The cerebellum in sagittal plane--anatomic-MR correlation: 2. The cerebellar hemispheres

Anything but Little: a Pictorial Review on Anatomy and Pathology of the Cerebellum

Despite its small size the cerebellum is an anatomically complex and functionally important part of the brain. Traditionally the cerebellum is viewed as a motor control structure entirely devoted to motor control and learning, but recent functional magnetic resonance imaging (fMRI) studies demonstrated significant involvement of the cerebellum in higher order cognitive functions. The anatomical complexity of the cerebellum is reflected by the several nomenclature systems that exist for the description of cerebellar anatomy. The cerebellum can be affected by a variety of pathological processes, including congenital, infectious and inflammatory, neoplastic, vascular, degenerative and toxic metabolic diseases. The purpose of this pictorial review is to (1) provide a general overview of cerebellar anatomy and function, (2) demonstrate normal cerebellar anatomy on imaging studies, and (3) illustrate both common as well as rare pathological conditions affecting the cerebellum.

Microneurosurgical Anatomic Study of the Horizontal Fissure of the Cerebellum

OBJECTIVE

The horizontal fissure of the cerebellum, which is the largest and most prominent fissure, has received less interest from anatomists and neurosurgeons. Hence, the current study aims to provide comprehensive detail about the horizontal fissure and its anatomic and surgical relationship with deeper structures such as the dentate nucleus and middle cerebellar peduncle for the benefit of the neurosurgeon.

METHODS

Ten whole formalin-fixed human cadaveric cerebellar hemispheres were obtained from human cadavers donated to the institution. Different parameters of the horizontal fissure were studied, such as length, depth (medial end, lateral end, and middle), sulcal and gyral variations (superficial and deep), and its close relationship, especially at depth, with the dentate nucleus and middle cerebellar peduncle.

RESULTS

The total length of the horizontal fissure on the right and the left side was 64.3 ± 7.9 mm (range, 53-77 mm) and 65.6 ± 8.01 mm (range, 53-79 mm), respectively. The medial third of the horizontal fissure, with the fewest vessels, was the most suitable place to puncture or start the dissection of the horizontal fissure. The surface projection of the center of the posterior border of the dentate nucleus lies within 20-21 mm on either side of the posterior midline along the horizontal fissure and is only approximately 4 mm deep from the horizontal fissure.

CONCLUSIONS

The anatomic measurements and relationships provided in this description of the horizontal fissure will serve as a tool for surgery selection and planning, as well as an aid to improve microneurosurgical techniques, with the final goal being better patient outcomes.

Anatomy of the Cerebral Cortex, Lobes, and Cerebellum

Strong foundational knowledge of the anatomy of the cerebral cortex, lobes, and cerebellum is key to guide the search for potential lesions based on clinical presentation and known focal neurologic deficits. This article provides an introduction and overview of cerebral cortical anatomy, including the key sulci that divide the 4 lobes of the cerebral cortex, as well as the major gyral and sulcal landmarks within each lobe. The organization of the cerebellum and its major anatomic constituents are also described. Commonly encountered anatomic variants and asymmetries in cerebral cortical anatomy are presented and discussed.

Suboccipital Transhorizontal Fissure Approach for Posterior Cranial Fossa Lesions: A Cadaveric Study and First Clinical Experience

BACKGROUND

Surgical treatment of pathological lesions in the deep cerebellar hemisphere, cerebellopontine angle (CPA), and fourth ventricle of the posterior cranial fossa (PCF) is challenging. Conventional neurosurgical approaches to these lesions are associated with risk of various complications. Mastery of efficient fissure dissection is imperative when approaching deep-seated lesions. The horizontal fissure (HF) is the largest and deepest fissure of the cerebellum.

OBJECTIVE

To conduct an anatomical study and introduce a novel suboccipital trans-HF (SOTHF) approach to access lesions of the deep cerebellar hemispheres, CPA, and upper fourth ventricle of the PCF.

METHODS

We performed a cadaveric dissection study focusing on anatomical landmarks and surgical feasibility of the SOTHF approach then implemented it in 2 patients with a deep cerebellar hemispheric tumor.

