Book - Vertebrate Zoology (1928) 10

From Embryology

Vertebrate Zoology G. R. De Beer (1928)

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Chapter X Lepus : A Warm-Blooded, Viviparous Chordate

Externals

The most obvious characteristic of the rabbit and of other mammals is the possession of hair, which, typically, forms a complete covering to the body. Hairs are more or less cylindrical epidermal structures, seated in little pits or follicles at the base of each of which is a papilla. The epidermal cells just above the papilla multiply actively and contribute new material to the hair, which in this way grows in length (see p. 234). The central axis of the hair is called the medulla, and surrounding this is the cortex (which is often pigmented), and a cuticle. The function of hair is to prevent loss of heat from the body by radiation, for mammals are warm- blooded (homothermous). It also serves for protection, and sometimes as a sensory tactile organ, as in the case of the vibrissas or " whiskers." The fingers and toes end in claws, likewise epidermal structures.


The skin is thicker than in the forms previously described. In the epidermis there is a great difference between the actively-growing cells at the base (stratum Malpighi), and the flat, horny cells on the surface (stratum corneum) which are continually being lost and replaced from the stratum Malpighi.


The dermis of the skin forms the basis of leather, and it commonly contains fat forming a layer which assists the animal in maintaining its internal heat. Beneath the skin are muscles which serve to move and shake it. In the region of the trunk these muscles form the panniculus carnosus ; in the head the skin muscles are concerned with movements of the eyebrows, lips, and external ears (platysma muscles) (see p. 278).


The skin in mammals is well supplied with glands of epidermal origin, and of which there are three kinds : sudoriparous, sebaceous, and mammary. The sudoriparous or sweat-glands, are small tubes which sink into the dermis from the surface, and end blindly after a certain number of coilings. They serve to excrete water which is obtained from the neighbouring blood-vessels, and in so doing they play an important part in the regulation of the temperature of the body. The water excreted is ordinarily converted into vapour, and thereby absorbs the latent heat required for this conversion from the body.


The sebaceous glands differ from the sweat-glands in that they branch repeatedly, and that their secretion is not an extracellular and liquid, but intracellular greasy substances which are pushed out in the loaded cells themselves. These glands are usually found opening into the hair-follicles, whence the greasy secretion spreads over the hair. Other glands of this type open to the surface along the edge of the eyelids (Meibomian glands), and into depressions at the sides of the anus (perineal glands). The secretion of the latter is responsible for the smell of the rabbit.


Mammary glands or milk glands, are also characteristic of the whole order Mammalia. They occur in both sexes, but are normally functional only in the female. They are branched tubes lying between the skin and the underlying muscles on the ventral surface of the body, and opening to the surface by nipples, of which there are in the rabbit about four pairs, corresponding to the usual number of young born in a litter.


The eyes have upper and lower eyelids and a small nictitating membrane. A noteworthy feature is the presence of external ears, or pinnae, which assist the sense of hearing, by concentrating the waves of sound.


The anus is at the root of the tail, and is separate from the urinogenital aperture, which is situated in front of it, and takes the form of a penis in the male or a vulva in the female. At the sides of the penis in adult males are the scrotal sacs which contain the testes. This ventral position of the testes is a new feature, peculiar to most adult mammals.

Skull

The skull has two occipital condyles, formed from the exoccipitals. The floor is formed by basioccipital, basisphenoid, presphenoid, and vomer, the latter representing the anterior portion of the parasphenoid of lower forms. The mesethmoid is perforated by a number of pores through which branches of the olfactory nerve run to the nasal sacs ; it is known as the cribriform plate. Anteriorly, the mesethmoid extends as the vertical septum nasi, which separates the cavities of the nasal capsules.


The roof of the skull is formed by the supraoccipital, parietals, frontals, and nasals. The bones of the auditory capsules are fused to form the periotics, which form the hinder part of the side of the brain-case. The remainder of the side is formed by the squamosals, alisphenoids (corresponding to the epipterygoids of reptiles), and orbitosphenoids. There is a small lachrymal bone near the front of the orbit.


