最高等的脊椎动物

1. 最高等的脊椎动物 —哺乳纲（Mammalia）

2. THE MAMMALS • Mammals, with their highly developed nervous system and numerous ingenious（精致的） adaptations, occupy （占据）almost every environment on earth that will support life. Despite their relatively small numbers (4,500 species as compared to 8,600 species of birds, well over 20,000 fishes and 900,000 insects), they are overall the most biologically successful group in the animal kingdom, with the possible exception of the insects. Many potentialities that dwelled（居住） more or less latently in other vertebrates are highly developed in mammals. Mammals are enormously diverse in size, shape, form, and function. They range in size from the diminutive pigmy shrew, with a body length of less than l.5 inches and weight of only a fraction of an ounce, to whales, exceeding l00 tons in weight. Commenting on the unequalled adaptive radiation of this group, one biologist has compared the evolution of mammals to a biologic application of Parkinson's law: they expand (like work) to fill all the available space (time). As a climax（顶点） to mammalian evolution there is man, a creature that we continue to hope will be an evolutionary success !

3. 哺乳动物全身被毛，体温恒定，胎生，具有胎盘(单孔类除外)，哺乳(具乳腺)，可以同其它动物相区别。四肢经扭转位于身体腹面；头骨合颞窝型，双枕髁，具有完整的次生硬腭和肌肉质软腭；下颌为单一齿骨，齿骨上着生槽生异型齿，为再生齿；颈椎恒为7块(极少数例外)；具汗腺；血液完全双循环，红血球无核，保留左体动脉弓；肺泡是气体交换的最终场所；具有肌肉质横膈；发展了外耳壳，听小骨3块；大脑发达且机能皮层化，发展了新小脑，有迅速学习的能力和灵活可塑的行为。哺乳动物全身被毛，体温恒定，胎生，具有胎盘(单孔类除外)，哺乳(具乳腺)，可以同其它动物相区别。四肢经扭转位于身体腹面；头骨合颞窝型，双枕髁，具有完整的次生硬腭和肌肉质软腭；下颌为单一齿骨，齿骨上着生槽生异型齿，为再生齿；颈椎恒为7块(极少数例外)；具汗腺；血液完全双循环，红血球无核，保留左体动脉弓；肺泡是气体交换的最终场所；具有肌肉质横膈；发展了外耳壳，听小骨3块；大脑发达且机能皮层化，发展了新小脑，有迅速学习的能力和灵活可塑的行为。 • 数百万年前新生代以来一直在地球上占有优势的哺乳类分布极为广泛，向北到北极冰盖的边缘，经过温带、沙漠地区和热带地区，直到南极洲边缘，同时也占据着海洋和主要河流。这种极强的适应性源于它们的高度发达的神经系统，复杂而完善的身体结构和生理功能，恒温和高代谢率，以及对后代的哺育和保护。哺乳类尽管相对来说它们的种数较少（4600种与鸟类的8600种，鱼类约20，000种，以及昆虫的800，000种相比），在动物界里可能除昆虫外，为生物学上最昌盛的一类。许多性能在其他脊椎动物或多或少尚是潜在的，但是在哺乳类是高度发达的。哺乳类的大小、形状、类型以及功能有极大的不同。它们的大小从体长少于4厘米，体重从仅几克的细小的 到体重超过100吨的鲸。