RESULTS

Anatomical feasibility of the SOTHF approach was demonstrated and compared with conventional approaches in the cadaveric study. Opening the suboccipital surface of the HF to create medial, intermediate, and lateral surgical corridors provided optimal viewing angles and wide access to the deep cerebellar hemispheres, CPA, and upper fourth ventricle without heavy cerebellar retraction. Sacrificing cerebellar neural structures and complex skull base techniques were not required to obtain adequate exposure. The SOTHF approach was successfully applied without complication in 2 patients with a deep cerebellar hemispheric tumor.

CONCLUSION

The HF is an important cerebellar fissure that provides a gateway to deep areas of the PCF. Further studies are needed to define and expand applications of the SOTHF approach.

Evaluation of the development of the posterior fossa in normal Chinese fetuses by using magnetic resonance imaging

The posterior fossa is an important brain structure containing the cerebellum, cerebral ventricle, and cistern. Early evaluation of the cerebellar structure and function may be valuable for early detection of fetal deformities. At present, no normal value for the fetal posterior fossa has been established yet. This study is aimed to investigate the development of the posterior fossa in normal Chinese fetuses by using magnetic resonance imaging (MRI).Pregnant women who need MRI scan were enrolled in our Hospital between January 2012 and December 2014. The fetal supero-inferior diameter (SID), anterio-posterior diameter (APD), cerebellar vermis area, cerebellar width (CW), cerebellar volume (CV), superior cerebellar cistern width, and cerebellomedullary cistern width were measured using MRI. Pearson's correlation analysis was used to detect the relationship between those parameters and gestational age. A regression analysis was performed for all parameters.A total of 92 participants were retrospectively enrolled finally. The results indicated SID, APD, cerebellar vermis area, CW, and CV were positively associated with gestational age, while no significant correlation was found between the superior cerebellar cistern width and cerebellomedullary cistern width and gestational age. Each equation was established.Our study demonstrated that MRI has the advantages over ultrasound imaging for prenatal evaluation of the fetal posterior fossa with multiple views. Normal value of the posterior fossa of Chinese fetuses was established in this study.

Dissecting the pathobiology of altered MRI signal in amyotrophic lateral sclerosis: A post mortem whole brain sampling strategy for the integration of ultra-high-field MRI and quantitative neuropathology

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is a clinically and histopathologically heterogeneous neurodegenerative disorder, in which therapy is hindered by the rapid progression of disease and lack of biomarkers. Magnetic resonance imaging (MRI) has demonstrated its potential for detecting the pathological signature and tracking disease progression in ALS. However, the microstructural and molecular pathological substrate is poorly understood and generally defined histologically. One route to understanding and validating the pathophysiological correlates of MRI signal changes in ALS is to directly compare MRI to histology in post mortem human brains.

RESULTS

The article delineates a universal whole brain sampling strategy of pathologically relevant grey matter (cortical and subcortical) and white matter tracts of interest suitable for histological evaluation and direct correlation with MRI. A standardised systematic sampling strategy that was compatible with co-registration of images across modalities was established for regions representing phosphorylated 43-kDa TAR DNA-binding protein (pTDP-43) patterns that were topographically recognisable with defined neuroanatomical landmarks. Moreover, tractography-guided sampling facilitated accurate delineation of white matter tracts of interest. A digital photography pipeline at various stages of sampling and histological processing was established to account for structural deformations that might impact alignment and registration of histological images to MRI volumes. Combined with quantitative digital histology image analysis, the proposed sampling strategy is suitable for routine implementation in a high-throughput manner for acquisition of large-scale histology datasets. Proof of concept was determined in the spinal cord of an ALS patient where multiple MRI modalities (T1, T2, FA and MD) demonstrated sensitivity to axonal degeneration and associated heightened inflammatory changes in the lateral corticospinal tract. Furthermore, qualitative comparison of R2* and susceptibility maps in the motor cortex of 2 ALS patients demonstrated varying degrees of hyperintense signal changes compared to a control. Upon histological evaluation of the same region, intensity of signal changes in both modalities appeared to correspond primarily to the degree of microglial activation.

CONCLUSION

The proposed post mortem whole brain sampling methodology enables the accurate intraindividual study of pathological propagation and comparison with quantitative MRI data, to more fully understand the relationship of imaging signal changes with underlying pathophysiology in ALS.

