Radiographic and Magnetic Resonance Imaging Identification of Thoracolumbar Spine Variants with Implications for the Positioning of the Conus Medullaris in Rhesus Macaques

The anatomy of the vertebral column in mammals may differ between species and between subjects of the same species, especially with regards to the composition of the thoracolumbar spine. We investigated, using several noninvasive imaging techniques, the thoracolumbar spine of a total of 44 adult rhesus macaques of both genders. Radiographic examination of the vertebral column showed a predominant spine phenotype with 12 rib‐bearing thoracic vertebrae and 7 lumbar vertebrae without ribs in 82% of subjects, whereas a subset of subjects demonstrated 13 rib‐bearing thoracic vertebrae and 6 lumbar vertebrae without ribs. Computer tomography studies of the thoraco‐lumbar spine in two cases with a pair of supernumerary ribs showed facet joints between the most caudal pair of ribs and the associated vertebra, supporting a thoracic phenotype. Magnetic resonance imaging (MRI) studies were used to determine the relationship between the lumbosacral spinal cord and the vertebral column. The length of the conus medullaris portion of the spinal cord was 1.5 ± 0.3 vertebral units, and its rostral and caudal positions in the spinal canal were at 2.0 ± 0.3 and 3.6 ± 0.4 vertebral units below the thoracolumbar junction, respectively (n = 44). The presence of a set of supernumerary ribs did not affect the length or craniocaudal position of the conus medullaris, and subjects with13 rib‐bearing vertebrae may from a functional or spine surgical perspective be considered as exhibiting12 thoracic vertebrae and an L1 vertebra with ribs. Anat Rec, 300:300–308, 2017. © 2016 Wiley Periodicals, Inc.     

A Computer-Assisted System for Diagnostic Workstations: Automated Bone Labeling for CT Images

Although accurate information on thoracolumbar bone structure is essential when computed tomography (CT) images are examined, there is no automated method of labeling all the vertebrae and ribs on a CT scan. We are developing a computer-aided diagnosis system that labels ribs and thoracolumbar vertebrae automatically and have evaluated its accuracy. A candidate bone was extracted from the CT image volume data by pixel thresholding and connectivity analysis. All non-bony anatomical structures were removed using a linear discriminate of distribution of CT values and anatomical characteristics. The vertebrae were separated from the ribs on the basis of their distances from the centers of the vertebral bodies. Finally, the thoracic cage and lumbar vertebrae were extracted, and each vertebra was labeled with its own anatomical number by histogram analysis along the craniocaudal midline. The ribs were labeled in a similar manner, based on location data. Twenty-three cases were used for accuracy comparison between our method and the radiologist’s. The automated labeling of the thoracolumbar vertebrae was concordant with the judgments of the radiologist in all cases, and all but the first and second ribs were labeled correctly. These two ribs were frequently misidentified, presumably because of pericostal anatomical clutter or high densities of contrast material in the injected veins. We are confident that this system can contribute usefully as part of a picture archiving and communication system workstation, though further technical improvement is required for identification of the upper ribs.     

Comments on the Serial Homology and Homologues of Vertebral Lateral Projections in Crocodylia (Eusuchia)

The literature on crocodylian anatomy presents the transverse process in an ambiguous meaning, which could represent all lateral expansions derived from the neural arch, including vertebrae from cervical to caudal series, or in a more restrictive meaning, being applied only to lumbar vertebrae. The lateral expansion of sacral and caudal vertebrae usually referred to as the transverse process has been discovered to be fused ribs, bringing more ambiguity to this term. Therefore, with the lack of a definition for transverse process and other associated terms, the present work aims to propose a nomenclatural standardization, as well as definitions and biological meaning, for vertebral rib related structures. Vertebra obtained from museum collections from a total of 87 specimens of 22 species of all extant Crocodylia genera were studied. All vertebrae, except cervical and first three dorsal, exhibit transverse processes. The transverse process is more developed in dorsal and lumbar vertebrae than in sacral and caudal vertebrae in which it is suppressed by the fused ribs. The serial homology hypotheses here proposed can also be aplied to other Crurotarsi and saurischian dinosaurs specimens. This standardization clarifies the understand of the serial homology among those homotypes, and reduces the ambiguity and misleadings in future work comparisons. Anat Rec, 301:1203–1215, 2018. © 2018 Wiley Periodicals, Inc.     