The margin to the upper jaw is formed by the premaxillae and the maxillae. From the maxillae the jugals extend backwards and meet a process (zygomatic) of the squamosal forming (with the jugal) the " cheek bone." The roof of the mouth is a false palate, formed by flat extensions of the maxillae and palatine bones meeting their fellows of the opposite side ventral to the true roof of the mouth, and enclosing the nasal passage. The pterygoids are small bones behind the palatines, and at the sides of the basisphenoid. The vomer representing the parasphenoid is covered over by the false palate. Large tympanic bullae lie beneath the periotic and contain the tympanic cavity. The nasal cavities contain a number of scroll- like turbinal bones (see Figs. 143, 149, and 150).


The wall of the skull is pierced by a number of holes or foramina through which nerves and blood-vessels pass out and enter. These foramina are commonly situated between different bones, for the nerve or blood-vessel developed first and the bones formed afterwards. At first these bones are small, but as they grow they meet one another forming sutures, and foramina are open sutures. Occasionally the bone grows all round the nerve or blood-vessel, and the foramen then pierces that bone. The most important foramina are those in the orbit and hinder region of the skull, and they are most conveniently studied in the skull of a young dog, in which the sutures between the bones are still plainly visible.


Fig. 60. Skull of a dog, seen from the left side and slightly from beneath.


The Roman figures close to the arrows indicate the cranial nerves which emerge from the several foramina. II, optic nerve through the optic foramen ; III, oculomotor nerve ; IV, trochlear nerve ; Vi, first or ophthalmic branch of the trigeminal nerve, and VI, abducens nerve, all emerging through the foramen lacerum anterius ; V2, second or maxillary branch of the trigeminal nerve, through the foramen rotundum ; V3, third or mandibular branch of the trigeminal nerve, through the foramen ovale ; VII, facial nerve, through the stylo-mastoid foramen ; IX, glossopharyngeal nerve ; X, vagus nerve ; XI, spinal accessory nerve, all emerging through the foramen lacerum posterius ; XII, hypoglossal nerve, through the condylar foramen ; a, alisphenoid ; ac, alisphenoid canal (lodging the so-called external carotid artery) ; bs, basisphenoid ; Et, opening into the tympanic bulla for the Eustachian tube ; flm, foramen lacerum medius through which the internal carotid artery enters the skull ; fp, false palate ; j, jugal ; /, lachrymal ; mp, mastoid process ; tip, nasal passage ; oc, occipital condyles ; os, orbitosphenoid ; p, parietal ; pi, palatine ; pm 4, fourth premolar modified into the carnassial tooth ; pt, pterygoid ; s, squamosal ; sp, styloid process ; tb, tympanic bulla.


The optic nerve (II) emerges through the optic foramen in the orbitosphenoid bone. Immediately behind this is the foramen lacerum anterius between the orbito-sphenoid and the alisphenoid. Through it pass the oculomotor (III), trochlear (IV), abducens (VI), and the ophthalmic branch of the trigeminal nerve (Vi). The maxillary branch of the tri- geminal (V2) emerges through the foramen rotundum in the alisphenoid, while the mandibular branch of the same nerve (V3) passes through the foramen ovale between the alisphenoid and the periotic. The Eustachian tube passes into the tympanic bulla through an opening between the latter and the basi- sphenoid. The foramen just median to this is the lacerum medius (likewise between the tympanic bulla and the basi- sphenoid), through which the internal carotid artery pierces the skull. The so-called external carotid artery which is given off from the internal carotid before the latter enters the skull, runs forwards through the alisphenoid canal. This is not a true foramen for it does not lead into the skull, but is a short tunnel in the alisphenoid. Its hind entrance is below the foramen ovale, and its anterior exit is confluent with the foramen rotundum. The facial nerve (VII) emerges through the stylo-mastoid foramen, between the hind face of the tympanic bulla and the periotic. Between the periotic and the exoccipital is a large elongated opening, the foramen lacerum posterius. Through this, pass the glossopharyngeal (IX), the vagus (X), and the spinal accessory (XI) nerves, and the internal jugular vein. The hypoglossal nerve (XII) passes through the condylar foramen in the exoccipital. The large hole at the back of the skull for the spinal cord is the foramen magnum.