4. 哺乳类与人类生活有着极为密切的关系，不仅在人类经济生活中起着重要作用，而且受到人类活动的巨大影响。哺乳类受到能从事繁重劳动的人类的影响较其他动物都多、人类驯养了许多哺乳类作为食物、衣着、驭兽及爱畜。每年在生物医学上的研究要用几百万的哺乳类，人们引进外来的哺乳类至新的生境，偶然也获得好的成果，但更常有的是意料不到的灾祸。虽然过去的历史曾提供我们许多的教训，但我们仍然继续过分地猎取有价值的野生哺乳类。有价值的鲸工业的全面崩溃是由于灭绝它的自然资源，现代世界上自我毁灭的一个典型例子。以为工业的每一环节仅关注今天的收获越多越好，好象明天的供应没有任何关系。在有些情况下，毁灭一个有价值的哺乳类自然资源是故意的，例如在印第安战争时期官方批准的（悲剧性的成功）灭除美洲野牛的政策，以饿死平原上的印第安人。虽然商业上的狩猎已减少，但由于不断增加的人口，不断地毁坏野生栖息地，影响及损害了哺乳类的区系。我们越来越意识到我们在这个行星上是有机界演化的最有权威者，使我们对我们的自然环境负有全部的责任。意识到我们的福利已经是并继续与其他哺乳类有着密切的关系，显然为了我们的利益，我们必须保存所有的哺乳类包括我们自己在内的自然环境。必须记住，自然界可以没有人类而存在，但是人类不能没有自然界而存在。哺乳类与人类生活有着极为密切的关系，不仅在人类经济生活中起着重要作用，而且受到人类活动的巨大影响。哺乳类受到能从事繁重劳动的人类的影响较其他动物都多、人类驯养了许多哺乳类作为食物、衣着、驭兽及爱畜。每年在生物医学上的研究要用几百万的哺乳类，人们引进外来的哺乳类至新的生境，偶然也获得好的成果，但更常有的是意料不到的灾祸。虽然过去的历史曾提供我们许多的教训，但我们仍然继续过分地猎取有价值的野生哺乳类。有价值的鲸工业的全面崩溃是由于灭绝它的自然资源，现代世界上自我毁灭的一个典型例子。以为工业的每一环节仅关注今天的收获越多越好，好象明天的供应没有任何关系。在有些情况下，毁灭一个有价值的哺乳类自然资源是故意的，例如在印第安战争时期官方批准的（悲剧性的成功）灭除美洲野牛的政策，以饿死平原上的印第安人。虽然商业上的狩猎已减少，但由于不断增加的人口，不断地毁坏野生栖息地，影响及损害了哺乳类的区系。我们越来越意识到我们在这个行星上是有机界演化的最有权威者，使我们对我们的自然环境负有全部的责任。意识到我们的福利已经是并继续与其他哺乳类有着密切的关系，显然为了我们的利益，我们必须保存所有的哺乳类包括我们自己在内的自然环境。必须记住，自然界可以没有人类而存在，但是人类不能没有自然界而存在。

5. Characteristics • l. Body covered with hair, but reduced in some • 2. Integument with sweat, sebaceous（脂肪）, and mammary glands • 3. Skeletal features of skull with two occipital condyles（枕髁）, seven cervical vertebrae （颈椎）(usually), and often an elongated tail • 4. Mouth with teeth on both jaws • 5. Movable eyelids and fleshy external ears • 6. Four limbs (reduced or absent in some) adapted for many forms of locomotion • 7. Circulatory system of a four-chambered heart, persistent left aorta, and nonnucleated（无核） red blood corpuscles

6. 8. Respiratory system of lungs and a voice box（喉） • 9. Muscular partition（肌隔） between thorax and abdomen • 10. Excretory system of metanephros kidneys and ureters （输尿管）that usually open into a bladder （膀胱） • 11. Nervous systems of a well-developed brain and l2 pairs of cranial nerves • l2. Warm blooded • l3. Cloaca（泄殖腔） present only in monotremes（单孔类） • l4. separate sexesl reproductive organs of a penis（阴茎）, testes (usually in a scrotum), ovaries, oviducts（输卵管）, and vagilla（子宫） • l5. Internal fertilization, eggs develop in a uterus with placental（胎盘） attachment (except monotremes), fetal （胎儿的）membranes (amnion（羊膜）, chorion（绒毛膜）, allantois（尿囊）) • l 6. Young nourished by milk from mammary glands

