268 STUDY OF PIG EMBRYOS. 



spinal accessory nerve, N.n, which arises from the cervical (in the figure upper) 

 end of the hind-brain and runs forward to join the vagus ganglion, N.io jug, 

 the jugular ganglion of the adult. 2, the characteristic relations of the anterior 

 cardinal vein to the trigeminal ganglion, A T .5. The vein is cut twice, Card' and 

 Ctf^sC, for it curves around the ganglion, passing on the inside of the ganglion 

 between it and the wall of the brain. The original vein persists throughout life 

 in- this position, and enlarges into the cavernous sinus of the adult. Inside or 

 mesially of the seventh to twelfth nerves the cardinal vein is obliterated, and 

 is replaced by a new vessel produced by "island formation" outside these nerves, 

 and designated as the vena capitis lateralis. It is, as it were, interpolated in the 

 course' of the original vein, and this interpolation is the principal factor in trans- 

 forming the embryonic anterior- cardinal into the adult jugular vein. In the 12 

 mm. pig the vena capitis ^ateralis is formed outside the otocyst and of the seventh 

 and eighth nerves. Later it extends by more island formations outside the ninth 

 to twelfth nerves also. The jugular, therefore, is to be defined as . the anterior 

 cardinal vein which, by successive island formations, has migrated to a new posi- 

 tion outside of the otocyst and cephalic ganglia. 3, to show the infundibular gland, 

 Inf, a small evagination from the ventral floor of the fore-brain, F.b. The evagi- 

 nation is really hollow, but the cavity does not appear in the section figured. It 

 enters into very close relations with another hollow evagination, which springs from 

 the dorsal roof of the oral cavity and is known as the hypophysis. The infundib- 

 ular gland and the. hypophysis become intimately associated with one another in 

 their further development and give rise to the pituitary body of the adult, the 

 gland becoming the posterior lobe the hypophysis the anterior lobe of that or- 

 gan. The- hypophysis may be best studied in sagittal sections (see page 292) .< 

 .sent section, figure 191, being at a lower level than figure 189, passes through' 

 ventral portion of the hind-brain and shows only a narrow part of the. cavity 

 of the fourth ventricle, Ven.iv. The three lavers in the wall, Md, of the hind-brain 

 are very distinct. At the anterior end o r <-ne hn.A 1 --^.v, ^ T -, P ~ ~ ^OK, < light 

 lines, Str, which are nv^eH KV n'-'-e-nbers. These lines have been identified as 

 the stria a/>. .,.,.(.#'. They need to be more accurately studied, however, for they 

 seem r^Oier to be fibers of the lateral root of the facial nerve. Close to the ante- 

 -sio/'section of the cardinal vein, Card", appear the minute fourth and third nerves, 

 which, however, are not indicated in the figure. Both lie close to the wall of the 

 vein on the side away from the trigeminal ganglioi . The fourth nerve lies nearer 

 ' outside of the embryo, the third nerve nearer the median plane. At about 

 ^e level as this part of the jugular vein, and very close to the wall of the 

 : * situal i ' -> loop of the internal Chrotic. Lower down, but not ciose 

 r n, Js the section of the lateral jugular. 



"" ''', // ntiil Optic i---j'igin"i : "\ (Fig. 192). The 

 <>r cervical JC gi n of the spinal cord, on 

 "" "-'rves. In this and the three 