The lower jaw consists of a single bone : the dentary, which articulates with the skull by means of the squamosal. This method of articulation is characteristic of mammals, and differs from the articular-quadrate articulation of all other Gnathostomes. Indeed, at first sight, the quadrate and articular appear to be absent from the mammalian skull. On the other hand, whereas other vertebrates have one single bone connecting the tympanic membrane with the fenestra ovalis of the auditory capsule, in the mammal there are three such bones. The one nearest to the fenestra ovalis is perforated and called the stapes (stirrup) ; it is homologous with the columella auris and hyomandibula of lower forms. The next bone is the incus, which is in reality the quadrate as can be shown by its embryological development ; and the last bone is the malleus which is in reality the articular. During the course of the evolution of the mammals, these bones have therefore undergone a remarkable change of function.


The " hyoid " is a small plate of bone, connected with the periotic by a number of small bones representing the hyoid arch (styloid process). The ceratobranchials of the 1st branchial arch are represented by the posterior horns of the hyoid (thyrohyoid). Elements of the remaining branchial arches are possibly represented in the cartilages of the larynx and of the trachea.


Teeth

Equally distinctive of mammals are the teeth, which are of different shape in the various regions of the mouth, a condition termed heterodont as distinct from the homodont condition of lower forms in which all the teeth are alike. A further distinction lies in the fact that the teeth are replaced once only in the mammal (diphyodont condition), and not repeatedly as in lower forms (polyphyodont). There are four kinds of mammalian teeth : incisors, canines, premolars, and molars. The incisors or cutting teeth are situated at the front of the mouth, those of the upper jaw are borne on the premaxillae. The next kind of tooth is the canine or tusk, but it is not present in the rabbit, which is a herbivorous animal. In the dog the canines are well developed ; that in the upper jaw is the most anterior tooth in the maxilla, and the canine of the lower jaw lies in front of that in the upper when the mouth closes. In the rabbit there is a long gap or diastema between the incisors and the premolars. Premolars and molars are often much alike, but their distinction lies in the fact that the premolars appear in two sets : " milk teeth " arising first and being replaced by permanent teeth as in the case of incisors and canines ; but there is only one generation of molars. Premolars and molars or grinding teeth, are the hindmost teeth to be carried on the maxillae in the upper jaw.


Bearing in mind the four different sorts of teeth, it is possible to describe the dentition of a mammal very simply and quickly by means of a " dental formula " : that of the rabbit is : .2033

The dental formula of the dog, on the other hand, is :

Most mammalian teeth grow to a certain size and then cease, as a result of the closing of the entrance into the pulp- cavity by the formation of " roots " or fangs. Some, however, retain the open pulp-cavities which are continuously supplying food material to the odontoblasts, as a result of which the tooth can go on growing throughout life. Such teeth are called " rootless," or " with persistent pulps," and examples are to be found in the incisors of the rabbit. As a rule, teeth which are subjected to perpetual wearing down owing to grinding or gnawing, or which can grow out of the mouth for unlimited distances such as the tusks of the elephant, are of this kind. In the rabbit, the lower incisors are kept in check by the upper ones, and vice versa ; but if one tooth through accident is lost or destroyed, the opposing tooth in the other jaw is no longer resisted in its growth. Under such circumstances it grows continuously and eventually kills the rabbit by preventing it from shutting its mouth. In a sense, it may be compared with the unruly growth of a tumour.


Skeleton. — The skeleton of mammals has a peculiarity in many of its bones which is not found in any other vertebrates. Several of the cartilage-bones, and especially the vertebrae and the bones of the limbs are composed of three pieces : a central shaft or diaphysis and an epiphysis at each end. Between the epiphyses and the diaphysis there are, in young mammals, portions of cartilage, and the bone is able to increase in length by adding on new bone to the diaphysis at each end. Eventu- ally, however, the epiphyses become firmly fused on to the diaphysis, and no further growth of the bone is then possible.



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Fig. 61. — Lepus : thoracic vertebras and ribs.


A, seen from in front ; B, seen from the left side, c, centrum of the vertebra ; ca, capitulum of the rib ; na, neural arch ; ns, neural spine ; r, rib ; t, tuberculum of the rib ; tp, transverse process.