7. 哺乳钢的特征概述如下： 1．体表复有毛发，但有些种类退化。 • 2．皮肤有汗腺、皮脂服以及乳腺。 • 3．头骨具二个枕骨髁，颈椎通常七枚，常有一伸长的尾。 • 4．口的上下颌具再出齿。 • 5．可活动的眼睑和肉质的外耳。 • 6．四肢（有些动物退化或缺失）适应于许多形式的运动。 • 7．循环系统是心脏分为四腔，仅有左动脉弓以及无核、双凹的红血球。 • 8．具肺的呼吸系统及有发声器。 • 9．在胸腔与腹腔之间有肌肉分隔（横隔膜）。 • 10．具后肾的排泄系统，输尿管通常与膀胱相通。 • 11．神经系统有高度发达的脑以及十二对脑神经。 • 12．内温或恒温。 • 13．仅单孔类有泄殖腔。 • 14．雌雄异体；生殖器官有阴茎、筹丸（通常在阴囊内）及卵巢、输卵管、阴道。 • 15．体内受精；卵以胎盘附着在子宫内发育（除单孔类外）；胎膜（羊膜，绒毛膜，尿囊）。 • 16以乳腺分泌的乳汁哺育幼儿。

8. 哺乳动物是全身披毛、运动快速、恒温胎生的脊椎动物，是脊椎动物中躯体结构、功能行为最为复杂的最高级动物类群。哺乳动物是全身披毛、运动快速、恒温胎生的脊椎动物，是脊椎动物中躯体结构、功能行为最为复杂的最高级动物类群。 哺乳动物是由具有某些两栖动物特征的原始爬行动物发展而来的，哺乳类在躯体结构上还保留着与两栖类相似的特征： 两栖类、哺乳类 爬行类、鸟类 头骨 双枕髁 单枕髁 皮肤腺 丰富 缺乏 排泄物 尿素 尿酸 由合颞窝类的一支古代爬行类即兽齿类于中生代三叠纪末期进化。因神经系统和感觉器官高度发达，有极高的体温调节能力，而具有极好的适应性，几乎生活于地球的每一个角落。是脊椎动物中最高等的一类。 由哺乳类中的灵长类进化出直立行走、大脑高度发达的人类。

10. 生物学特征： 全身被毛，体温恒定，胎生，具胎盘（单孔类除外），哺乳（具乳腺）；四肢经扭转位于身体腹面；头骨合颞窝型，双枕髁，具完整次生硬腭和肌肉质软腭；齿骨生有槽生异型齿，为再生齿；颈椎恒为7块；具汗腺；完全双循环，红血球无核，保留左体动脉弓；肺泡是气体交换的最终场所；具肌肉质横膈；有外耳壳，听小骨3块；大脑发达且机能皮层化。

12. 1 胎生和哺乳： 胎生和哺乳对后代的发育和生长具有完善、有利的保护。从受精卵、胚胎、胎儿产出、至幼仔自立的整个过程均有母兽的良好的保护，使后代的成活率大为提高，而使哺乳类在生存竞争中占有较高的起点，在地球上的生存和发展中具有较大的优势。

13. （1）胎生 受精卵发育为羊膜卵。除产卵的单孔类外，受精卵在进入母体子宫后植入子宫壁中，其绒毛膜和尿囊与母体子宫内膜结合形成胎盘，胎儿发育过程中所需营养和氧气以及排泄的废物是通过胎盘来传递的。受精卵发育至胎儿成熟（妊娠期）在母体子宫内进行。母兽在此期间可自由摄食，胎儿也得到保护。 胎生的有袋类的胎盘不是真正的胎盘，而是卵黄囊胎盘，幼仔发育不完全即产出，需在母体腹部育儿袋中含着母兽的乳头继续发育直至成熟。

15. （2）哺乳 母体具有乳腺和乳头。乳腺是高度特化的汗腺，母兽以乳汁哺育幼仔。母兽在对幼仔哺乳的同时对其保护，包括营造巢穴。 卵生原兽亚纲动物具乳腺而无乳头，母兽孵化出的幼兽舔食母兽腹部乳腺区分泌的乳汁。

16. 1.胎生、哺乳及其意义 大多数哺乳动物的生殖方式为胎生，胚胎在母体内发育时通过胎盘吸取母体血液中的营养物质和氧气，同时把排泄物送入母体内。 1.1 胎盘 胎盘是胚胎的绒毛膜和尿囊膜与母体子宫内膜结合形成的特殊结构。 哺乳动物胚胎的绒毛膜和尿囊膜相愈合，并形成许多指状突起（绒毛）嵌入母体的子宫内膜，胚胎与母体之间坂这层膜所隔开，胎儿、母体的两套血液循环下不相通，营养物质、呼吸气体和代谢废物靠弥散作用进行交换