C7 vertebra homeotic transformation in domestic dogs – are Pug dogs breaking mammalian evolutionary constraints?

The number of cervical vertebrae in mammals is almost constant at seven, regardless of their neck length, implying that there is selection against variation in this number. Homebox (Hox) genes are involved in this evolutionary mammalian conservation, and homeotic transformation of cervical into thoracic vertebrae (cervical ribs) is a common phenotypic abnormality when Hox gene expression is altered. This relatively benign phenotypic change can be associated with fatal traits in humans. Mutations in genes upstream of Hox, inbreeding and stressors during organogenesis can also cause cervical ribs. The aim of this study was to describe the prevalence of cervical ribs in a large group of domestic dogs of different breeds, and explore a possible relation with other congenital vertebral malformations (CVMs) in the breed with the highest prevalence of cervical ribs. By phenotyping we hoped to give clues as to the underlying genetic causes. Twenty computed tomography studies from at least two breeds belonging to each of the nine groups recognized by the Federation Cynologique Internationale, including all the brachycephalic ‘screw‐tailed’ breeds that are known to be overrepresented for CVMs, were reviewed. The Pug dog was more affected by cervical ribs than any other breed (46%; P < 0.001), and was selected for further analysis. No association was found between the presence of cervical ribs and vertebral body formation defect, bifid spinous process, caudal articular process hypoplasia/aplasia and an abnormal sacrum, which may infer they have a different aetiopathogenesis. However, Pug dogs with cervical ribs were more likely to have a transitional thoraco‐lumbar vertebra (P = 0.041) and a pre‐sacral vertebral count of 26 (P < 0.001). Higher C7/T1 dorsal spinous processes ratios were associated with the presence of cervical ribs (P < 0.001), supporting this is a true homeotic transformation. Relaxation of the stabilizing selection has likely occurred, and the Pug dog appears to be a good naturally occurring model to further investigate the aetiology of cervical ribs, other congenital vertebral anomalies and numerical alterations.     

Functional adaptations of facet geometry in the canine thoracolumbar and lumbar spine (Th10-L6).

The shape, size and transverse distance between contralateral caudal articular processes of caudal thoracic and lumbar vertebrae were evaluated in a sample of 140 macerated canine spines to contribute to the understanding of the development of vertebral misalignment. Dogs were grouped as large, chondrodystrophic, and small breeds. In large dogs, caudal articular surfaces were adapted to higher body-weight by the presence of larger articular surfaces. Additionally, caudal articular surfaces not only covered the lateral, but also the ventral (i.e. ventral facet) and/or caudal aspects (i.e. caudal facet) of the caudal articular processes. Ventral facets, which increase loading capacity of the zygapophyseal joints in normally aligned vertebrae, were more frequently observed in large breeds (p < 0.001) than in small and chondrodystrophic breeds. With some exceptions, caudal facets primarily were present in large dogs, especially at the vertebral levels L3-L5. Their formation is induced by extension of the normal lykphotic thoracic and lumbar spine causing lumbar lordosis and denotes loss in elasticity of the bow-and-string-construction of the trunk. Because ventral and caudal facets are consistent with convexity of the caudal articular surfaces, which to a great degree was noted to result in formation of ball-and-socket joints, spinal stability is supposed to be decreased under such conditions.     

Role of somite patterning in the formation of Weberian apparatus and pleural rib in zebrafish

In the vertebrate body, a metameric structure is present along the anterior–posterior axis. Zebrafish tbx6^−/− larvae, in which somite boundaries do not form during embryogenesis, were shown to exhibit abnormal skeletal morphology such as rib, neural arch and hemal arch. In this study, we investigated the role of somite patterning in the formation of anterior vertebrae and ribs in more detail. Using three-dimensional computed tomography scans, we found that anterior vertebrae including the Weberian apparatus were severely affected in tbx6^−/− larvae. In addition, pleural ribs of tbx6 mutants exhibited severe defects in the initial ossification, extension of ossification, and formation of parapophyses. Two-colour staining revealed that bifurcation of ribs was caused by fusion or branching of ribs in tbx6^−/−. The parapophyses in tbx6^−/− juvenile fish showed irregular positioning to centra and abnormal attachment to ribs. Furthermore, we found that the ossification of the distal portion of ribs proceeded along myotome boundaries even in irregularly positioned myotome boundaries. These results provide evidence of the contribution of somite patterning to the formation of the Weberian apparatus and rib in zebrafish.     

