Jeffrey T. Laitman

The new face of gross anatomy.

The nature of anatomy education has changed substantially over the past decade due to both a new generation of students who learn differently from those of past years and the enormity of advances in anatomical imaging and viewing. At Mount Sinai School of Medicine, our anatomy courses have been designed to meld classic dissection with the tools physicians and surgeons will use tomorrow. We introduce students to the newest technologies available for viewing the body, such as minimally invasive approaches, ultrasonography, three‐dimensional visualizations, multi‐axial computerized image reconstructions, multi‐planar magnetic resonance imaging, and plastinated prosections. Students are given a hands‐on, team‐building experience operating laparoscopes in the laboratory. A great strength of our program is the important and active participation by faculty from 15 different basic and clinical departments, including several chairs and voluntary faculty. This interdisciplinary approach brings to our students direct, one‐on‐one encounters or presentations by our finest physicians and surgeons and our core anatomy faculty. In addition, the presence of many teaching assistants drawn from upper classmen and advanced graduate students adds an additional, vibrant dimension. Our anatomy programs for medical/graduate students and postgraduates are structured around three simple principles: (1) it is a privilege to teach, (2) we enlist only passionate teachers, and (3) it is our role to instill appreciation and respect for human form. Anat Rec (New Anat) 269:81–88, 2002.

The anatomy, physiology, acoustics and perception of speech: essential elements in analysis of the evolution of human speech

Abstract Inferences on the evolution of human speech based on anatomical data must take into account its physiology, acoustics and perception. Human speech is generated by the supralaryngeal vocal tract (SVT) acting as an acoustic filter on noise sources generated by turbulent airflow and quasi-periodic phonation generated by the activity of the larynx. The formant frequencies, which are major determinants of phonetic quality, are the frequencies at which relative energy maxima will pass through the SVT filter. Neither the articulatory gestures of the tongue nor their acoustic consequences can be fractionated into oral and pharyngeal cavity components. Moreover, the acoustic cues that specify individual consonants and vowels are “encoded”, i.e., melded together. Formant frequency encoding makes human speech a vehicle for rapid vocal communication. Non-human primates lack the anatomy that enables modern humans to produce sounds that enhance this process, as well as the neural mechanisms necessary for the voluntary control of speech articulation. The specific claims of Duchin (1990) are discussed.

Specializations of the human upper respiratory and upper digestive systems as seen through comparative and developmental anatomy

The human upper respiratory, or aerodigestive, tract serves as the crossroads of our breathing, swallowing and vocalizing pathways. Accordingly, developmental or evolutionary change in any of these functions will, of necessity, affect the others. Our studies have shown that the position in the neck of the mammalian larynx is a major factor in determining function in this region. Most mammals, such as our closest relatives the nonhuman primates, exhibit a larynx positioned high in the neck. This permits an intranarial larynx to be present and creates largely separate respiratory and digestive routes. While infant humans retain this basic mammalian pattern, developmental descent of the larynx considerably alters this configuration. Adult humans have, accordingly, lost separation of the respiratory and digestive routes, but have gained an increased supralaryngeal region of the pharynx which allows for the production of the varied sounds of human speech. How this region has changed during human evolution has been difficult to assess due to the absence of preserved soft-tissue structures. Our studies have shown that the relationship between basicranial shape and laryngeal position in living mammals can be a valuable guide to reconstruct the region in ancestral humans. Based on these findings we have examined the basicrania of fossil ancestors—from over two million years ago to near recent times—and have reconstructed the position of the larynx and pharyngeal region in these early forms. This has allowed us insight into how our ancestors may have breathed and swallowed, and when the anatomy necessary for human speech evolved.

Advances in understanding the relationship between the skull base and larynx with comments on the origins of speech

The position of upper respiratory structures, such as the larynx, has proven to be of great importance in determining an animal’s breathing, swallowing and vocalizing abilities. Studies on living mammals have also shown that the shape of the basicranium is related to the position of the larynx. This information has been of value in using the skull base as a means to reconstruct the upper respiratory tract of fossil hominids. Ongoing comparative and experimental studies of this region are adding new information on the mechanical relationship of the skull base to contiguous areas of the respiratory tract. For example, examination of the region in mammals disparate from humans, such as cetaceans, and experimental work on the region in rats, is adding new data on how the larynx and skull base may functionally interact.