18. 1.1 胎盘 有二种类型 1）蜕膜胎盘： 胎盘的绒毛膜和尿囊膜与子宫内膜联系紧密，愈合在一起，分娩时绒毛膜连带子宫内膜一起脱出体外，生产时大量出血，如人。 2）无蜕膜胎盘 胎盘的绒毛膜和尿囊膜与子宫内膜联系不紧密，胎儿出生时胎 盘与子宫内膜容易分离，分娩时子宫内膜不受伤害，无大量出血现象。如牛、羊。 胎生为胚胎发育提供了营养、保护及恒温发育条件，使外界环境对胎儿不良影响减少到最小程度。

21. 1 体形多样化 兽形：四肢发达，有尾，适合奔跑，陆生。 鲸形：躯体流线型。鱼形，四肢退化，呈浆状，水生。 蝙蝠型：前肢特化成翼，具皮膜，能飞翔。 适应辐射：来源于同一祖先的动物在扩大生存范围和占领分布区的过程中，因遭迂到不同的环境条件逐渐形成了不同的适应器官的现象称为适应辐射

22. Integument and derivatives（衍生物） • The mammalian skin and its modifications especially distinguish mammals as a group. As the interface between the animal and its environment, the skin is strongly molded by the animal's way of life. In general the skin is thicker in mammals than in other classes of vertebrates, although it is made up of the two typical divisions--epidermis and dermis (corium) (see Fig. 29-l, B). Among the mammals the dermis becomes much thicker than the epidermis. The epidermis varies in thickness. lt is relatively thin where it is well protected by hair, but in places subject to much contact and use, such as the palms or soles, its outer layers become thick and cornified with keratin（角蛋白）.

23. HAlR. Hair is especially characteristic in mammals, although man himself is not a very hairy creature and in the whales it is reduced to only a few sensory bristles on the snout. The hair follicle from which a hair grows is an epidermal structure even though it lies mostly in the dermis and subdermal（皮下） (subcutaneous) tissues (Fig. 29- l, B). The hair grows continuously by rapid proliferation of cells in the base of the follicle（小囊）. As the hair shaft（毛干） is pushed upward, new cells are carried away from their source of nourishment and turn into the dense type of keratin (hard keratin) that constitutes nails（指甲）, claws（爪）, hooves（角）, and feathers as well as hair. On a weight basis, hair is by far the strongest material in the body. It has a tensile strength comparable to rolled aluminum, which is nearly twice as strong, weight for weight, as the strongest bone. Hair is more than just a cylinder of keratin. It consists of three layers: the medulla or pith（髓质） in the center of the hair, the cortex（皮层） with pigment granules next to the medulla, and the outer cuticle（表皮） composed of imbricated scales .The hair of different mammals shows a considerable range of structure. It may be deficient in cortex, such as the brittle hair of the deer, or it may be deficient in medulla, such as the hollow, air filled hairs of the wolverine, so favored by northerners for trimming the hoods of parkas because it resists frost accumulation. The hairs of rabbits and some others are scaled to interlock when pressed together. Curly hair, such as that of sheep, grows from curved follicles.

24. Each hair follicle（毛囊） is provided with a small strip of muscle that, when contracted, pulls the hair upright. These erector muscles are under the control of the sympathetic nervous system. During certain emotional states (fear and excitement), many animals erect their hair, particularly that on the neck and between the shoulders, thus increasing the apparent size of the body. In man, contraction of the erector muscles after excitement or cold stimulation causes the hair to stand up and the skin to dimple in above the muscle attachment. The result is "goosenesh." • Mammals characteristically have two kinds of hair forming the pelage: (l) dense and soft under hair for insulation and (2) coarse and longer guard hair（针毛） for protection against wear and to provide coloration. The under hair traps a layer of insulating air: in aquatic animals such as the fur seal, otter, and beaver, it is so dense that it is almost impossible to wet it. In water the guard hairs wet and mat down over the under hair, forming a protective blanket . A quick shake when the animal emerges flings off the water and leaves the outer guard hair almost dry.

