OF THE BRAIN OF THE HUMAN EMBRYO. 35 



deduced. In order to facilitate a review of the successive steps in this interesting 

 metamorphosis, there have been assembled on plate 1, figures 13 to 21, a series of 

 simplified sketches, and through the aid of these it is hoped that the steps that 

 are outhned in the following summary can be readily identified. 



In the primary type of circulation the arrangement for the drainage of the 

 capillaries of the head (figs. 13, 14) consists bilaterally of one main channel, the 

 "primary head-vein," that starts in the region of the midbrain, runs caudalward 

 alongside of the brain-tube, and terminates at the duct of Cuvier. The primary 

 head-vein is composite in origin. That portion of it rostral to the vagus nerve is 

 an intrinsic vein of the head; the remaining caudal portion is in reahty a neck-vein 

 and constitutes the anterior cardinal vein — eventually the internal jugular vein. 

 Together these portions form a continuous channel, the primary head-vein, into 

 which the blood from the capillary sheet immediately investing the brain-tube is 

 drained by means of anastomosing venous loops. These loops are arranged more 

 or less in the form of three plexuses — the anterior dural plexus, the middle 

 dural plexus, and the posterior dural plexus. Other small tributaries which are 

 not all shown in the figures empty into the primary head- vein, thereby draining 

 the structures ventral and lateral to the brain-tube, such as the nerve-ganglion 

 masses and the maxillary and mandibular gill-bars. A large one comes from the 

 eye region and eventually is modified into the ophthalmic vein. 



From this simple group of drainage channels are eventually derived all the 

 adult venous sinuses. The metamorphosis which they undergo is based on a series 

 of circulatory adjustments that are made necessary by certain changes in their 

 environment, the two most conspicuous being the changes in the region of the 

 cartilaginous capsule of the labyrinth and the still greater changes involved in the 

 growth and marked alteration in the form of the brain. Among the factors involved 

 in these circulatory adjustments may be mentioned the reduction of plexuses into 

 simple channels, the conversion of channels into plexuses, the total obliteration 

 of established channels, and the change in position of channels. Under this latter 

 phenomenon there is to be recognized a "passive migration," where there is a change 

 in the position of the vein-wall itself, due to the movement of its environment, 

 which exerts a flexion or traction force upon it. We also recognize a "spontaneous 

 migration," where there is a change in position of the blood-stream only, where in a 

 circumfluent manner the blood-stream develops a new channel in the adjacent loops 

 of the plexus, with a corresponding dwindling of the previously used channel. 

 The "replacement channel" might be mentioned as another type of spontaneous 

 migration, in which the venous channels are changed in position and direction in 

 this process of adjustment. In the replacement channel there is the formation of 

 a new channel and the obliteration of an old one, as in other types of spontane- 

 ous migration. It, however, differs from them in that it is not a gradual and 

 progressive change in position, but an abrupt and immediately complete one. 

 Furthermore, the new channel lacks the morjihological characteristics of the 

 old one. With these various factors in mind one can readily follow the .stei:)s by 

 which the primary head-vein and its tributaries gradually merge into adult dural 

 sinuses. 