The epiphyses can still, however, be recognised as distinct from the diaphysis.


Vertebral Column

In all mammals the number of cervical vertebrae is seven (three species only form an exception to this rule). The first is the centrum-less atlas, and the second is the axis bearing the centrum of the atlas in the form of the odontoid peg. The cervical vertebrae have vertebrarterial canals formed by the fusion of the tuberculum of the rib to the transverse process of the vertebrae, and the capitulum of the rib to the centrum. The thoracic vertebrae are usually a dozen in number, and each is related to a pair of ribs with which it articulates by tubercular and capitular facets. Behind the thoracic region are the lumbar vertebrae, usually seven in number, and characterised by their large transverse processes. Next comes the sacral region which is attached to the ilium of the pelvic girdle, and the caudal region with vertebrae which become simpler in structure as they approach the tip.


Fore Limb and Girdle

The pectoral girdle is formed by the scapula, which bears a ridge, the acromion, and a small coracoid process representing the coracoid of lower forms. There is no separate coracoid. The clavicle is slender, and joins the acromion to the sternum. The arm is made up of the usual bones : humerus, radius, and ulna, three proximal carpals (scaphoid or radiale, lunar or intermedium, cuneiform or ulnare) ; one central carpal (centrale), and four distal carpals (trapezium, trapezoid, magnum, and unciform) make up the wrist. There are five metacarpals, and the phalanges are 2, 3, 3, 3, 3 in number on the respective fingers.


Hind Limb and Girdle

The pelvic girdle is formed of the usual three bones : ilium, ischium, and pubis, on each side. The ilium runs forwards and upwards from the acetabulum to the sacrum, instead of backwards as in reptiles. The pubis meets its fellow from the opposite side in the middle line, forming the pubic symphysis ; and a large obturator foramen separates the pubis from the ischium of its own side.


The femur has a large head, which fits into the acetabular cavity of the pelvic girdle, and three processes or trochanters, which serve for the attachment of muscles. The tibia is large, but the fibula is small and fused on to the tibia. Covering the front side of the joint between femur and tibia is a small bone, the patella or knee-cap.


The proximal tarsal bones are two in number : the astra- galus, and the calcaneum (heel-bone). There is one centrale or navicular, and three distal tarsals. The latter are, the mesocuneiform (2nd tarsal), ectocuneiform (3rd tarsal), and the cuboid (fused 4th and 5th tarsals). The rabbit is specialised in having lost the endocuneiform (ist tarsal), otherwise its tarsus is easily comparable to that of Triton. The foot has four toes, the ist or hallux having disappeared. There are consequently four metatarsals, and the digital formula for the number of phalanges is o, 3, 3, 3, 3.


It is common to find small irregular bones on the under or palmar side of the joints of several of the fingers and toes, and covering certain joints of the arm and leg. These are the sesamoid bones. They arise in connexion with the insertion of tendons on to bones. Examples are the patella, and the pisiform which underlies the joint between the ulna and the cuneiform bone of the wrist. Sesamoids are important functionally, but they have not much significance in com- parative anatomy, since they are not constant from group to group.


Sternum and Ribs

The breast-bone or sternum is sub- divided into six sections, called sternebrae, and a posterior piece called the xiphisternum. The most anterior of these (manu- brium) is attached to the clavicles. The first seven ribs articulate ventrally with the sternum. The dorsal part of each of these ribs is bony, the ventral part cartilaginous. The next two ribs are attached ventrally not to the sternum but to the seventh rib, and the remainder end freely and are not attached to any skeleton.


Coelom

An important characteristic of mammals is the fact that the perivisceral coelomic cavity is completely divided into two by a transverse partition, anteriorly convex, the diaphragm. The lungs are in front of this diaphragm, enclosed in the pleural cavities. The pericardium is also in front of the diaphragm, but its cavity is separated from that of the pleural cavities. The remaining viscera lie in the general peritoneal cavity behind the diaphragm (see Fig. 127).