26. 特 点 1）表皮角质层发达，真皮加厚，含有丰富的血管、神经和感觉末稍，能感受温度、压力和疼觉。 2）皮下脂肪层发达，有隔热保温作用。 3）披毛： 表披毛是哺乳动物区别于其它脊椎动物的最显著特征之一。 毛发的表皮角质化的产物，与爬行类的角质鳞片及鸟类的羽毛是同源物。 具有保温、保护和触觉作用

27. （4）皮肤腺特别发达 皮肤腺来源于表皮生长层 ，有皮脂、汗腺、味腺和乳腺4 种类型： 皮脂腺：分泌皮脂以润滑皮肤和毛发，防止干燥和浸润。 汗腺：分泌汗液，通过出汗可调节体温，排灌部分尿素。 味腺：为汗腺和皮脂腺的变形，能分泌特殊物质以吸引异 性、识别同种个体或用以自卫。 乳腺：为变态的汗腺，由若干乳腺集中心一定区域称为乳区 哺乳类乳腺根椐乳头的有无及开口情况可分为三种类型： 无乳头型：乳腺似汗腺，无乳头，如鸭嘴兽 真乳头型：如乳腺管都于乳头表面，如灵长类 假乳头型：乳头内有一空隙，称乳管，乳腺管开口于乳腺管基部，如牛。

28. 皮肤及其它皮肤衍生物 皮肤：表皮和真皮组成，加厚，表皮角质层发达。 • 表皮：分角质层和生长层，表皮角质层发达，不断脱落 • 真皮：加厚，由致密结缔组织构成，内有交错排列的胶原纤 • 维和弹性纤维，真皮具极强的韧性。 • 皮下组织：舒松结缔组织，缓冲，多脂肪 皮肤衍生物是脊椎动物中最复杂、多样的，功能多样化（保护、调节体温、感受刺激、分泌、排泄） • 毛 • 趾端表皮衍生物 • 头部表皮和真皮特化产物 • 皮肤腺

29. （1）毛 哺乳类特有，表皮角质化形成，与角质鳞片及羽毛同源，组成： 毛根：真皮毛囊内，末端膨大成毛球，内有真皮的毛乳头伸入，含丰富毛细血管以供给毛生长所需养料；毛囊内有皮脂腺开口，分泌油脂润泽毛发和皮肤；毛囊基部有竖毛肌附着，收缩时使毛直立，可调节体温； 毛干：为髓质、皮质和鳞片层 髓质：多孔，保温； 皮质：坚韧，细胞含色素； 鳞片层：角质细胞，保护； 作用： 保温和调温。

30. 哺乳类有二种类型的毛： • （1）稠密且柔软的绒毛，用于绝热的； • （2）粗且长的针毛，用于防磨损并提供色彩。 • 绒毛保持一层绝缘空气，在水生动物中如海豹、水獭和海狸的绒毛，是如此稠密，几乎不可能弄湿，在水中，粗毛铺在绒毛的上面形成保护毯。当动物跳出水面时，迅速抖动，把水抛掉，外层的粗毛几近干燥。 • 大部分哺乳类一年有二次换毛，一次在春季，一次在秋季。通常夏毛衣较冬毛衣薄得多，并常常是不同的颜色。几种北方的肉食性融（如黄鼠狼）的冬毛衣是白色的，而夏毛衣是有色的。以前曾认为北极动物的白色冬毛皮可以减少辐射的损失而保持身体的热量，但最近的研究表明黑色和白色毛皮的辐射热是相等的。冬季北极动物呈白鱼只是单纯的伪装与雪地环境相一致。北美变色的野兔一年换毛三次：白色冬毛衣替换为棕灰色的夏毛衣。而后替换为深灰色的秋毛衣，不久又脱落，从底下长出白色冬毛衣。

31. COLORATlON. We referred above to the cryptic white coloration of arctic mammals, an obviously useful adaptation for an all white environment. Outside of the arctic, most mammals wear somber colors for protective purposes. Often the species is marked with "salt--and--pepper" coloration or a disruptive pattern that helps to make it inconspicuous in its natural surroundings. Examples are the leopard's spots, the stripes of the tiger, and the spots of fawns. Zoologists have long wondered what adaptive purpose, if any, is served by the clearly defined black and white pattern of zebras. Although the zebra would appear to be a conspicuous target for predators, naturalists in Africa report that its stripes tend to blur the outline of the animal when it moves, making it difficult to distinguish from the background and from the more uniformly colored species of the African plains. Other mammals, for example, skunks, advertise their presence with conspicuous warning coloration.

