole is invested by connective tissue, whichalso penetraters between the vessels of the so-called gland, and
. Quain's elements of anatomy . omposed almostentirely of a plexus of minute arteries, which are derived in the one case directlyfrom the carotid, in the other from the middle sacral. The plexiform vessels areinvested by one or more layers of granular polygonal cells, apparently like thosefound in the interstitial tissue of some other organs (testis, ovary, thyroid, supra-renal bodies), and probably modifications of the more widely distributed plasma-cells of the connective tissue. The whole is invested by connective tissue, whichalso penetraters between the vessels of the so-called gland, and itself containsnumerous granular ceEs. The true nature and function of these peculiar struc-tures is entirely unknown. DEVELOPMENT OP BLOOD-VESSELS. The first vessels which appear in the ovum are formed in the meso-derm, and the process subsequently goes on in the same layer and in itsderivatives in all parts of the animal body. New vessels, also, are formedin the healing of wounds, in the restoration of lost parts, and in the. Fig. 191.—Paet of the network op developing blood-vessels in the vascular AKEA OP THE GUINEA-PIG. (E. A, S.) II, Uood-corpviscles becoming free in an enlarged and hollowed out part of the net-work.^ The smaller figure on the left represents a of the larger figure, more highlymagnified ; d, a nucleus undergoing division. production of adventitious growths. The process is in every caseessentially the same. The first vessels of the embryo, both of the chick and mammal, areformed in the vascular area, and originate from some of the cells of the 198 BLOOD-VESSELS. Fig. 192.
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