32 



DEVELOPMENTAL ALTERATIONS IN THE VASCULAR SYSTEM 



Evans, 1912, figs. 399 and 400). According to him it is thus originally paired 

 and bilaterally symmetrical. According to the present writer, it is not until 

 later that we can speak of a superior sagittal sinus. It is not until the plexuses, 

 described by Evans, have anastomosed across the median line and have formed a 

 longitudinal network in the meshes of which an asymmetrical channel is finally 

 established, that we can speak of a superior sagittal sinus. In some cases two or 

 more larger longitudinal channels are formed in the mesh, as is shown in figure 10; 

 but in such cases they are not strictly bilaterally symmetrical. 



Owing to the growth of the cerebral hemispheres in 20 mm. embryos, there is 

 formed a well-marked cerebral longitudinal fissure which is occupied by embryonic 

 tissue. This rapidly takes the form of the adult falx cerebri. It is in the dorsal 

 part of this loose dural tissue that the meshes of the sagittal i^lexus are found. 



TELENCEPHALON 



PLEXUS 

 CUTIS 



Figure 10. 



Caniera-lucida sketch of the vertex of the 

 head of a human embryo 27.5 mm. long 

 (Carnegie Collection, No. 1458), in 

 which the lilood-vessels were conspic- 

 uous through a natural injection. It 

 is the exception where two channels are 

 found so nearlj' the same size. From 

 a single specimen, like this, one might 

 incorrectly gain the impression that the 

 superior sagittal sinus is at first bilat- 

 erally symmetrical. Even here, how- 

 ever, it can be seen that Ijoth of the 

 larger channels drain caudally to the 

 right side. The minute anastomosing 

 loops that connect the two main chan- 

 nels with each other and with the chan- 

 nels in the more ventral ijaits of the 

 falx are not shown. On each side can 

 be seen the serrated upper margin of 

 the advancing capillary plexus of the 

 skin. Above this margin the skin is as 

 yet nonvascular. On account of the 

 dilatation of these vessels along this 

 .advancing edge the margin is conspic- 

 uous. 



MESENCEPHALON 



SIN. TRANS. 

 RECT. 



At this time it can be seen that one or more larger channels are opening along the 

 dorsal mid-line, which will form the superior sagittal sinus, and connected with 

 them by anastomosing loops is a more ventrally situated large channel that con- 

 stitutes the sinus rectus. This latter extends forward and drains the lower part 

 of the falx. It has two converging limbs in front that drain the choroidal masses 

 of the hemispheres. In figure 26 a portion of the right hemisphere is removed to 

 expose the region of the falx cerebri. Here the straight sinus can be seen as dif- 

 ferentiated out from the sagittal plexus, with which it anastomoses freely at its 

 caudal end in a plexiform manner. Anteriorly the straight sinus bifurcates, enters 

 the choroidal fissure on each side, and terminates in the sinus-like choroidal bodies. 

 These, on the other hand, are fed from the caudal end by the choroidal arteries. 



On coming to embryos 50 mm. long, figure 9 (Carnegie Collection, No. 458, 

 54 mm.), we find that here the superior sagittal sinus is established, at least in part. 

 In its cephalic portion there is a large characteristic channel, lacking only the dural 