Differentiation and classification of thoracolumbar transitional vertebrae

The literature states that transitional vertebrae at any junction are characterized by features retained from two adjacent regions in the vertebral column. Currently, there is no published literature available that describes the prevalence or morphology of thoracolumbar transitional vertebrae (TLTV). The aim of this study was to identify the qualitative characteristics of transitional vertebrae at the thoracolumbar junction and establish a technique to differentiate the various subtypes that may be found. A selection of vertebral columns from skeletal remains (n = 35) were evaluated in this study. Vertebrae were taken based on features that are atypical for vertebrae in each relative region. The transitional vertebrae were qualitatively identified based on overlapping thoracic and lumbar features of vertebrae at the thoracolumbar junction. The following general overlapping characteristics were observed: aplasia or hypoplasia of the transverse process, irregular orientation on the superior articular process and atypical mammillary bodies. The results show that the most frequent location of the transitional vertebrae was in the thoracic region (f = 23). The second most frequent location was in the lumbar region (f = 10). In two specimens of the selection (f = 2), an additional 13th thoracic vertebra was present which functioned as a transitional vertebra. This study concluded that one can accurately identify the characteristics of transitional vertebrae at the thoracolumbar junction. In addition, the various subtypes can be differentiated according to the region in the vertebral column the vertebra is located in and the relative number of vertebral segments in the adjacent regions of the vertebral column. This provides a qualitative tool for researchers to differentiate the transitional vertebrae from distinctly different typical thoracic or lumbar vertebrae at the thoracolumbar junction.     

Vertebral column regionalisation in Chinook salmon,Oncorhynchus tshawytscha

Teleost vertebral centra are often similar in size and shape, but vertebral‐associated elements, i.e. neural arches, haemal arches and ribs, show regional differences. Here we examine how the presence, absence and specific anatomical and histological characters of vertebral centra‐associated elements can be used to define vertebral column regions in juvenile Chinook salmon (Oncorhynchus tshawytscha). To investigate if the presence of regions within the vertebral column is independent of temperature, animals raised at 8 and 12 °C were studied at 1400 and 1530 degreedays, in the freshwater phase of the life cycle. Anatomy and composition of the skeletal tissues of the vertebral column were analysed using Alizarin red S whole‐mount staining and histological sections. Six regions, termed I–VI, are recognised in the vertebral column of specimens of both temperature groups. Postcranial vertebrae (region I) carry neural arches and parapophyses but lack ribs. Abdominal vertebrae (region II) carry neural arches and ribs that articulate with parapophyses. Elastic‐ and fibrohyaline cartilage and Sharpey's fibres connect the bone of the parapophyses to the bone of the ribs. In the transitional region (III) vertebrae carry neural arches and parapophyses change stepwise into haemal arches. Ribs decrease in size, anterior to posterior. Vestigial ribs remain attached to the haemal arches with Sharpey's fibres. Caudal vertebrae (region IV) carry neural and haemal arches and spines. Basidorsals and basiventrals are small and surrounded by cancellous bone. Preural vertebrae (region V) carry neural and haemal arches with modified neural and haemal spines to support the caudal fin. Ural vertebrae (region VI) carry hypurals and epurals that represent modified haemal and neural arches and spines, respectively. The postcranial and transitional vertebrae and their respective characters are usually recognised, but should be considered as regions within the vertebral column of teleosts because of their distinctive morphological characters. While the number of vertebrae within each region can vary, each of the six regions is recognised in specimens of both temperature groups. This refined identification of regionalisation in the vertebral column of Chinook salmon can help to address evolutionary developmental and functional questions, and to support applied research into this farmed species.     

