28 THE DEVELOPMENTAL ALTERATIONS IN THE VASCULAR SYSTEM 



It is clearly subdivided into three separate systems: (1) the suijerficial system 

 belonging to the integument and soft parts, (2) the dural sj'stem lying between the 

 dura and bone, and (3) the cerebral system. All three are originally outgrowths 

 of the same capillary j^lexus. The separation of the dural veins and the cerebral 

 veins we have traced through ste]) by step. The sujierficial vessels in embryos 

 20 mm. long are already separated off from the dural system by the membranous 

 and cartilaginous cranium. They appear first in the lower parts of the head, 

 where, in consequence of the earlier maturation of this region, they are originally 

 separated off from the deeji system and are in the form of a plexus that gradually 

 spreads upward over the vault. They maintain a few anastomoses with the dural 

 system, which constitute the so-called emissary veins. One of these is shown in 

 figures 6 and 27. Aside from the channel maintained through the orbit, the chief 

 drainage from the superficial system is through the external jugular vein, which is 

 jiictured by J^alzer (1895) as already ]iresent in guinea-pig embryos 20 mm. long. 



On examining the dura in embryos 50 mm. long it will be seen that for the 

 greater part it closely invests the interior of the developing cranium and is relatively 

 poor in blood-vessels. Tliis is true especially in those portions where the carti- 

 laginous and bony cranium is more advanced in its differentiation, as in the base of 

 the skull and in the frontal, temporal, and lower occipital regions. In other regions 

 the dura projects within the cranial cavity, being separated from the future bony 

 skull bj'' a layer of areolar tissue, in the meshes of which are found the large blood- 

 channels and their tributaries. The largest area of this kind is situated over the 

 midbrain, extending from the caudal margin of the cerebral hemispheres to the 

 cerebellum. This area extends laterally down to the base of the skull, narrowing 

 as it does so. It constitutes what is known later as the tentorium cerebelli, and in 

 it is included the greater jiart of the dural venous sj^stem. A basal extension of 

 the tentorium widens out in the region of the semilunar ganglion and in its meshes 

 is formed the cavernous sinus. A thinner area of the same tissue extends caudal- 

 ward from the cavernous sinus, median to the otic capsule, to join the jugular region. 

 The slender plexus of veins extending through this constitutes the inferior petrosal 

 sinus. Along all the sinuses we find this same areolar meshwork. It is not to be 

 confused with the developing arachnoid tissue, from which it is everywhere separated 

 by the dura. Blood-vessels supjilying and draining the brain are also found in 

 the arachnoid at this time and in some regions they are quite numerous, such as 

 the region of the Sylvian fissure and along the more ventral parts of the midbrain 

 and hindbrain. These cerebral vessels are everywhere separated and distinct 

 from the dural blood-channels, with the exception of the few points where they 

 empty into the big dural channels, as occurs in the adult. The connection between 

 the dural system and the cerebral system is no longer by a multiple anastomosis of 

 small vessels, but instead by isolated larger veins. 



Examination of figures 6 and 27 shows that we have in fetuses at this time an 

 arrangement of the dural venous system that in most respects follows the adult 

 arrangement. The cavernous sinus still has a simpler character than is found in 

 the adult. It is situated median and ventral to the semilunar ganglion and has 