32. An interesting aspect of color is the pair of rump patches of the pronghorn antelope, which are composed of long white hairs erected by special muscles. When alarmed, the animal can flash these patches in a manner visible for a long distance. They may be used as a warning signal to other members of the herd. The well--known "flag" of the Virginia white-tailed deer serves a similar purpose. • Mammalian coloration is principally caused by pigmentation in the hair, although in a few, bare surfaces of skin may be found with bright hues, such as in the cheeks and ischial callosities of the mandrill (a species of baboon), which may be attributable to pigment or to blood capillaries in the skin. At least two types of chromatophores are found in mammals --melanophores (black and brown pigment) and xanthophores (red and yellow pigment). Pigment granules may also lie outside the regular pigment cells. Although the color of hair may fade to some extent, any noticeable change in the color of a mammal's fur coat must be brought about by molting. • Albinism, or a lack of pigment, may happen in most kinds of mammals, as also may melanism, or an excess of black pigment.

33. 哺乳类的色彩主要是由于毛发的色素，少数裸露皮肤的表面有发亮的色彩，如西非洲的狒狒的面颊及坐骨的联股，这可能是由于皮肤内的色素或毛细血管所形成。哺乳类至少有二种载色体——黑色素细胞（黑及褐色素）及黄色素细胞（红及黄色素）。正规的色素细胞外可能也有色素颗粒。毛的颜色虽然在某种程度上也许会褪色，但哺乳类毛皮颜色显著的改变必然是脱毛引起的。哺乳类的色彩主要是由于毛发的色素，少数裸露皮肤的表面有发亮的色彩，如西非洲的狒狒的面颊及坐骨的联股，这可能是由于皮肤内的色素或毛细血管所形成。哺乳类至少有二种载色体——黑色素细胞（黑及褐色素）及黄色素细胞（红及黄色素）。正规的色素细胞外可能也有色素颗粒。毛的颜色虽然在某种程度上也许会褪色，但哺乳类毛皮颜色显著的改变必然是脱毛引起的。 • 白化（Albinism）或缺少色素，在哺乳类的大部分种类中可能都会遇到，但不要与白色相混合。白色是北极哺乳动物和鸟类的色彩。白化者有红眼睛和淡红色皮肤，而白色冬毛衣的北极哺乳类有黑色的眼睛，其耳尖、鼻子及尾尖常为黑色。黑化或黑色素过多也偶然在动物中见到。

34. 毛的衍生物 • 哺乳动物的毛发衍变以适应许多不同的用途。例如公猪的刚毛，大多数动物口鼻部的须以及豪猪类的棘。 • 感觉毛 • 许多哺乳类的须，一般不恰当地称之为“髯”（络腮胡子）实际上是感觉毛，有特殊的感觉作用。每一毛囊基部的毛球有一大的感觉神经。须的最轻微的移动都能引起神经末梢产生冲动传至脑的特殊感觉区，夜间活动的和穴居动物的须毛特别长。在海豹，它们是明显地作为一个远距离的感觉器官能觉察其他物体或鱼类路过时，在水中引起的波浪或湍流的压力。常在混浊水中猎取食物的海豹，其视力退化。研究发现盲海豹如同正常海豹一样的肥胖与健康。 • 刺甲胄 • 豪猪、刺谓、针鼹以及其他一些哺乳类生出有效且危险的刺甲胄；北美豪猪的刺被击中时能于基部断裂，加上末端有向后的尖钩深刺于受害者。为使迟钝的初遇者如狗了解它们的利害，这时豪猪使刺发出卡喀卡塔声并凸出刺上的白色标志朝向它们的干扰着。 • 的甲壳有着十分不同的来源。鳞片是真皮来源的小骨骼，被以韧且角化的表皮。毛发从鳞片之间与无鳞的身体腹部长出。