The cat vertebral column: stance configuration and range of motion

This study examined the configuration of the vertebral column of the cat during independent stance and in various flexed positions. The range of motion in the sagittal plane is similar across most thoracic and lumbar joints, with the exception of a lesser range at the transition region from thoracic-type to lumbar-type vertebrae. The upper thoracic column exhibits most of its range in dorsiflexion and the lower thoracic and lumbar in ventroflexion. Lateral flexion is limited to less than 5° at all segments. The range in torsion is almost 180° and occurs primarily in the midthoracic region, T4-T11. Contrary to the depiction in most atlases, the standing cat exhibits several curvatures, including a mild dorsiflexion in the lower lumbar segments, a marked ventroflexion in the lower thoracic and upper lumbar segments, and a profound dorsiflexion in the upper thoracic (above T9) and cervical segments. The curvatures are not significantly changed by altering stance distance but are affected by head posture. During stance, the top of the scapula lies well above the spines of the thoracic vertebrae, and the glenohumeral joint is just below the bodies of vertebrae T3-T5. Using a simple static model of the vertebral column in the sagittal plane, it was estimated that the bending moment due to gravity is bimodal with a dorsiflexion moment in the lower thoracic and lumbar region and a ventroflexion moment in the upper thoracic and cervical region. Given the bending moments and the position of the scapula during stance, it is proposed that two groups of scapular muscles provide the major antigravity support for the head and anterior trunk. Levator scapulae and serratus ventralis form the lateral group, inserting on the lateral processes of cervical vertebrae and on the ribs. The major and minor rhomboids form the medial group, inserting on the spinous tips of vertebrae from C4 to T4. It is also proposed that the hypaxial muscles, psoas major, minor, and quadratus lumborum could support the lumbar trunk during stance. Received: 2 January 1997 / Accepted: 23 September 1997     

Microarchitecture of cetacean vertebral trabecular bone among swimming modes and diving behaviors

Cetaceans (dolphins, whales, and porpoises) are fully aquatic mammals that are supported by water's buoyancy and swim through axial body bending. Swimming is partially mediated by variations in vertebral morphology that creates trade-offs in body flexibility and rigidity between axial regions that either enhance or reduce displacement between adjacent vertebrae. Swimming behavior is linked to foraging ecology, where deep-diving cetaceans glide a greater proportion of the time compared to their shallow-diving counterparts. In this study, we categorized 10 species of cetaceans (Families Delphinidae and Kogiidae) into functional groups determined by swimming patterns (rigid vs. flexible torso) and diving behavior (shallow vs. deep). Here, we quantify vertebral trabecular microarchitecture (a) among functional groups (rigid-torso shallow diver (RS), rigid-torso deep diver (RD), and flexible-torso deep diver (FD)), and (b) among vertebral column regions (posterior thoracic, lumbar, caudal peduncle, and fluke insertion). We microCT scanned vertebral bodies, from which 1-5 volumes of interest were selected to quantify bone volume fraction (BV/TV), specific bone surface (BS/BV), trabecular thickness (TbTh), trabecular number (TbN), trabecular separation (TbSp), and degree of anisotropy (DA). We found that BV/TV was greatest in the rigid-torso shallow-diving functional group, smallest in flexible-torso deep-diving species, and intermediate in the rigid-torso deep-diving group. DA was significantly greater in rigid-torso caudal oscillators than in their flexible-torso counterparts. We found no variation among vertebral regions for any microarchitectural variables. Despite having osteoporotic skeletons, cetacean vertebrae had greater BV/TV, TbTh, and DA than previously documented in terrestrial mammalian bone. Cetacean species are an ideal model to investigate the long-term adaptations, over an animal's lifetime and over evolutionary time, of trabecular bone in non-weight–bearing conditions.     

Changes in the surface of the superior articular joint from the lower thoracic to the upper lumbar vertebrae

Thirty-two human vertebral columns were selected from the Kanazawa Collection at the University of Kanazawa, Japan. The superior articular joint surface was categorised into the thoracic type and the lumbar type, and the pattern of the change from one type to the other in the lower thoracic and upper lumbar region was examined. In 21 of 32 cases (66%), the change from the thoracic to the lumbar type occurred over 2 vertebral segments, either between the 12th thoracic and 1st lumbar vertebrae (44%) or between the 11th and 12th thoracic vertebrae (22%). In the remaining 11 cases (34%), the change occurred over 3 vertebral segments, with a transitional type of articular surface. The change from the thoracic to lumbar type of articular surface has been believed to occur over 2 vertebral segments, but occurs over 3 segments in as many as 34% of the articular surfaces.     

