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Internal features.It is divisible, on the basis of cytoarchitectonic, chemoarchitec-tonic and functional criteria, into three bilateral longitudinal columns: median, medial, and lateral.Proper nuclei of the myelencephalon are as fol- The inferior olivary complex (nuclei olivares infe-riores) is an irregularly crenated mass of grey matter with a medially directed hilum, through which numerous fibres enter and leave the nuclei.It is divisible, on the basis of cytoarchitectonic, chemoarchitec-tonic and functional criteria, into three bilateral longitudinal columns: median, medial, and lateral.Axons from the spinal nucleus of the trigemi-nal nerve cross at the midline with the contralateral fibres, compose the trigeminal lemniscus (lemniscus trigeminalis) and ascend to the contralateral thala-mus.Proper nuclei of the myelencephalon are as fol- The inferior olivary complex (nuclei olivares infe-riores) is an irregularly crenated mass of grey matter with a medially directed hilum, through which numerous fibres enter and leave the nuclei.The rhomboid fossa (fossa rhomboidea) is composed by the dorsal surfaces of the pons and the cranial half of the medulla oblongata which are divided by the medullary striae of the fourth ventricle (striae medullares ventriculi quarti).The inferior salivatory nucleus nucleus salivato-rius inferior [IX/ is situated near the upper pole of the vagal nucleus, just above the pontomedullary junction which sends preganglionic parasympathetic fibres into the glossopharyngeal nerve for the control of the parotid salivary gland.Caudal fibres link the vagus [X] and cranial accessory XI] nerves and are distributed to the pharyngeal constrictors, intrinsic laryngeal muscles and striated muscles of the palate and upper ocsophagus.The decus-sation of the medial lemniscus, or sensory decussation (decussatio lemnisci medialis) is located dorsal to the pyramids and ventral to the central grey mat-ter.The decus-sation of the medial lemniscus, or sensory decussation (decussatio lemnisci medialis) is located dorsal to the pyramids and ventral to the central grey mat-ter.It extends caudally to the first cervical spinal segment and rostrally to the open part of the medulla under the trigone of the THE BRAIN vagus nerve, or vagal trigone (trigonum nervi vagi) of the floor of the fourth ventricle.The vagal nucleus controls the non-striated muscle of the viscera of the thorax (heart, bronchi, lungs and oesophagus) and abdomen (stomach, liver, pancreas, spleen, small intestine and proximal part of the colon).The inferior olivary nuclei receive afferents from specific sources and project axons to the cerebellum as the olivocerebellar tract (ascending), to the spinal cord as the olivospinal fibres (descending).Second-order axons emerge from the nuclei as internal arcuate fibres (fibrae arcuatae internae), at first curving ventrolaterally around the central grey matter and then ventromedially between the spinal tract of the trigeminal nerve and the central grey matter.The inferior olivary nuclei receive afferents from specific sources and project axons to the cerebellum as the olivocerebellar tract (ascending), to the spinal cord as the olivospinal fibres (descending).Second-order axons emerge from the nuclei as internal arcuate fibres (fibrae arcuatae internae), at first curving ventrolaterally around the central grey matter and then ventromedially between the spinal tract of the trigeminal nerve and the central grey matter.The nucleus ambiguus (nucleus ambiguus [IX, X, XII) is a group of large motor neurons, situated deep in the medullary reticular formation on the level of the rhomboid fossa.The solitary nuclei, or nuclei of the solitary tract (nuclei tractus solitarii [VII, IX, X/) lie ventrolat-eral to the dorsal nucleus of the vagus nerve.The solitary nuclei are intimately related to, and receive fibres from the solitary tract, which carries afferent fibres from the facial [VII], glossopharyngeal [IX] and vagus [X] nerves.Primary spinal propriocep-tive afferents (first-order afferent fibres) synapse with multipolar neurons in the nuclei.

Internal features. The bulb contains white substance and grey substance. Bodies of neurons constitute the nuclei of cranial nerves and proper nuclei of the myelencephalon. Nuclei project to the surface.
The rhomboid fossa (fossa rhomboidea) is composed by the dorsal surfaces of the pons and the cranial half of the medulla oblongata which are divided by the medullary striae of the fourth ventricle (striae medullares ventriculi quarti). A posterior median sulcus separates right and left halves of the floor of the fourth ventricle. The medial eminences are positioned aside of the groove.
The prominent nucleus of the hypoglossal nerve (nucleus nervi hypoglossi /XIII) lies near the midline in the dorsal medullary grey matter. It is approximately 2 cm long. Its rostral part is located beneath the trigone of the hypoglossal nerve, or hypoglossal trigone (trigonum nervi hypoglossi) in the floor of the fourth ventricle, and its caudal part extends into the closed part of the bulb. Motor neurons within the nuclei correspond to the structural and functional divisions of tongue muscles.
The posterior, or dorsal nucleus of the vagus nerve nucleus posterior nervi vagi (X) lies dorsolateral to the hypoglossal nucleus. It extends caudally to the first cervical spinal segment and rostrally to the open part of the medulla under the trigone of the
THE BRAIN
vagus nerve, or vagal trigone (trigonum nervi vagi) of the floor of the fourth ventricle. The vagal nucleus controls the non-striated muscle of the viscera of the thorax (heart, bronchi, lungs and oesophagus) and abdomen (stomach, liver, pancreas, spleen, small intestine and proximal part of the colon).