The diaphragm, which is of course pierced by the alimentary canal, the aorta, and the inferior vena cava, divides the trunk effectively into thoracic and abdominal regions. It is muscular, and plays an important part in the process of respiration, assisting the ribs in increasing the capacity of the thoracic box, and causing air to rush into the lungs. It is developed in part from the transverse septum. The scrotal sacs contain a portion of coelomic cavity, lined with ccelomic epithelium or peritoneum termed tunica vaginalis.


The alimentary canal is supported by a dorsal mesentery. The latter, in the region of the stomach, is called the great omentum, and is pulled ventrally and backwards so as to enclose a sac (the omental bursa) which communicates with the general peritoneal cavity on the right side of the stomach by the foramen of Winslow. In the rabbit the great omentum is small, but in other forms it is extensive and laps over the ventral side of several coils of the intestine. Fat is often found deposited in the great omentum, and especially in the Pig - The pleural cavities each surround a lung, which is suspended in them by a mesentery. The coelomic epithelium of a pleural cavity is called the pleura, and it is divided into visceral (or splanchnic) and parietal (or somatic) parts. The parietal pleura lines the outer wall of the pleural cavity, the anterior face of the diaphragm, and in the middle line comes into contact with its fellow from the opposite side to form the mediastinal septum. The visceral pleura continues from the parietal and covers over the lung. When the ribs are lifted, the pleural cavities increase their volume, and since the space between the visceral and parietal pleura is a closed one, expansion of the parietal pleura is necessarily accompanied by expansion of the visceral pleura and of the lung which it covers. If the thoracic box were punctured and air could get into the pleural cavities, the visceral pleura and the lungs would fail to expand. The pericardium lies ventral and median to the pleural cavities.


Alimentary System

The original edge to the mouth is represented by the gums, in which the teeth are set. Outside these, fleshy lips are developed. The roof of the mouth is formed by the false palate, due to the extension inwards of a shelf from the maxilla and palatine bone on each side. In this region it is called the hard palate, and it is continued posteriorly a short distance by the soft palate, in which there is no bone. The false palate encloses the nasal passage between itself and the true roof of the mouth, and this passage opens into the mouth behind the soft palate, by the secondary choana. The floor of the mouth is occupied by a large soft tongue. Four pairs of salivary glands secrete into the mouth. They are, the parotid (just in front of the ear), the submaxillary (behind the angle of the mouth), the infra-orbital (below and behind the cheek-bone, and the sublingual (on the inner side of the lower jaw).


The Eustachian tubes open into the mouth near the opening of the nasal passage, and behind them are the tonsils which are remnants of the 2nd pair of visceral (1st branchial) gill pouches.


The pharynx connects with the larynx by the glottis, and this opening is protected by a flap, the epiglottis. In breathing, the soft palate is dropped, thus allowing air to come in through the nasal passage and into the mouth, and the epiglottis is raised, allowing the air to enter the larynx on its way to the lungs. When swallowing is taking place, the soft palate is raised, closing the nasal passage, and the epiglottis is forced down and bars the way into the larynx.


The oesophagus runs through the diaphragm to the cardiac portion of the stomach. The other or pyloric portion of the stomach opens into the duodenum, the opening being surrounded by a sphincter muscle. The duodenum receives the bile-duct and the pancreatic duct. The liver is large, and fits close up against the posterior face of the diaphragm. It is connected with the stomach by the lesser omentum (mesen- tery), and with the floor of the peritoneal cavity by a small ventral mesentery, the falciform ligament. The gall-bladder is green in colour. The pancreas lies in the mesentery which stretches between the two arms of a loop formed by the duodenum.