Hox genes specify vertebral types in the presomitic mesoderm

We show here that expression of Hoxa10 in the presomitic mesoderm is sufficient to confer a Hox group 10 patterning program to the somite, producing vertebrae without ribs, an effect not achieved when Hoxa10 is expressed in the somites. In addition, Hox group 11-dependent vertebral sacralization requires Hoxa11 expression in the presomitic mesoderm, while their caudal differentiation requires that Hoxa11 is expressed in the somites. Therefore, Hox gene patterning activity is different in the somites and presomitic mesoderm, the latter being very prominent for Hox gene-mediated patterning of the axial skeleton. This is further supported by our finding that inactivation of Gbx2, a homeobox-containing gene expressed in the presomitic mesoderm but not in the somites, produced Hox-like phenotypes in the axial skeleton without affecting Hox gene expression.     

Posterior distraction forces of the posterior longitudinal ligament stratified according to vertebral level

Introduction Although considered significant in resisting midline intervertebral disc herniation, the posterior longitudinal ligament (PLL) has had relatively few studies performed regarding its morphology and function. We performed the present experiment to discern the amount of posterior tensile force necessary to disrupt the PLL at each vertebral level. Materials and methods Twenty-five adult cadavers underwent laminectomies of vertebrae C1 to S1. After removal of the spinal cord, nerve roots, and dura mater, the PLL was identified for each vertebral level and a steel wire placed around its waist in the midline and a tensile gauge attached and posterior tension applied perpendicular to the spine. Forces necessary to failure of the PLL were noted for each vertebral level. Results The PLL was found to be stronger in the thoracic spine compared to the cervical and lumbar vertebrae (P < 0.05). Dividing the vertebral levels in this manner, we found an average posterior distraction force to failure of 48.3 N in the cervical region, 61.3 N in the thoracic region, and 48.8 N in the lumbar region. Conclusions These findings support clinical observations that thoracic disc herniation is rare. We hypothesize that this clinical observation is partially due to a stronger PLL in the thoracic spine.     

Novel spinal dysplasia in two generations

We report on a father and son with a previously undescribed skeletal abnormality and severe short stature. Antenatal sonographic evaluation of the propositus (son), obtained due to maternal pre-eclampsia, suggested an abnormal spine. At birth, no congenital anomalies were noted and transition to extra-uterine life was smooth. Radiographs performed five days after birth showed spina bifida, hemivertebrae in the mid-thoracic region, and widened lumbar in-terpedicular distances. MRI of the lower thoracic and lumbar vertebrae documented crescent-shaped appearance of the affected vertebrae and abnormally narrow A-P diameter of the vertebral bodies. Intervertebral discs were small, and the posterior elements, as well as the spinous processes of the affected vertebrae, were markedly hypoplas-tic. However, there was no narrowness of the spinal canal, and the limbs were unaffected. CT scan with three-dimensional reformatting of the thoracic and lumbar vertebrae documented unusual sagittal clefting of all of the vertebral bodies, which has previously been undescribed. The father had severe kyphoscoliosis and a height of 131.6 cm (?7.5 S.D.). Radiograph-ically, he was found to have multiple segmentation anomalies and diminished A-P diameter of his affected vertebral bodies. The multiple vertebral anomalies are the probable cause for the father's severe kyphoscoliosis. The pattern of inheritance suggests that an autosomal dominant gene is responsible for this condition and that father represennts a de novo mutationn. These radiographic abnormalities have not been described previously and represent a new form o vertebral spinal dysplasia. © 1995 Wiley-Liss, Inc.     