35. （2） 鳞片、爪、指甲、蹄 哺乳类指趾端多具爪，为表皮角质鞘 功能：防御、抓捕、攀缘、挖掘等 灵长类爪：演变为指甲 有蹄类爪：演变为蹄 穿山甲大型表皮角质鳞片 犰狳的真皮骨质板等

37. HORNS AND ANTLERS. Three kinds of horns or hornlike substances are found in mammals. True horns found in ruminants, for example, sheep and cattle, are hollow sheaths of keratinized epidermis that embrace a core of bone arising from the skull. Horns are not normally shed, are not branched (although they may be greatly curved), and are found in both sexes. The horns of North American pronghorn antelope are unique in that they are shed each year after the breeding season. But unlike the shedding of deer antlers, the new horn replaces the old by growing up inside and pushing it off. Antlers of the deer family are entirely bone when mature. During their annual growth, antlers develop beneath a covering of highly vascular soft skin called "velvet." When growth of the antlers is complete just prior to the breeding season, the blood vessels constrict and the stag tears off the velvet by rubbing the antlers against trees . The antlers are dropped after the breeding season. New buds appear a few months later to herald the next set of antlers. For several years each new pair of antlers is larger and more elaborate than was the previous set. The annual growth of antlers places a strain on the mineral metabolism since during the growing season a large moose or elk must accumulate 50 or more pounds of calcium salts from its vegetable diet. A third kind of horn is that of the rhinoceros. Hairlike horny fibers arise from dermal papillae and are cemented together to form a single horn

38. （3） 角（有蹄类具有） • 洞角：为反刍动物的角，如羊及牛，是由表皮角质化形成的中空的鞘，内含来源于真皮的骨质心。角正常时不更换，不分叉（虽然它们可能有大幅度的弯曲），且两性都有。唯一的北美叉角羚羊的角每年生殖季节后更换一次，但与鹿角的更换不一样，新角由内部长出并将外鞘推离以替换旧的。 实角：真皮骨质角，分叉，多为雄性有，每年脱换一次（如鹿角）。新生鹿角被有带茸毛的皮肤（鹿茸），后干枯脱落。 • 表皮角：由表皮角蛋白纤维形成（犀牛角）

42. 1）表皮角质层发达，真皮加厚，含有丰富的血管、神经和感觉末稍，能感受温度、压力和疼觉。 2）皮下脂肪层发达，有隔热保温作用。 3）披毛： 表披毛是哺乳动物区别于其它脊椎动物的最显著特征之一。 毛发的表皮角质化的产物，与爬行类的角质鳞片及鸟类的羽毛是同源物。 具有保温、保护和触觉作用

43. The eccrine glands (sweat glands) are found only in hairless regions (foot pads, etc.) in most mammals, although in some apes and in man they are scattered all over the body and are important devices for heat regulation. These glands have developed by the time of birth and are true secretory,or merocrine, glands, that is, the cell remains intact or is not destroyed in the process of secretion. Their secretory coils are restricted to the dermal region. Sweat glands are used mainly to regulate the body temperature. They are common on such animals as the horse and man but are greatly reduced on the carnivores (cats) and are entirely lacking in shrews, whales, and others. Dogs are now known to have sweat glands all over the body. ln human beings, racial differences are pronounced. • Blacks, who have more than whites, can withstand warmer weather. Lacrimal, or tear, glands keep the surface of the eye moist and clean. • Apocrine glands are larger than eccrine glands and have longer and more winding ducts.

44. Their secretory coil is in the subdermis. They always open into the follicle of a hair or where a hair has been. Phylogenetically they are much older than the eccrine gland and are found in all mammals, some of which have only this kind of gland. Blacks have more apocrine glands than do whites, and women have twice as many as men. They develop about the time of sexual puberty and are restricted (in man) to the axillae, mons pubis, breasts, external auditory canals, prepuce, scrotum, and a few other places. Their secretion is not watery like ordinary sweat (eccrine gland) but is a milky, whitish or yellow secretion that dries on the skin to form a plastic-like film. Only the tip of the secretory cell is destroyed in the process of secretion. Their secretion is not involved in heat regulation, but their activity is known to be correlated with certain aspects of the sex cycle, among other possible functions.