Orthostatic hypotension in acute cerebellar infarction

To investigate the frequency and pattern of orthostatic hypotension (OH) associated with acute isolated cerebellar infarction, and to identify the cerebellar structure(s) potentially responsible for OH, 29 patients (mean age 60.0) with acute isolated cerebellar infarction performed a standard battery of autonomic function tests including the head up tilt test using Finapres for recording of the beat-to-beat BP response during the acute period. Cerebellar infarction related OH was defined as fall in BP (>20 mmHg systolic BP) on tilting in patients without any disease(s) that could potentially cause autonomic dysfunction, or in patients who had a potential cause of autonomic dysfunction, but showed the absence of OH during a follow-up test. The severity and distribution of autonomic dysfunction were measured by the composite autonomic severity score (CASS). Nine patients (31 %) had OH (range 24-53 mmHg) on tilting during the acute period. Most patients (7/9) had a remarkable decrement in systolic BP immediately upon tilting, but OH rapidly normalized. Mean of maximal decrease in systolic BP during head up tilt test was 37.0 mmHg. The OH group showed mild autonomic dysfunctions (CASS, 3.7) with adrenergic sympathetic dysfunction appearing as the most common abnormality. Lesion subtraction analyses revealed that damage to the medial part of the superior semilunar lobule (Crus I) and tonsil was more frequent in OH group compared to non-OH group. Cerebellar infarction may cause a brief episode of OH. The medial part of the superior semilunar lobule and tonsil may participate in regulating the early BP response during orthostasis.

Automated MRI cerebellar size measurements using active appearance modeling

Although the human cerebellum has been increasingly identified as an important hub that shows potential for helping in the diagnosis of a large spectrum of disorders, such as alcoholism, autism, and fetal alcohol spectrum disorder, the high costs associated with manual segmentation, and low availability of reliable automated cerebellar segmentation tools, has resulted in a limited focus on cerebellar measurement in human neuroimaging studies. We present here the CATK (Cerebellar Analysis Toolkit), which is based on the Bayesian framework implemented in FMRIB's FIRST. This approach involves training Active Appearance Models (AAMs) using hand-delineated examples. CATK can currently delineate the cerebellar hemispheres and three vermal groups (lobules I-V, VI-VII, and VIII-X). Linear registration with the low-resolution MNI152 template is used to provide initial alignment, and Point Distribution Models (PDM) are parameterized using stellar sampling. The Bayesian approach models the relationship between shape and texture through computation of conditionals in the training set. Our method varies from the FIRST framework in that initial fitting is driven by 1D intensity profile matching, and the conditional likelihood function is subsequently used to refine fitting. The method was developed using T1-weighted images from 63 subjects that were imaged and manually labeled: 43 subjects were scanned once and were used for training models, and 20 subjects were imaged twice (with manual labeling applied to both runs) and used to assess reliability and validity. Intraclass correlation analysis shows that CATK is highly reliable (average test-retest ICCs of 0.96), and offers excellent agreement with the gold standard (average validity ICC of 0.87 against manual labels). Comparisons against an alternative atlas-based approach, SUIT (Spatially Unbiased Infratentorial Template), that registers images with a high-resolution template of the cerebellum, show that our AAM approach offers superior reliability and validity. Extensions of CATK to cerebellar hemisphere parcels are envisioned.

Is there relationship between brain atrophy and higher incidence of hip fracture in old age?--A preliminary study

PURPOSE

The studies on the correlation between incidence of fall and brain atrophy have been going on to find out the cause of fall and its prevention. The purpose of this study was to explore the relationship between incidence of hip fracture and brain volume, measured by magnetic resonance image.

MATERIALS AND METHODS

A total of 14 subjects with similar conditions (age, height, weight, and past history) were selected for this study. Fracture group (FG) was consisted of 5 subjects with intertrochanteric fracture. Control group (CG) had 9 subjects without intertrochanteric fracture. MRI-based brain volumetry was done in FG and CG with imaging software. Total brain (tBV), absolute cerebellar volumes (aCV) and relative cerebellar volumes (rCV) were compared between two groups. Student t-test was used to statistically analyze the results.

RESULTS

In FG, average tBV, aCV and rCV were 1034.676±38.80, 108.648±76.80 and 10.50±0.72 cm³, respectively. In CG, average tBV, aCV and rCV were found to be 1106.459±89.15, 114.899±98.06 and 10.39±0.53 cm³, respectively, having no statistically significant difference (p>0.05).

