8 THE DEVELOPMENTAL ALTERATIONS IN THE VASCULAR SYSTEM 



vessels lie bilaterally close along the brain-A\all, at first either in the form of a 

 single, slender, longitudinal tube, as seen along the hindlwain, or a plexiform space 

 as seen near the forebrain and midbrain. Soon after the primordial vessels are 

 estabUshed, endothehal buds sprout from their walls and in conjunction with them 

 form an irregular endothelial vascular meshwork which tends to spread over the 

 surface of the brain-wall, especially in the region of the forebrain and midbrain. 

 There is thus formed a plexiform system which constitutes a germinal bed of endo- 

 thelium rather than a circulatory apparatus. 



During the second developmental period the primordial blood-vessel plexus 

 of the head slowly resolves itself into veins, arteries, and capillaries, and becomes 

 architecturally suited to the circulatory flow of the blood. That portion of the 

 plexus which lies against the brain spreads out as a flattened capillary sheet, con- 

 forming everywhere to the shape of the brain-wall and its attached ganglia and 

 sense-organs. The more superficial part of the plexus develops a coarser mesh and 

 forms larger channels, which tend to unite into continuous trunks and gradually, 

 by virtue of their communications, can be recognized as definite arteries and veins. 

 The intermediate loops of the plexus maintain the anastomosis between the deep 

 capillary sheet and the more superficial trunks, forming tributaries of the veins 

 and branches of the arteries. The second period thus establishes the primary 

 type of the circulation of the head, in which there is a capillary bed, fed by arterial 

 branches from the aortic system and drained bilaterally by a continuous venous 

 trunk which extends back to the venous end of the heart. 



The third period is inaugurated by the differentiation of the membranous 

 skull, the dura mater, and the arachnoid-pial membranes. As a result of this stratifi- 

 cation of the tissues of the head, the more ventral of the anastomosing channels 

 that connect the deep capillary plexus with the superficial vessels become closed 

 off and there is a general separation or cleavage of the vessels immediately sur- 

 rounding the brain-wall from those belonging to the membranous skull and its 

 coverings. This process begins at the base of the skull and extends bilaterally 

 upward toward the middle fine of the vault, in which region the communications 

 between the deeper and more superficial systems are to some extent maintained. 

 In this way the cerebral vessels are gradually separated from the dural vessels and 

 in a similar manner the superficial vessels of the head become isolated b.y the laying 

 down of the primordium of the membranous skidl, after which their course of 

 development is quite independent of that of the dural and cerebral systems. 



By the time the third period is well under way it is overlapped by the fourth 

 period, under which we include the remarkable series of adjustments in the arrange- 

 ment of the blood-vessels, in adaptation to the developmental alterations in the 

 form, size, and condition of the structures of the head region. The brain is one of 

 the chief factors in this process. The marked change in its form, and especially 

 the prolonged relative growth of the cerebral hemispheres, render necessary a 

 continuous series of alterations in the blood-channels that extend far into the late 

 fetal stages. In the earlier stages a fundamental change results from the growth 

 of the labyrinth and its cartilaginous capsule, whereby a mechanical obstruction is 