Acoustic Models of Sound Production and Propagation

Acoustic models based on physics and mathematics may yield significant advances in the understanding of sound production, propagation, and interaction associated with whales and dolphins. Models can be used to estimate the limits of intensity and frequency that are physically possible given the anatomy of a species. Models can also tell us what kind of anatomical structures would be necessary in order to produce sound having specific characteristics. Models can be used to clarify what type of measurements should be performed to answer specific questions. Many areas of bioacoustics stand to benefit from simulation of sound propagation through biological tissues and the media surrounding them. However, accurate modeling of biological subjects with complex anatomical features is extremely challenging, and few modern studies exist of sound production and propagation in whales and dolphins.

Discovery of a low frequency sound source in Mysticeti (baleen whales): anatomical establishment of a vocal fold homolog.

The mechanism of mysticete (baleen whale) vocalization has remained a mystery. Vocal folds (true vocal “cords”), the structures responsible for sound production in terrestrial mammals, were thought to be absent in whales. This study tests the hypothesis that the mysticete larynx possesses structures homologous to vocal folds and that they are capable of sound generation. Laryngeal anatomy was examined in 37 specimens representing 6 mysticete species. Results indicate the presence of a U‐shaped fold (U‐fold) in the lumen of the larynx. The U‐fold is supported by arytenoid cartilages, controlled by skeletal muscles innervated by the recurrent laryngeal nerve, is adjacent to a diverticulum (laryngeal sac) covered with mucosa innervated by the superior laryngeal nerve, and contains a ligament—conditions that also define the vocal folds of terrestrial mammals and, therefore, supports homology. Unlike the vocal folds of terrestrial mammals, which are perpendicular to airflow, the mysticete U‐fold is oriented parallel to airflow. U‐fold adduction/abduction and elevation/depression may control airflow, and vibration of its edges may generate sounds. The walls of the laryngeal sac can expand and contract, may serve as a resonant space, and may also propagate vibrations generated by movements of the supporting arytenoid cartilages. The extensive musculature surrounding the laryngeal sac may enable rapid and forceful expulsion of air from the lumen of the sac into other respiratory spaces, or maintain a constant sac volume despite the effects of ambient pressure (e.g., changes during diving or ascent). The size and complexity of the mysticete larynx indicates an organ with multiple functions, including protection during breathing/swallowing, regulation of airflow and pressures in the respiratory spaces, and sound generation. The presence of a vocal fold homolog offers a new insight into both the mechanism of sound generation by mysticetes and the divergent evolution of odontocete and mysticete cetaceans. Anat Rec, 290:745–759, 2007.

Sisters of the Sinuses: Cetacean Air Sacs

This overview assesses some distinguishing features of the cetacean (whale, dolphin, porpoise) air sac system that may relate to the anatomy and function of the paranasal sinuses in terrestrial mammals. The cetacean respiratory tract has been modified through evolution to accommodate living in water. Lack of paranasal sinuses in modern cetaceans may be a diving adaptation. Bone‐enclosed air chambers are detrimental, as their rigid walls may fracture during descent/ascent due to contracting/re‐expanding air volumes. Flexible‐walled “sinuses” (extracranial diverticula) are a logical adaptation to diving. Odontocetes (toothed whales) exhibit several pairs of paranasal air sacs. Although fossil evidence indicates that paranasal sinuses occur in archaeocetes (ancestors/relatives of living cetaceans), it is not known whether the paranasal sacs derive from these sinuses. Sac pigmentation indicates that they derived from invaginations of the integument. Unlike sinuses, paranasal sacs are not circumferentially enclosed in bone, and therefore can accommodate air volume changes that accompany diving pressure changes. Paired pterygoid sacs, located ventrally along the cetacean skull, connect the pharynx and middle ear cavities. Mysticetes (baleen whales) have a large midline laryngeal sac. Although cetacean air sacs do not appear to be homologous to paranasal sinuses, they may serve some analogous respiratory, vocal, or structural functions. For example, these sacs may participate in gas exchange, thermoregulation, resonance, and skeletal pneumatization. In addition, they may subserve unique aquatic functions, such as increasing inspiratory volume, mitigating pressure‐induced volume change, air shunting to reduce respiratory dead space, and facilitating underwater sound production and transmission. Anat Rec, 291:1389–1396, 2008.

