OF THE BRAIN OF THE HUMAN EMBRYO. 



17 



plastic model of the region. Such a model is shown in figures 24 and 25, plate 3, 

 being a wax-plate reconstruction made from an embryo 11.5 mm. long (Carnegie 

 Collection, No. 544). Mall (1905) has pictured about the same stage in his figure 

 9, and this stage is also pictured by Markowski (1911) in his figure 1. 



In their main points all of these embryos correspond rather closely, and ap- 

 parently the vascular system at this time does not show any great variation. A 

 large venous channel is formed in the region lateral to the diencephalon and passes 



PLEXUS MEDIALIS 



V CAPITIS PRIMA 



VESICULA AUDITIVA 



PLEXUS POST. 



Figure 1. 

 Drawing of the primary head-vein and its main tributaries in a human embryo 13.8 mm. 

 long (Carnegie Collection, No. 940). The tributaries are arranged in the form of three plex- 

 uses — the anterior, middle, and posterior dural plexuses. The trunk of the posterior plexus 

 passes through the foramen jugulare in order to empty into the primary head-vein, and 

 this point marks the junction of the intrinsic head portion of the primary head-vein wnth the 

 neck portion or anterior cardinal vein. The latter eventually becomes the internal jugular 

 vein. Enlarged about 10 diameters. 



backward median to the trigeminal nerve and lateral to the otic capsule through 

 the region of the future middle ear, where it bends sharply downward in the neck 

 region to finally empty into the duct of Cuvier. All the veins of the cranial region 

 drain into this main channel. This constitutes the primary head-vein with which 

 we are already familiar. This vein at 14 mm. differs from its condition at 4 mm. 

 only in being more deflected in its course by the structures through which it threads 

 its way. It still forms a fairly direct and efficient drainage-channel. It was this 