CONCLUSION

There was no significant difference between the fracture and control groups. Patients with neurologic disease such as cerebellar ataxia definitely have high incidence of fall that causes fractures and have brain changes as well. However, FG without neurologic disease did not have brain volume change. We consider that high risk of fall with hip fracture might decrease brain function which is not obvious to pickup on MRI.

Rhombencephalosynapsis: embryopathology and management strategies of associated neurosurgical conditions with a review of the literature

OBJECT

Rhombencephalosynapsis (RS) is a rare congenital posterior fossa malformation characterized by dorsal fusion of the cerebellar hemispheres, hypogenesis or agenesis of the vermis, and fusion of the dentate nuclei and superior cerebellar peduncles. The objective of this institutional study is to review the clinical conditions associated with RS and analyze the varied biological profile of this unique condition.

METHODS

The study data were collected retrospectively from the medical records of patients at Rainbow Babies and Children's Hospital. After required institutional review board approval, the authors obtained information regarding the cases of RS reviewed by the Departments of Radiology, Genetics, and/or Pediatric Neurology. Medical charts were systematically reviewed, and 9 patients were analyzed in detail.

RESULTS

The authors describe 6 cases of RS and 3 cases of partial RS. This case series demonstrates an association between RS and symptomatic hydrocephalus (7 of 9 patients) and RS and Chiari malformation (5 of 9 patients). Patients with symptomatic hydrocephalus underwent endoscopic third ventriculostomy or ventriculoperitoneal shunt insertion. One of the patients with an associated Chiari malformation underwent foramen magnum decompression.

CONCLUSIONS

The authors present a large case series of RS. Patients with RS often had hydrocephalus and/or a Chiari Type I or II hindbrain malformation. Neuroimaging findings of RS are presented along with hypotheses to explain the embryopathology of this unusual condition.

Association of Chiari malformation type I and tethered cord syndrome: preliminary results of sectioning filum terminale

OBJECTIVE

The pathogenesis of CM-I is incompletely understood. We describe an association of CM-I and TCS that occurs in a subset of patients with normal size of the PCF.

METHODS

The prevalence of TCS was determined in a consecutively accrued cohort of 2987 patients with CM-I and 289 patients with low-lying cerebellar tonsils (LLCT). Findings in 74 children and 244 adults undergoing SFT were reviewed retrospectively. Posterior cranial fossa size and volume were measured using reconstructed 2D computed tomographic scans and MR images. Results were compared to those in 155 age- and sex-matched healthy control individuals and 280 patients with generic CM-I. The relationships of neural and osseus structures at the CCJ and TLJ were investigated morphometrically on MR images. Intraoperative CDU was used to measure anatomical structures and CSF flow in the lumbar theca.

RESULTS

Tethered cord syndrome was present in 408 patients with CM-I (14%) and 182 patients with LLCT (63%). In 318 patients undergoing SFT, there were no significant differences in the size or volume of the PCF as compared to healthy control individuals. Morphometric measurements demonstrated elongation of the brain stem (mean, 8.3 mm; P < .001), downward displacement of the medulla (mean, 4.6 mm; P < .001), and normal position of the CMD except in very young patients. Compared to patients with generic CM-I, the FM was significantly enlarged (P < .001). The FT was typically thin and taut (mean transverse diameter, 0.8 mm). After SFT, the cut ends of the FT distracted widely (mean, 41.7 mm) and CSF flow in the lumbar theca increased from a mean of 0.7 cm/s to a mean of 3.7 cm/s (P < .001). Symptoms were improved or resolved in 69 children (93%) and 203 adults (83%) and unchanged in 5 children (7%) and 39 adults (16%) and, worse, in 2 adults (1%) over a follow-up period of 6 to 27 months (mean, 16.1 months +/- 4.6 SD). Magnetic resonance imaging 1 to 18 months after surgery (mean, 5.7 months +/- 3.8 SD) revealed upward migration of the CMD (mean, 5.1 mm, P < .001), ascent of the cerebellar tonsils (mean, 3.8 mm, P < .001), reduction of brain stem length (mean, 3.9 mm, P < .001), and improvement of scoliosis or syringomyelia in some cases.

