180 



BLOOD-VESSELS. 



Erectile, or cavernous tissue. — By this; term is understood a peculiar stnictm-e. 

 fonning the principal part of certain organs which are cai)able of being rendered 

 turgid, or erected, by distension with blood. It consists of dilated and freely 

 intercommimicating branches of veins, into which arteries poiu' their blood, 

 occupj-ing the areolaj of a network foiTaed by fibrous, elastic, and probably con- 

 tractile bands, named trabeculaj, and enclosed in a distensible fibrous envelope. 

 This pecidiar arrangement of the blood-vessels scarcely deserves to be regarded as 

 constituting a distinct texture, though reckoned as such by some WTiters : it is 

 restricted to a very few parts of the body, and in these is not altogether uniform 

 in character ; the details of its stnicture will, therefore, be considered with the 

 special description of the organs in which it occurs. 



Fig. 121. 



DEVELOPMENT OF BLOOD-VESSELS. 



The first vessels which appear are formed within the ovum, in the 

 germinal membrane, and the process subsequently goes on in growing 

 parts of the animal body. New vessels, also, are formed in the healing 



of wounds and sores, in 

 the OT'ganisation of effused 

 lymph, in the restoration of 

 lost parts, and in the pro- 

 duction of adventitious 

 growths. The process is in 

 every case essentially the 

 same, although more or less 

 modified a.ccording to cir- 

 cumstances. 



The first vessels of the 

 embryo chick, those, namely, 

 which produce the vascular 

 area, originate fi'om nu- 

 cleated cells belonging to 

 the middle germinal layer. 

 "Within these cells vacuoles 

 are formed and, increasing 

 in size, a cavity filled with 

 fluid is in this way produced 

 in the interior of the cell 

 (fig. 121,^). Blood-corpus- 

 cles (d) are found at an early period within the cavity : the mode in 

 which they become developed has been already described (p. 41). The 

 cells, whilst these changes are going on, increase largely in size so as to 

 form vesicles, visible to the naked eye as minute reddish specks, which 

 have been known since the time of Pander as "blood-islands." They 

 are at first isolated, as the name implies, but after a time send out pro- 

 cesses which unite with those of neighbouring cells, and the cavities 

 becoming extended into these processes a network of vessels is by 

 this means produced.* 



The wall(Z/) of these primary vessels is therefore composed at first 

 merely of the protoplasm of the original embryonic cells with a few 



* We are liero mainly following Klein's description (Wiener Sitzungsb., Ixiv. 1870), 

 but accordin;; to the more recent account given by Balfour (Quarterly Journal of 

 Micioscoiiic Science, July, 1873), the cells first unite by means of processes into a proto- 

 plasmic network, -witliin the substance of which, by the multiplicatiou of the cell-nuclei, 

 blood-corpuscles subsetiucutly become developed. 



Fig. 121.— Cells from Middle Layer op Chick's 

 Blastoderm undergoing development into 

 Blood- Vessels. Magnified. 



d, blood-coi-puscles (from Klein). 



