OCR Text |
Show 1887.] PROF. T. J. PARKER ON CARCHARODON RONDELETII. 37 rest.) are large and folded. The metacoele or fourth ventricle (mt. cce.) is covered by a thick tela vasculosa (tel.vasc.) and its floor is marked by five well-marked grooves. The epencephalon or cerebellum (epen.) is large, divided into lobes by transverse sulci, and prolonged forwards so as to cover all the median portion of the mesencephalon as well as backwards over the anterior sixth of the metencephalon. It contains a large epiccele (cerebellar ventricle), which communicates by a comparatively small aperture or ostium with the metacoele. The mesencephalon presents dorsally a pair of large optic lobes, or optencephala (opten.), and contains a spacious mesoccele (fig. 21, (mes.cce.), on the middle of the floor of which is a small aperture (x) leading into the diaccele or third ventricle (di.cce.). This apparently unusual arrangement no doubt indicates that the optic lobes have overlapped the posterior portion of the diencephalon. The diencephalon (dien.) is short; its cavity, the diaccele (di.cce.), is arched over posteriorly by a narrow bridge of nervous matter, but for the most part is covered in only by the thick vascular velum interpositum (vel.int.). O n its floor is a longitudinal groove (y) leading both into the mesoccele and into the infundibulum, which is short and bears a large trilobed hypophysis or pituitary body (hyp.). No haematosac (saccus vasculosus) was apparent, and the conarium or pineal body was not observed. On the ventral surface of the diencephalon are small rounded lobi inferiores (lob.inf.). In the foetal brain, as well as in that of Lamna, the diencephalon is quite concealed in a view from above, the anterior faces of the optic lobes being quite vertical and in close contact with the posterior face of the cerebrum (fig. 29). The cerebrum is a large, transversely elongated mass, consisting of the fused prosencephala or cerebral hemispheres (prosen.), the line of junction between which is marked both above and below by a distinct groove. Each prosencephalon is also divisible into a large dorso-lateral and a smaller ovoidal ventral lobe (fig. 22). The cerebrum contains well-developed lateral ventricles or proso-cceles (fig. 21, prs.cce.), communicating each by a foramen of Monro (for.M.) with a small triangular space, the aula, lying immediately cephalad of the diaccele proper, and consisting of the cavity of the basi-cerebrnm or unpaired portion of tbe protencephalon (embryonic fore-brain) left by the budding-off of the cerebral hemispheres. A choroid plexus (ch.plx.) is continued into each prosocoele from the velum interpositum. On the inner wall of the cavity is a large ovoidal elevation (a), and a smaller one (6) occurs on its floor. The rhinencepbalon consists, as already stated, of a greatly elongated crus, and of a bulb in apposition with the olfactory sac. A cavity, the rhinocoele (rh.cce.) is continued into the crus from the corresponding lateral ventricle. The first four cerebral nerves present no special features of importance. The fifth, seventh, and eighth arise, as usual, close together, having between them four principal roots. The trigeminal (figs. 20, 22, and 23, v) arises by two roots-an |