CONCLUSIONS

Chiari malformation type I/TCS appears to be a unique clinical entity that occurs as a continuum with LLCT/TCS and is distinguished from generic CM-I by enlargement of the FM and the absence of a small PCF. Distinctive features include elongation and downward displacement of the hindbrain, normal position of the CMD, tight FT, and reduced CSF flow in the lumbar theca. There is preliminary evidence that SFT can reverse moderate degrees of tonsillar ectopia and is appropriate treatment for cerebellar ptosis after Chiari surgery in this cohort.

Midline medulloblastoma versus astrocytoma: the position of the superior medullary velum as a sign for diagnosis

PURPOSE

We wish to describe the position of the superior medullary velum (SMV) in midline posterior fossa tumours as a sign in helping to distinguish between midline medulloblastoma and midline astrocytoma.

MATERIAL AND METHODS

Sagittal T1-weighted MRI images of 21 consecutive patients with histologically documented posterior fossa midline astrocytomas (nine cases) and medulloblastomas (12 cases) were reviewed, with respect to the position of the velum medullare superius.

RESULTS

In all medulloblastomas the SMV was superiorly dislocated; in eight astrocytomas it was anteriorly and/or inferiorly disclocated; only in one astrocytoma the SMV presented upward dislocation.

CONCLUSION

In the differential diagnosis between medulloblastoma and astrocytoma the upward dislocation of the SMV is strongly suggestive of medulloblastoma.

Volume of cerebellar vermis in monozygotic twins discordant for combat exposure: lack of relationship to post-traumatic stress disorder

Several functional neuroimaging studies have implicated the cerebellar vermis in post-traumatic stress disorder (PTSD), but there have been no structural neuroimaging studies of this brain structure in PTSD. We utilized magnetic resonance imaging (MRI) with manual tracing to quantify the volumes of three divisions of the mid-sagittal vermis, and their total, within an identical, co-twin control design that employed Vietnam veterans discordant for combat exposure in Vietnam. Each structure's volume was significantly correlated between twins, indicating a partial familial determination: for anterior superior vermis, r=0.73; for posterior superior vermis, r=0.47; for inferior posterior vermis, r=0.51; and for total vermis, r=0.57. There were no significant differences between the PTSD and non-PTSD veterans for any vermis volume, and no significant main effects or interactions when their non-combat-exposed co-twins were added to the analyses. Thus, the results do not support the structural abnormality of cerebellar vermis in combat-related PTSD.

Evaluation of morphological plasticity in the cerebella of basketball players with MRI

Cerebellum is a key structure involved in motor learning and coordination. In animal models, motor skill learning increased the volume of molecular layer and the number of synapses on Purkinje cells in the cerebellar cortex. The aim of this study is to investigate whether the analogous change of cerebellar volume occurs in human population who learn specialized motor skills and practice them intensively for a long time. Magnetic resonance image (MRI)-based cerebellar volumetry was performed in basketball players and matched controls with V-works image software. Total brain volume, absolute and relative cerebellar volumes were compared between two groups. There was no significant group difference in the total brain volume, the absolute and the relative cerebellar volume. Thus we could not detect structural change in the cerebellum of this athlete group in the macroscopic level.

Stimulation sous-thalamique dans la maladie de Parkinson sévère

The subthalamic nucleus (STN) is the main target of deep brain stimulation (DBS) treatment for severe idiopathic Parkinson's disease. But there is still no clear information on the location of the effective contacts (used during the chronic phase of stimulation). Our aim was to assess the anatomical structures of the subthalamic area (STA) involved during chronic DBS. Ten patients successfully treated were included. The surgical procedure was based on direct STN targeting (stereotactic MRI based) pondered by the acute effects of intraoperative stimulation. We used a formaldehyde-fixed human specimen to compare by matching MRI images obtained at 1.5 Tesla (performed in clinical stereotactic conditions) and at very high field at 4.7 Tesla. This allowed accurate analysis of the anatomy of the STA and retrospective precision of the location of the center of effective contacts which were located within the STN in 4 patients, at the interface between the STN and the ZI and/or FF in 13, at the interface between ZI and FF in 2 and between the STN and the substantia nigra in one. These results were consistent with the literature, revealing the implication of neighboring structures, especially the zona incerta and Forel's Field, in the clinical benefit.

