Harper C J, McLellan W A, Rommel S A, Gay D M, Dillaman R M, Pabst D A
Department of Biology and Marine Biology, University of North Carolina, Wilmington, North Carolina 28403, USA.
J Morphol. 2008 Jul;269(7):820-39. doi: 10.1002/jmor.10628.
The melon is a lipid-rich structure located in the forehead of odontocetes that functions to propagate echolocation sounds into the surrounding aquatic environment. To date, the melon's ability to guide and impedance match biosonar sounds to seawater has been attributed to its unique fatty acid composition. However, the melon is also acted upon by complex facial muscles derived from the m. maxillonasolabialis. The goal of this study was to investigate the gross morphology of the melon in bottlenose dolphins (Tursiops truncatus) and to describe how it is tendinously connected to these facial muscles. Standard gross dissection (N = 8 specimens) and serial sectioning (N = 3 specimens) techniques were used to describe the melon and to identify its connections to the surrounding muscles and blubber in three orthogonal body planes. The dolphin forehead was also thin-sectioned in three body planes (N = 3 specimens), and polarized light was used to reveal the birefringent collagen fibers within and surrounding the melon. This study identified distinct regions of the melon that vary in shape and display locally specific muscle-tendon morphologies. These regions include the bilaterally symmetric main body and cone and the asymmetric right and left caudal melon. This study is the first to identify that each caudal melon terminates in a lipid cup that envelopes the echolocation sound generators. Facial muscles of the melon have highly organized tendon populations that traverse the melon and insert into either the surrounding blubber, the connective tissue matrix of the nasal plug, or the connective tissue sheath surrounding the sound generators. The facial muscles and tendons also lie within multiple orthogonal body planes, which suggest that the melon is capable of complex shape change. The results of this study suggest that these muscles could function to change the frequency, beam width, and directionality of the emitted sound beam in bottlenose dolphins. The echolocation sound propagation pathway within the dolphin forehead appears to be a tunable system.
鲸蜡器官是一种富含脂质的结构，位于齿鲸的前额，其功能是将回声定位声音传播到周围的水生环境中。迄今为止，鲸蜡器官引导生物声纳声音并使其与海水实现阻抗匹配的能力一直归因于其独特的脂肪酸组成。然而，鲸蜡器官还受到源自上颌鼻唇肌的复杂面部肌肉的作用。本研究的目的是调查宽吻海豚（Tursiops truncatus）鲸蜡器官的大体形态，并描述其如何通过肌腱与这些面部肌肉相连。使用标准大体解剖（N = 8个标本）和连续切片（N = 3个标本）技术来描述鲸蜡器官，并在三个相互垂直的身体平面中确定其与周围肌肉和鲸脂的连接。还在三个身体平面中对海豚前额进行了薄切片（N = 3个标本），并使用偏振光来揭示鲸蜡器官内部和周围的双折射胶原纤维。本研究确定了鲸蜡器官中形状各异且呈现局部特定肌腱形态的不同区域。这些区域包括左右对称的主体和锥体以及不对称的左右尾侧鲸蜡器官。本研究首次确定每个尾侧鲸蜡器官都终止于一个脂质杯，该脂质杯包裹着回声定位声音发生器。鲸蜡器官的面部肌肉具有高度有组织的肌腱群，这些肌腱群穿过鲸蜡器官并插入到周围的鲸脂、鼻栓的结缔组织基质或声音发生器周围的结缔组织鞘中。面部肌肉和肌腱也位于多个相互垂直的身体平面内，这表明鲸蜡器官能够进行复杂的形状变化。本研究结果表明，这些肌肉可能起到改变宽吻海豚发出声束的频率、波束宽度和方向性的作用。海豚前额内的回声定位声音传播路径似乎是一个可调节的系统。
