本篇文章共分 5 段，從蛇的頰窩器官位置、功能、運作方式，到蛇利用頰窩器官所能得到的立體感知能力一一加以說明陳述。

本篇考題英文原文與對應之中文翻譯整理如下。練習作答解題時若有對語意不清楚之處，請仔細查閱對照，以提升閱讀理解能力。

The Pit Organs of Snakes 蛇的頰窩器官

1. 頰窩器官的功能

   It is thought that the facial pits or pit organs on the head of some snakes are specialized infrared (heat) receptors. When a rattlesnake strikes, the direction of the strike seems to be guided by the infrared radiation from its prey. A rattlesnake strikes only at warm-blooded prey, and when the prey is dead and at room temperature, the snake will not strike. However, a blindfolded snake strikes correctly at a dead rat that is pulled across the cage, provided the rat is warmer than the surroundings. Blindfolded, the snake cannot be guided by vision; nor is it guided by the sense of smell, for it will strike correctly even at a moving, cloth-wrapped electric bulb. The pit organs are evidently involved in sensing the location of warm objects. All snakes that have pit organs feed preferentially on warm-blooded prey, and this further supports the view that these organs are infrared sensors. In the rattlesnake the pit organs are located, one on each side, between the nostril and the eye: they are connected to many nerves, and this in itself suggests a sensory role for the organ.

   人們認為，一些蛇的面部頰窩或頭部的頰窩器官是專門的紅外（熱）感受器。當響尾蛇攻擊時，其攻擊的方向似乎是由獵物的紅外輻射引導的。響尾蛇只攻擊溫血的獵物，當獵物死亡且處於室溫時，蛇不會攻擊。然而，蒙住眼睛的蛇會正確地攻擊被拉過籠子的死老鼠，只要老鼠的溫度比周圍的環境高。蒙著眼睛的蛇不能用視覺來引導，也不能用嗅覺來引導，因為它甚至會正確地攻擊一個移動的、用布包裹的電燈泡。頰窩器官顯然參與了對溫暖物體位置的感知。所有具備頰窩器官的蛇都優先以溫血的獵物為食，這進一步支持了這些器官是紅外傳感器的觀點。在響尾蛇中，頰窩器官位於鼻孔和眼睛之間，每側各一個：它們與許多神經相連，這本身就表明該器官的感覺作用。

2. 神經系統反應

   The sensitivity of the facial pit has been examined by recording the activity in the nerve leading from the organ. A variety of stimuli, such as sound, vibration, or light of moderate intensity (with the infrared part of the spectrum filtered out), has no detectable effect on the activity in the nerve. However, if objects of a temperature different from the surroundings are brought into the receptive field around the head, there is a striking change in nerve activity, regardless of the temperature of the intervening air.

   通過記錄來自該器官的神經活動，對面部頰窩的敏感性進行了研究。各種刺激，如聲音、振動或中等強度的光（過濾掉光譜中的紅外線部分），對這些神經的活動沒有可察覺的影響。然而，如果將溫度與周圍環境不同的物體帶入頭部周圍的感受區，則神經活動會發生驚人的變化，而不管中間空氣的溫度如何。

3. 頰窩器官運作方式

   How is the infrared radiation sensed? The pit is covered by a thin transparent membrane, and it has been suggested that a rise in temperature in the pit behind the membrane could cause an expansion of the gas with a consequent deformation of the membrane. This in turn could be sensed by a suitable receptor. This hypothesis is highly improbable, for a cut in the membrane that opens the pit to the outside air causes no loss in responsiveness, a result that is incompatible with the hypothesis that a pressure change is sensed.

   紅外線輻射是如何被感知的？頰窩被一層薄薄的透明膜所覆蓋，有人認為，膜後面的頰窩溫度升高可能導致氣體膨脹，從而使膜發生變形。這反過來又可以被一個合適的受體感應到。此一假設的可能性非常低，因為在膜上一道會使頰窩與外界空氣相通的切口，並不會導致反應性的喪失，此一結果與感覺到壓力變化的假設是不相容的。

4. 紅外輻射感應

   We are left with two other possibilities to consider: Either the effect is photochemical, which means that the infrared radiation is absorbed by a specific compound, analogous to the light-sensitive pigments in the eye, or the pit organs are sensitive to the slight temperature rise caused when infrared radiation reaches it. The infrared radiation emitted from a mammalian body has low quantum energy, which makes any photochemical effect on a pigment extremely unlikely. Pure infrared radiation can be produced by a laser, and experiments with such radiation of known wavelength provide strong evidence that the mode of reception in the facial pit organ is entirely thermal.

   我們只剩下另外兩種可能性需要考慮。要麼是光化學效應，這意味著紅外輻射被一種特殊的化合物吸收，類似於眼睛中的光敏色素，要麼是頰窩器官對紅外輻射到達它時引起的輕微溫度上升敏感。哺乳動物身體發出的紅外輻射具有較低的量子能量，這使得對色素的任何光化學作用都極不可能。純粹的紅外輻射可以由雷射產生，用這種已知波長的輻射進行的實驗提供了強有力的證據，證明面部頰窩器官的接收方式完全是透過熱的。

5. 頰窩器官的立體感應

   Can the pit organs be used for perception of the infrared source in the way our two eyes are used for stereoscopic vision? This seems likely, not only from observations of the precision with which a snake can strike, but also from studies of its brain activity. When infrared radiation falls on the facial pit organ, electric activity can be recorded from the optic tectum, the part of the brain with which the optic nerve is connected. This in itself is suggestive; although the nerves from the pit organ are completely separate from the optic nerves, the same part of the brain seems to handle visual information, which is known to be stereoscopic, and infrared information. The optic tectum has left and right parts, located on either side of the brain. Each part receives input from each eye via the optic chiasma, the point at which the optic nerves from the left and right eye meet. Many of the neurons in the tectum respond to stimulation of the pit organ on the opposite side of the head. This is reminiscent of the way information from the eyes is handled; the crossover of the optic nerve in the optic chiasma is essential for stereoscopic vision and interpretation of distance. Information from the two pit organs is apparently coordinated and interpreted in a similar way, a conclusion in agreement with recorded changes in the neural activity in the tectum when the infrared source is in a position to irradiate both pits at once. It therefore appears that the facial pits indeed provide stereoscopic perception and substantially aid in the precision of estimating the location of prey.

   頰窩器官能否像我們的兩隻眼睛用於立體視覺那樣，用於感知紅外源？這似乎是有可能的，這不僅來自於對蛇的精確打擊的觀察，而且也來自於對蛇的大腦活動的研究。當紅外輻射落在面部頰窩器官上時，可以從視頂蓋上記錄到電活動，視頂蓋是大腦中與視神經相連的部分。這本身就說明了問題；儘管來自頰窩器官的神經與視神經完全分開，但大腦的同一部位似乎在處理視覺信息（已知是立體的）和紅外信息。視頂蓋有左右兩部分，分別位於大腦的兩側。每一部分都通過視交叉點接收來自每隻眼睛的輸入，視交叉點位於來自左眼和右眼的視神經交匯處。視頂蓋中的許多神經元對頭部另一側的頰窩器官的刺激作出反應。這讓人聯想到處理來自眼睛的信息的方式；視神經在視交叉點中的交叉對立體視覺和解釋距離至關重要。來自兩個頰窩器官的信息顯然是以類似的方式進行協調和解釋的，這一結論與記錄到的當紅外源同時照射兩個頰窩器官時視頂蓋上的神經活動變化兩相一致。因此，看來面部頰窩確實提供了立體感，並對估計獵物位置的準確性有很大幫助。