ISTHMUS RHOMBENCEPHALI.
The isthmus rhombencephali forms the transition from the mid-brain to rhomb encephalon, which latter is subdivided into the metencephalon and the myelen cephalon.
To the isthmus belong the brachia conjunctiva, the velum medullare anterius and the trigonum lemnisci, which structures collectively constitute the dorsal part of the isthmus. Ventral are the cerebral peduncles. The isthmus surrounds the upper end of the fourth ventricle.
The brachia conjunctiva cerebelli, crura cerebelli ad cerebrum, or superior cerebellar peduncles, form two flattened cylindrical columns that emerge from the cere bellum. They embrace the anterior medullary velum, converge forward and come together behind the quadrigeminal plate. At the sides, the brachia conjunctiva border on the pontile peduncles, separated from the latter by the sulcus lateralis mesencephali, which runs at first toward the corpus geniculatum lateralis and then laterally.
The velum medullare anterius is a thin medullary sheet that stretches between the brachia conjunctiva or the superior cerebellar peduncles. Dorsally it is covered by and fused with the lingula of the cerebellum and assists in roofing in the anterior part of the fourth ventricle (Fig. 79). From the narrow front end of the velum arises the frenulum
veli medullaris anterioris that extends toward the inferior colliculi.
In advance of the front end of the brachium conjunctivum, lies a triangular field, the trigonum lemnisci. It is usually distinguishable from the whiter brachium by its gray color. Laterally, the trigonum borders on the cerebral peduncle, separated by the sulcus lateralis mesencephali, in front it is bounded by the brachium quadrigeminum inferius and the inferior colliculi. The area contains the fibre-tracts of the fillet or lem niscus and, deeply placed, the nucleus lemnisci lateralis. Occasionally one notes delicate white fibre-strands that pass from the sulcus mesencephali lateralis over the brachium, par ticularly in the vicinity of the quadrigeminal bodies. Some of the strands bend medially at right angles and pass backward through the anterior medullary velum. These fibrae arciformes belong to a bundle that ascends from the spinal cord to the cerebellum, the tractus spino-cerebellaris ventralis or Gowers' tract (page 161).