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Fig. 62. — Lepus : dissection of the vascular system seen from the ventral side. a, aorta ; ab, adrenal body ; al, anterior laryngeal nerve (branch of vagus) ; am, anterior mesenteric artery ; c, carotid artery ; ca, cceliac artery ; eg, anterior mesenteric sympathetic ganglion ; en, cervical nerve ; d, diaphragm ; da, ductus arteriosus ; dn, depressor nerve (branch of vagus) ; i, innominate artery ; isg, posterior cervical sympathetic ganglion ; /, jugular vein ; /, larynx ; o, oesophagus ; p, pulmonary trunk ; pa, pulmonary artery ; ph, phrenic nerve ; pv, pulmonary vein ; ra, renal artery ; rd, ramus descendens (branch of hypoglossal nerve) ; rl, recurrent laryngeal nerve (branch of vagus) ; rv, renal vein ; s, stomach ; sa, subclavian artery ; ssg, anterior cervical sympathetic ganglion ; sv, subclavian vein ; sy, sympathetic nerve-chain ; t, thymus gland ; th, thyroid gland ; tr, trachea ; v, vagus nerve ; vet, vena cava inferior ; vg, vagus ganglion ; XII, hypo- glossal nerve.


The small intestine is lined with countless finger-shaped processes called villi which absorb the products of digestion. Along the wall of the intestine are masses of lymphatic tissue known as Peyer's patches, from which lymphocytes pass into the cavity of the intestine. In the rabbit the small intestine is over two yards long, and it ends in a chamber called the sacculus rotundus, with which the ccecum and the large intestine connect. The ccecum or blind gut ends blindly as its name implies, and at its extremity is the vermiform appendix which contains much lymphatic tissue. The ccecum is a structure commonly found in herbivorous animals, for in it cellulose is digested with the help of bacteria. It is usual to find it reduced or absent in carnivorous forms. The large intestine or colon connects with the caecum near the opening of the sacculus rotundus, and leads to the rectum and anus.


Respiratory System

Most of the structures concerned with respiration have already been described in connexion with the mouth and the pleural cavities. The larynx is protected by a number of cartilages (thyroid, cricoid, and arytenoid) to which muscles are attached. Internally, it contains the vocal cords. The trachea which is kept open by cartilaginous rings, leads from the larynx to the point where the two bronchi arise. Each bronchus leads to a lung, and becomes subdivided into larger and larger numbers of increasingly smaller air-spaces. The mammalian lung is not a vascular hollow sac such as the lung of the newt or the lizard ; its cavity is repeatedly sub- divided so that it appears to be filled with spongy tissue in which blood-capillaries circulate, surrounded on all sides by the minute air-spaces. The surface of contact between air- spaces and blood-vessels is very great ; in man, for example, it is about thirty times the area of the body-surface.


Vascular System

The heart contains four chambers, two auricles and two ventricles. The truncus arteriosus has been split into two, right down to its base. One of these vessels opens out of the right ventricle and leads to the lungs ; it is the pulmonary artery. The other opens out of the left ventricle and is the aorta which leads to the carotid arteries and the systemic arch. The two superior venae cavae and the inferior vena cava open directly into the right auricle ; there is no sinus venosus. The pulmonary veins open into the left auricle. Guarding the opening between the right auricle and right ventricle is the tricuspid valve ; the corresponding opening between the left auricle and left ventricle is guarded by the mitral valve.* The openings of the aorta and pulmonary artery are guarded by semi-lunar valves.


The systemic (4th arterial) arch persists only on the left side. On the right, it is represented only by the short in- nominate artery from which the right carotid and right sub- clavian arteries arise. On the left side these two arteries arise from the systemic arch, which, passing back and up round the left side of the gut, becomes the dorsal aorta. The dorsal aorta gives off the following arteries : coeliac (to stomach, liver, duodenum, and spleen) ; anterior mesenteric (to small intestine and colon) ; posterior mesenteric (to rectum) ; all of which run ventrally in the mesentery to the several viscera. Between the anterior and the posterior mesenteric arteries, the dorsal aorta also gives off the renal arteries to the kidneys, and the genital arteries to the gonads. In the case of males in which the testes have descended into the scrotal sacs, the latter arteries are of considerable length. Posteriorly the dorsal aorta divides into the iliac arteries which supply the hind legs, and the caudal artery. .The 5th arterial arch dis- appears, but the 6th is represented by the pulmonary. Originally, as in the fish, the 6th arterial arch communicated with the lateral dorsal aorta, and this communication is present in the mammalian embryo, on the left side, in the form of the ductus arteriosus. In the adult the ductus arteriosus loses its function (which is important in the embryo) and degenerates into a ligament ductus Botalli.