Role of the cerebellum in implicit motor skill learning: a PET study

To depict neural substrates of implicit motor learning, regional cerebral blood flow was measured using positron emission tomography (PET) in 13 volunteers in the rest condition and during performance of a unimanual two-ball rotation task. Subjects rotated two balls in a single hand; a slow rotation (0.5 Hz) was followed by two sessions requiring as rapid rotation as possible. The process was repeated four times by a single hand (Block 1) and then by the opposite hand (Block 2). One group of volunteers began with the right hand (n = 7), and the other with the left (n = 6). Performance was assessed by both quickness and efficiency of movements. The former was assessed with the maximum number of rotation per unit time, and the latter with the electromyographic activity under constant speed of the movement. Both showed learning transfer from the right hand to the left hand. Activation of cerebrum and cerebellum varied according to hand. Activation common to both hands occurred in the bilateral dorsal premotor cortex and parasagittal cerebellum, right inferior frontal gyms, left lateral cerebellum and thalamus, supplementary motor area, and cerebellar vermis. The left lateral cerebellum showed the most prominent activation on the first trial of the novel task, and hence may be related the early phase of learning, or "what to do" learning. Left parasagittal cerebellum activity diminished with training both in first and second blocks, correlating inversely with task performance. This region may therefore be involved in later learning or "how to do" learning. The activity of these regions was less prominent with prior training than without it. Thus the left cerebellar hemisphere may be related to learning transfer across hands.

Odorant-induced and sniff-induced activation in the cerebellum of the human

Functional magnetic resonance imaging was used to test whether odorants induce activation in the cerebellum of the human. The odorants vanillin and propionic acid both induced significant activation, primarily in the posterior lateral hemispheres. Activation was concentration-dependent, greater after stimulation with higher concentration odorants. By contrast, the action of sniffing nonodorized air induced significant activation in the anterior cerebellum, primarily in the central lobule. These findings demonstrate that the cerebellum plays a role in human olfaction. A hypothesis is proposed whereby the cerebellum maintains a feedback mechanism that regulates sniff volume in relation to odor concentration.

Lobular patterns of cerebellar activation in verbal working-memory and finger-tapping tasks as revealed by functional MRI

The lobular distributions of functional activation of the cerebellum during verbal working-memory and finger movement tasks were investigated using functional magnetic resonance imaging (fMRI). Relative to a rest control, finger tapping of the right hand produced ipsilateral-increased activation in HIV/HV [Roman numeral designations based on Larsell's () nomenclature] and HVI and weaker activation in HVIII that was stronger on the ipsilateral side. For a working-memory task, subjects were asked to remember six (high load) or one (low load) visually presented letters across a brief delay. To assess the motoric aspects of rehearsal in the absence of working memory, we asked the subjects to repeatedly read subvocally six or one letters at a rate that approximated the internally generated rehearsal of working memory (motoric rehearsal task). For both tasks, bilateral regions of the superior cerebellar hemispheres (left superior HVIIA and right HVI) and portions of posterior vermis (VI and superior VIIA) exhibited increased activation during high relative to low load conditions. In contrast, the right inferior cerebellar hemisphere (HVIIB) exhibited this load effect only during the working-memory task. We hypothesize that HVI and superior HVIIA activation represents input from the articulatory control system of working memory from the frontal lobes and that HVIIB activation is derived from the phonological store in temporal and parietal regions. From these inputs, the cerebellum could compute the discrepancy between actual and intended phonological rehearsal and use this information to update a feedforward command to the frontal lobes, thereby facilitating the phonological loop.

The role of the transcerebellar view in the detection of fetal central nervous system anomaly

The current resolution of antenatal ultrasonography permits the sonologist to visualize and critically evaluate the cerebellum and cisterna magna. Abnormalities in the size and shape of these structures will assist the sonologist in the detection of the majority of central nervous system malformations.

Isolated rhombencephalosynapsis diagnosed by magnetic resonance imaging

A case of human rhombencephalosynapsis diagnosed in vivo by magnetic resonance imaging (MRI) is presented that demonstrates fusion of the dentate nuclei and agenesis of the cerebellar vermis. To our knowledge, this condition in its isolated form has been previously diagnosed only at necropsy. The developmental features of the cerebellum are discussed and correlation with cerebellar function is also presented. Recognition of this anomaly in the living patient may be useful in further understanding the clinical significance of segmentation of the rhombencephalon.