This nucleus is a general visceral efferent nucleus and is the largest parasympathetic nucleus in the brainstem. The nucleus ambiguus (nucleus ambiguus [IX, X, XII) is a group of large motor neurons, situated deep in the medullary reticular formation on the level of the rhomboid fossa. Rostral fibres emerging from it join the glossopharyngeal nerve [X]. Caudal fibres link the vagus [X] and cranial accessory XI] nerves and are distributed to the pharyngeal constrictors, intrinsic laryngeal muscles and striated muscles of the palate and upper ocsophagus. The motor nuclei of cranial nerves receive corticonuclear fibres (fibrae corticonucleares) of pyramidal pathways for conscious motor activity.
The inferior salivatory nucleus nucleus salivato-rius inferior [IX/ is situated near the upper pole of the vagal nucleus, just above the pontomedullary junction which sends preganglionic parasympathetic fibres into the glossopharyngeal nerve for the control of the parotid salivary gland.
The solitary nuclei, or nuclei of the solitary tract (nuclei tractus solitarii [VII, IX, X/) lie ventrolat-eral to the dorsal nucleus of the vagus nerve. The solitary nuclei are intimately related to, and receive fibres from the solitary tract, which carries afferent fibres from the facial [VII], glossopharyngeal [IX] and vagus [X] nerves. These fibres enter the tract in descending order and send gustatory information from the lingual and palatal mucosa. They also convey visceral impulses from the pharynx [IX and
X| and from the oesophagus and abdominal alimentary canal [X].
The spinal nucleus and spinal tract of the trigemi-nal nerve are visible ventrolateral to the dorsal grey columns as the trigeminal tubercle. They are continuous with the substantia gelatinosa of the spinal cord. Axons from the spinal nucleus of the trigemi-nal nerve cross at the midline with the contralateral fibres, compose the trigeminal lemniscus (lemniscus trigeminalis) and ascend to the contralateral thala-mus.Proper nuclei of the myelencephalon are as fol-
The inferior olivary complex (nuclei olivares infe-riores) is an irregularly crenated mass of grey matter with a medially directed hilum, through which numerous fibres enter and leave the nuclei. The inferior olivary nuclei receive afferents from specific sources and project axons to the cerebellum as the olivocerebellar tract (ascending), to the spinal cord as the olivospinal fibres (descending). The nuclei constitute the subcortical balance centre.
The gracile and cuneate nuclei (nucleus gracilis et nucleus cuneatus) serve as relays between the spinal cord and higher levels. Primary spinal propriocep-tive afferents (first-order afferent fibres) synapse with multipolar neurons in the nuclei. They are part of the pathway that is the major route for discriminative aspects of tactile and proprioceptive sensa-tion. Second-order axons emerge from the nuclei as internal arcuate fibres (fibrae arcuatae internae), at first curving ventrolaterally around the central grey matter and then ventromedially between the spinal tract of the trigeminal nerve and the central grey matter. The fibres decussate in the midline thereafter forming the medial lemniscus (lemniscus me-dialis) which ascends to the thalamus. The decus-sation of the medial lemniscus, or sensory decussation (decussatio lemnisci medialis) is located dorsal to the pyramids and ventral to the central grey mat-ter. The medial lemniscus ascends from the decus-sation as a flattened tract, near the median raphe.
Numerous islets of grey matter are scattered centrally in the ventrolateral medulla, an area intersected by nerve fibres that run in all directions.
This is the reticular formation formatio reticula-ris), which is polymodal, diffuse or non-specific in function. The reticular regions are often regarded as phylogenetically ancient, representing a primitive nerve network upon which more anatomically organized, functionally selective, connections have developed during evolution. The reticular formation is a continuous core that traverses the whole brainstem, and is continuous below with the reticular intermediate spinal grey laminae. It is divisible, on the basis of cytoarchitectonic, chemoarchitec-tonic and functional criteria, into three bilateral longitudinal columns: median, medial, and lateral.Proper nuclei of the myelencephalon are as fol-
The inferior olivary complex (nuclei olivares infe-riores) is an irregularly crenated mass of grey matter with a medially directed hilum, through which numerous fibres enter and leave the nuclei. The inferior olivary nuclei receive afferents from specific sources and project axons to the cerebellum as the olivocerebellar tract (ascending), to the spinal cord as the olivospinal fibres (descending). The nuclei constitute the subcortical balance centre.
The gracile and cuneate nuclei (nucleus gracilis et nucleus cuneatus) serve as relays between the spinal cord and higher levels. Primary spinal propriocep-tive afferents (first-order afferent fibres) synapse with multipolar neurons in the nuclei. They are part of the pathway that is the major route for discriminative aspects of tactile and proprioceptive sensa-tion. Second-order axons emerge from the nuclei as internal arcuate fibres (fibrae arcuatae internae), at first curving ventrolaterally around the central grey matter and then ventromedially between the spinal tract of the trigeminal nerve and the central grey matter. The fibres decussate in the midline thereafter forming the medial lemniscus (lemniscus me-dialis) which ascends to the thalamus. The decus-sation of the medial lemniscus, or sensory decussation (decussatio lemnisci medialis) is located dorsal to the pyramids and ventral to the central grey mat-ter. The medial lemniscus ascends from the decus-sation as a flattened tract, near the median raphe.
Numerous islets of grey matter are scattered centrally in the ventrolateral medulla, an area intersected by nerve fibres that run in all directions.
This is the reticular formation formatio reticula-ris), which is polymodal, diffuse or non-specific in function. The reticular regions are often regarded as phylogenetically ancient, representing a primitive nerve network upon which more anatomically organized, functionally selective, connections have developed during evolution. The reticular formation is a continuous core that traverses the whole brainstem, and is continuous below with the reticular intermediate spinal grey laminae. It is divisible, on the basis of cytoarchitectonic, chemoarchitec-tonic and functional criteria, into three bilateral longitudinal columns: median, medial, and lateral.