45. The most common apocrine glands include the scent glands, found in all terrestrial species. Their location and function vary greatly. Some are defensive in nature, others convey information to members of the same species, and still others are involved in the mating process. These glands are often located in the preorbital, metatarsal, and interdigital regions (deer): preputial region on the penis (muskrats, beavers, canine family, etc.): base of tail (wolves and foxes): and anal region (skunks, minks, weasels). These last, the most odoriferous of all glands, open by ducts into the anus and can discharge their secretions forcefully for several feet. During the mating season many mammals give off strong scents for attracting the opposite sex. Man also is endowed with scent glands. But civilization has taught us to dislike our own scent, a concern that has stimulated a lucrative deodorant industry to produce an endless output of soaps and odormasking compounds.

46. （4）皮肤腺特别发达 皮肤腺来源于表皮生长层 ，有皮脂、汗腺、味腺和乳腺4 种类型： 皮脂腺：分泌皮脂以润滑皮肤和毛发，防止干燥和浸润。 汗腺：分泌汗液，通过出汗可调节体温，排灌部分尿素。 味腺：为汗腺和皮脂腺的变形，能分泌特殊物质以吸引异 性、识别同种个体或用以自卫。 乳腺：为变态的汗腺，由若干乳腺集中心一定区域称为乳区 哺乳类乳腺根椐乳头的有无及开口情况可分为三种类型： 无乳头型：乳腺似汗腺，无乳头，如鸭嘴兽 真乳头型：如乳腺管都于乳头表面，如灵长类 假乳头型：乳头内有一空隙，称乳管，乳腺管开口于乳腺管基部，如牛。

48. Mammary glands, which provide the name for mammals, are probably modified apocrine glands, although recent studies suggest that they may have derived from sebaceous glands (see below). Whatever their evolutionary origin, they occur on all female mammals and on most, if not all, male mammals, on the latter they are often covered by hair. They develop by the thickening of the epidermis to form a milk line along each side of the abdomen in the embryo. On certain parts of these lines the mammae appear, while the intervening parts of the ridge disappear. They secrete milk for the nourishment of the young. Milk varies in composition in rapidly growing young it may contain 40% fat (seals, walruses). ln the human female, the mammary glands begin at puberty to increase in size because of fat accumulation and reach their maximum development in about the twentieth year. The breasts (or mammae) undergo additional development during pregnancy. ln other mammals, the breasts are swollen only periodically when they are distended with milk during pregnancy and subsequent nursing of the young. The outlets of the gland are by elevated nipples (absent in monotremes) . The glands are located on the thorax of primates, bats, and a few others but on the abdomen or inguinal region in other mammals. Nipples vary in number from two in the human being, horse, bat, etc., to 25 in the opossum. The number is not always constant in the same species.

49. 乳腺 哺乳类的名称由之而来，可能是顶装分泌腺衍生的，而最近的研究认为其可能是由皮脂腺衍生而来。无论是由哪一种进化起源，它存在于所有的雌性哺乳动物和大多数（如不是全部）雄性哺乳类，在雄性，它们是无活性，常复以毛发。在胚胎时期沿着腹部的两侧，由表皮增厚发育形成乳房线，在线上的某些部位出现乳房，而介于隆起的其他增厚部位消失。 • 人类女性乳腺在青春期由于脂肪的堆积开始增大，约发育至二十岁到顶峰。乳房于妊娠时得到额外的发育。其他哺乳类的乳房仅在怀孕及哺育幼儿期间充满乳汁时周期性地膨胀。腺的开口是通过高突的乳头（单孔类没有乳头）。灵长类、蝙蝠以及少数其他动物的腺体位于胸部，但在其他哺乳类位于腹部或腹股沟处。乳头的数目不一，从人类、马、蝙蝠及其他动物的二个到考拉熊（一种有袋类）的二十四个。同一种类中，数目也常不恒定 。