Mechanisms of Middle ear aeration: Anatomic and physiologic evidence in primates

Proper aeration is a prerequisite for normal middle ear function in all terrestrial mammals. Our previous studies in primates provided anatomic evidence of neural circuits between the middle ear, brain, and eustachian tube by which central respiratory neurons can control middle ear aeration. Yet mechanisms that regulate middle ear aeration remain poorly understood. This study extends our research by examining maturation of these neural circuits, and investigating their underlying physiology. Ultrastructural examination of tympanic nerves, the afferent limb of the neural circuit, in an age‐graded series of cynomolgus monkeys (Macaca fascicularis) showed substantial differences between newborn, young, and adult animals. These included a twofold increase in average myelin thickness, and greater than threefold increase in the ratio of myelinated to un‐myelinated fibers from newborn to adult animals. These marked developmental changes may translate into functional differences in regulation of middle ear aeration in young animals, and possibly explain the extraordinarily high incidence of middle ear disease in early childhood. In physiologic experiments, bilateral electromyographic responses were recorded from eustachian tube muscles, the efferent limb of the neural circuit, in adult monkeys after ipsilateral stimulation of the tympanic nerve. Response latencies were 9 to 28 msec, similar to those of other multi‐synaptic bilateral brainstem reflexes. These physiologic data strongly suggest a concept of active control of middle ear aeration by respiratory neurons in the brain.

The Human Aerodigestive Tract and Gastroesophageal Reflux: An Evolutionary Perspective

In order to appreciate fully the nature of supraesophageal complications of gastroesophageal reflux in humans, it is essential to view the problem within an evolutionary framework. Examination of the aerodigestive tract anatomy of our mammalian relatives shows that this region in humans is highly derived as compared to other mammals. Among the specializations that adult humans exhibit is a caudal position of the larynx, which results in a permanently expanded oropharynx. These anatomical features underlie our distinctive breathing and swallowing patterns and provide the substrate that allows for the production of articulate speech. While the selection factors that have shaped human evolution obviously favored our derived aerodigestive tract, aspects of this anatomy appear particularly unsuited to accommodate gastroesophageal reflux. Indeed, our unique aerodigestive tract morphology may predispose us to an array of supraesophageal complications of gastroesophageal reflux.

Climatic Effects on the Nasal Complex : A CT Imaging, Comparative Anatomical, and Morphometric Investigation of Macaca mulatta and Macaca fascicularis

Previous studies exploring the effects of climate on the nasal region have largely focused on external craniofacial linear parameters, using dry crania of modern human populations. This investigation augments traditional craniofacial morphometrics with internal linear and volumetric measures of the anatomic units comprising the nasal complex (i.e., internal nasal cavity depth, maxillary sinus volumes). The study focuses on macaques (i.e., Macaca mulatta and Macaca fascicularis) living at high and low altitudes, rather than on humans, since the short residency of migratory human populations may preclude using them as reliable models to test the long‐term relationship of climate to nasal morphology. It is hypothesized that there will be significant differences in nasal complex morphology among macaques inhabiting different climates. This study integrated three different approaches: CT imaging, comparative anatomy, and morphometrics—in an effort to better understand the morphological structure and adaptive nature of the nasal complex. Results showed statistically significant differences when subsets of splanchnocranial and neurocranial variables were regressed against total maxillary sinus volume for particular taxa. For example, basion–hormion was significant for M. fascicularis, whereas choanal dimensions were significant only for M. mulatta. Both taxa revealed strong correlation between sinus volume and prosthion to staphylion distance, which essentially represents the length of the nasal cavity floor—and is by extension an indicator of the air conditioning capacity of the nasal region. These results clearly show that climatic effects play a major role in shaping the anatomy of the nasal complex in closely related species. The major influence upon these differing structures appears to be related to respiratory‐related adaptations subserving differing climatic factors. In addition, the interdependence of the paranasal sinuses with other parts of the complex strongly indicates a functional role for them in nasal complex/upper respiratory functions. Anat Rec, 291:1420–1445, 2008.