OF THE BRAIN OF THE HUMAN EMBRYO. 19 



terior cerebral veins. The last one empties into the main channel through a single 

 trunk, but the other two groups tend to maintain the character of the original 

 plexus and usually have multiple openings into the primary head-vein. Further- 

 more, due to the cleavage effect of the dura, which tends to separate them from the 

 deeper vessels, the veins forming these three groups belong chiefly to the dura 

 mater and the tissues forming the membranous cranium. There is, therefore, an 

 advantage in adopting for the temporary description of this period of develop- 

 ment a terminology something like that of Markowski (1911). In doing so, a 

 distinction between the lateral and mesial portions will not be made, but the three 

 groups as given by Mall will be retained. We shall thus speak of the anterior, middle, 

 and posterior dural plexuses, or (more formally) plexus durce. matris anterior, plexus 

 durce matris medialis, and plexus dura' fnatris posterior, as they are indicated in 

 figures 1, 24, and 25. In these figures only the larger channels of the plexus are 

 shown and it is to be understood that an intervening smaller venous mesh con- 

 nects them more or less completely. 



These three plexuses are not exactly uniform as regards their pattern, but 

 from the very first they exhibit an individuality that seems to correspond to the 

 difference in structure of the areas which they drain. The anterior plexus is 

 modified along its oral margin in adaptation to the form of the bulging hemisphere. 

 The middle plexus has two larger trunks, one of which drains the capillaries on the 

 oral surface of the cerebellar plate; the other drains the anterior part of the roof 

 of the fourth ventricle. It is this latter one that was pictured by His (1904) and 

 incorrectly described as the sinus transversus (p. 121). In the posterior plexus 

 there can be recognized usually a single channel that is larger than the others and 

 that tends to cross the dorso-median line to anastomose with the plexus of the opposite 

 side, the main channel of the other side being correspondingly smaller, thus giving 

 rise to a bilateral asymmetry. Another place at which a larger channel is frequently 

 seen crossing dorsally over the median line in an asymmetrical manner is at the 

 junction of the midbrain and hindbrain. A third favorable place of this kind is 

 over the diencephalon along the caudal margin of the cerebral hemisphere. Here 

 is formed the beginning of the transverse sinus, which consequently shows an 

 asymmetrical relation to the superior sagittal sinus, as will be pointed out later. 



In the ventral portions the dural plexuses are more or less completely separated 

 from the deeper-lying plexus or capillary sheet that closely invests the wall of the 

 neural tube, in which are developed the cerebral veins and main arterial supply. 

 In tracing the plexus dorsalward toward the median line, we find an increasing 

 frequency of communication between the two, and near the median line they are so 

 intimately connected that it is impossible to distinguish between them; in other 

 words, in this region the cleavage between these two layers is not yet estabUshed. 

 The manner in which the tips of the dural plexuses communicate with the capillary 

 sheet of the brain-wall is indicated in figures 24 and 25, where a portion of the sheet 

 is drawn in over the occipital pole of the cerebral hemisphere. It is to be remem- 

 bered that a capillary meshwork of this kind invests the central nervous system 

 everywhere, the pattern of the mesh varying somewhat according to the region. 



