

DEVELOPMENT OF THE VEINS. 



1039 



At a later period that portion of the vein which lay medial to the otic vesicle and the 7th, 

 8th, 9th, 10th, and llth cerebral -nerves has disappeared and has been replaced by a new channel, 

 which is placed lateral to the otic vesicle and the 7th, 8th, 9th, 10th, and llth cerebral nerves. 

 The new channel extends from the semilnnar ganglion of the trigeminal nerve to the upper end 

 of the extra-cranial part of the anterior cardinal vein, that is, to the upper end of the internal 

 jugular vein. This secondary vessel follows the course of the facial nerve and in part of its 

 extent it is extra-cranial. 



In the human embryo the stage in which the primitive stem vein lies to the medial side of 

 the otic vesicle and the 7th, 8th, 9th, 10th, and llth nerves does not seem to occur. At all 

 events in a 3 mm. embryo in the Edinburgh University Collection, and in a 4 mm. embryo in 

 the Collection of the Carnegie Institution of Washington (Streeter), the second stage, in which 

 the posterior part of the cranial portion of the primitive vein lies lateral to the otic vesicle and 

 the 7th, 8th, 9th, 10th, and llth cerebral nerves, is already present and there is no indication 

 of a vein medial to the otic vesicle. 



In the human embryo, therefore, the primitive venous stem in the head region, on each side, 

 consists of an anterior portion medial to the semilunar ganglion of the trigeminal nerve, and a 

 posterior portion lateral to the otic vesicle and the 7th, 8th, 9th, 10th, and llth cerebral nerves ; 

 for this stem Streeter has suggested the convenient term "primary head vein" (Fig. 834). 



The most anterior tributaries of the primary head vein are derived from the region of the 

 otic vesicle and remnants of them become converted into the ophthalmic vein, but in addition to 

 the anterior tributaries there are numerous dorsal or upper tributaries which become arranged 

 in three main groups : an anterior plexus associated with the regions of the fore-brain and the 



Longitudinal anastomoses of vessels 

 of anterior and middle plexuses 



Veins of 

 anterior plexus 



Veins of 

 anterior plexus 



Primary head vein 



FIG. 836 A. DIAGRAM OP A TRANSVERSE 

 SECTION OF THE SECONDARY FORE- 

 BRAIN AND THE VENOUS PLEXUSES. 



Longitudinal 

 anastomosis 



nterventricular 

 .men 



Third ventricle 



Inferior cerebral vein 



FIG. 836 B. DIAGRAM OF A TRANSVERSE SECTION OF THE 

 BRAIN SHOWING THE FOLDING OF THE UPPER PARTS OF 

 THE PLEXUSES BETWEEN THK CEREBRAL HEMISPHERES. 



mid-brain ; a middle plexus associated with the cerebellar region of the hind-brain ; and a 

 posterior plexus associated with the region of the medulla oblongata (Fig. 834). 



The vessels of each plexus tend to run together as they approach the stem of the primary 

 head vein and so three stems are formed, the anterior, middle, ana posterior ; they were described 

 by Mall in 1904 (Fig. 834). This condition persists until the embryo attains a length of about 

 18 mm. when an anastomosis forms, above the otic vesicle, between the stems from the middle 

 and posterior plexuses (Fig. 835), and at the same time that part of the primary head vein which 

 lay lateral to the otic vesicle and the 7th, 8th, 9th, 10th, and llth cerebral nerves disappears. 



By the time the embryo has become 21 mm. long the anastomosis mentioned has become 



very important, and a separation has occurred between the lower and the upper portions of the 



anterior stem tributary ; therefore, at that period, the blood from the eye region flows backwards 



to the anterior end of the primary head vein, then upwards along what was the lower part of 



the middle stem tributary, next backwards along the anastomosis above the otic region to the 



posterior stem tributary, down which it passes to the upper part of the extra-cranial portion of 



the anterior cardinal vein which has now become the internal jugular vein (Fig. 835). At this 



, time the blood from the anterior and middle plexuses reaches the supra-otic anastomosis through 



( the upper or dorsal part of the middle stem tributary (Fig. 835). 



In the meantime the subdural and subarachnoid spaces have been forming, and with the 

 formation of those spaces the main parts of the venous plexuses are carried away from the brain, 

 ith the membrane which will be transformed into the dura mater, but in part the plexuses 

 still retain their connections with the piamater on the brain surface, and they afterwards 

 establish new connections with the veins which appear on the surfaces of the developing 

 cerebral hemispheres. In the meantime on each side the upper or dorsal tributaries of the 

 anterior and the middle plexus anastomose together (Fig. 835). 