学龄期儿童胸椎经“椎弓根-肋骨”单元内固定的数字化测量

文题释义： 学龄期儿童：从入小学到青春期发育开始之前为学龄期,一般指6或7-12岁。在经历过第1次生长高速期(出生到3岁)后,此时身高及体质量为稳定生长期,除生殖系统外,其他系统器官发育均接近成人水平。大脑皮质理解、分析,认知能力增强,社会心理进一步发育,在小学的教育影响下,学龄期儿童的认识、观察、注意、记忆、想象、思维、语言等方面不断发展。从11-12岁女孩便开始进入青春期,男孩进入青春期的时间则相对晚一两年。 椎弓根-肋骨单元：椎弓根与其外侧自前向后的肋椎关节、肋骨头、肋横突关节和肋横突韧带共同构成“椎弓根-肋骨”单元。第1,11,12肋头只与其相对应椎体的肋凹及椎间盘相关节;第2-9肋骨的肋头位于上下两个椎体之间;第10肋头有的也和相邻的两椎体相关节;上7个肋骨的肋结节与相应的胸椎横突尖前面的肋凹相关节;第8-10肋结节较近于肋骨的下缘,与相应的胸椎横突尖的上缘相关节;第11,12肋与胸椎存在有肋椎关节,无肋横突关节。 背景：经椎弓根螺钉内固定已广泛应用于腰椎,且在胸椎中固定的应用已逐渐被接受。但由于考虑到上胸椎椎弓根狭窄,特别是在T₃-T₉之间,椎弓根置钉几乎都会穿破皮质伤及邻近重要结构的风险,为避免出现严重的并发症,有学者提出经肋横突关节和肋椎关节至椎体的椎弓根外入路,之后又有人设计了类似的进钉方法,提供足够不穿出肋骨的安全路径。目前,现有的研究多集中于成人中、上胸椎。 目的：测量学龄期儿童胸椎经椎弓根-肋骨单元螺钉内固定的相关解剖参数,探讨其在不同年龄段、不同性别之间的发育规律和形态特征,为临床提供理论依据。 方法：选择7-12岁学龄期儿童胸椎67例,无骨质破坏、肿瘤、畸形、退变、骨折等脊柱疾患及既往无脊柱相关手术病史,行螺旋CT扫描后三维重建,观测椎弓根-肋骨单元的形态结构,测量其横径、长度、内倾角及经椎弓根-肋骨单元置钉安全角度范围并进行统计分析,探讨其在解剖学上置钉的可行性。所有儿童的监护人对试验方案均知情同意,且得到医院伦理委员会批准。 结果与结论：①胸椎“椎弓根-肋骨”单元横径随年龄增加而增大,随椎序的增加呈先减少后增加的趋势,同年龄组内男性大于女性;②经“椎弓根-肋骨”单元钉道长度在不同年龄组中差异均有显著性意义(P < 0.05),随年龄的增长出现明显增高趋势,随椎序的增加呈先增后减趋势;③经“椎弓根-肋骨”单元置钉最小和最大内倾角得出安全角度范围为18°-25°,其中置钉安全范围最大位于T₁,其次为T₁₀,最小位于T₄和T₅;④由此可见,胸椎经椎弓根-肋骨单元置钉安全角度范围儿童较成人窄,在行椎弓根-肋骨单元置钉时若参照成人的标准可能会引起严重的神经血管损伤,需根据术前CT结果进行个体化置钉。 ORCID: 0000-0002-1977-3180(和雨洁) 中国组织工程研究杂志出版内容重点：人工关节;骨植入物;脊柱;骨折;内固定;数字化骨科;组织工程     

Progressive vertebral fusion of unknown etiology: a case report

We report a 16-year-old male with progressive vertebral fusion of the cervical, thoracic, and lumbar vertebrae; irregular vertebral body surfaces; coronal clefting of the vertebral bodies of the thoracic and lumbar spine; absence of one cervical vertebra; and a few other minor, nonspinous abnormalities. All laboratory findings have been within normal limits. To our knowledge, the particular findings in this patient have not been reported previously.     

Lower axial skeleton of the musk shrew (Suncus murinus)

Macroscopic structure as well as pre- and postnatal development of the lumbar, sacral, and caudal vertebrae of the musk shrew (Suncus murinus, Insectivora) were observed. The lumbar vertebrae possess two pairs of unusual processes, hyperapophyses and hypapophyses. The hyperapophyses are located on the dorsal surface of the caudal articular processes of all the lumbar vertebrae, whereas the hypapophyses are found on the caudal part of the ventral surface of the bodies in the first few lumbar vertebrae. The former gives attachment to the Mm. rotatores lumborum and the latter to the Mm. psoas major and minor. The articular processes of the lumbar vertebrae are oriented more horizontally compared with those in other mammals. The sacrum is very narrow transversely due to poor development of the ventrolateral wing. The auricular surface includes cranial parts of the wing and of the fused vertebral arches as well as the cranial articular process of the first sacral vertebra. In the caudal vertebrae, chevron bones are H-shaped when viewed ventrally, and give attachment to tendons of the caudal muscles. This report describes the relationships between the structural peculiarities of the lower axial skeleton and the locomotive habits of the musk shrew.