The vena cava superior of each side is made up of the jugular and subclavian veins, and opens into the right auricle. In some forms the left superior vena cava is connected with the right by a transverse innominate vein, and so loses its own opening into the right auricle. The left vena cava superior also receives at its base the thoracic duct which connects with the system of lymphatic vessels. The posterior cardinal veins are represented by the azygos (right) and hemiazygos (left) veins of the wall of the thorax. The hemiazygos connects with the azygos, which opens into the right superior vena cava. The connexion between the hemiazygos and the left superior vena cava has been lost.

  • The number of flaps which these valves possess should be obvious : the tricuspid has three, and the mitral two.

The ductus arteriosus is also called the


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Fig. 63. — Diagram showing the relations of the arterial arches and the branches of the vagus nerve in : A, Scyllium, and B, Lepus ; seen from the left side.


al y anterior laryngeal nerve ; bv 1, 3, 4, and 6, blood-vessels (arterial arches) running in the first, third, fourth, and sixth visceral arch ; ca y carotid arch ; d, ductus arteriosus ; da, dorsal aorta ; Et, Eustachian tube ; g 1, first gill-slit ; h, heart ; m, mouth ; pa, pulmonary artery ; rl, recurrent laryngeal nerve ; s, spiracle ; sa, systemic arch ; t, tonsil ; v, vagus nerve ; vb 1,4, first, fourth branch of the vagus nerve.


The walls of the heart itself are drained by veins, called coronary veins, which open into the right auricle.

The veins from the hind legs (iliac and femoral veins) run into the inferior vena cava, which also receives the genital vein from the gonads, the renal veins from the kidneys and the hepatic veins from the liver, and runs into the right auricle. Blood from the stomach and intestine is carried to the liver by the hepatic portal vein : there is no renal portal vein. The blood of the mammals differs from that of all other animals in that in the adult, the red blood-corpuscles have no nuclei. Instead of being biconvex, the red corpuscles here are bicon- cave. The source of supply of new blood-corpuscles in late embryonic and in adult life is in the red marrow which is situated in the central cavity of a number of bones. In addition, lymphocytes are produced in the lymph- glands, which also serve as blood-filters. It is possible that blood- corpuscles may also be formed in the spleen.


Like birds, mammals are warm-blooded, or homothermous.


Urino-genital System

The kidneys are asymmetrically placed. They are metanephric structures, connected by the ureters with the urinary bladder.


In the female the Mullerian ducts persist while the Wolffian ducts disappear together with the mesonephros (traces of the latter may persist as the epoophoron and paroophoron). The ovaries are close to the anterior end of the Mullerian ducts or oviducts, which open into the peritoneal cavity by the Fallopian tubes. The base of each oviduct is enlarged and specialised to form the uterus, in which the young embryos develop, for mammals are viviparous. The two uteri are close together, and they open into the single median vagina. The bladder is just ventral to the vagina and connects with it to form the vestibule which communicates with the exterior by the vulva.


The vestibule is dorsal to the pubic symphysis, and ventral to the anus, with which it has no connexion. There is therefore no cloaca.


In the male, the Mullerian ducts disappear except for the uterus masculinus, which lies dorsal to the bladder. The testes are connected with the epididymis, representing the mesonephros of their own side. From the epididymis the vas deferens or Wolffian duct leads to the base of the bladder on its dorsal side, close to the prostate gland. The bladder and vasa deferentia lead into a tube, the urethra, which runs through and opens to the exterior at the end of the penis.




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Fig. 64. — Lepus : dissection of the female urinogenital system seen from the ventral side.


For explanation of lettering, see Fig. 65.



The testes arise near the roof of the peritoneal cavity, suspended by mesenteries. When here, they are said to be in the abdominal position, for later on they descend ventrally and backwards into the scrotal sacs. The spermatic cords, containing the artery from the dorsal aorta, show the path taken by the testes in their descent ; they passed median and ventral to the ureters as is shown also by the course of the

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Fig. 65. — Lepus : dissection of the male urinogenital system seen from the ventral side. a, anus ; ab, ad, adrenal body ; b, bladder ; da, dorsal aorta ; e, epi- didymis ; g, gubernaculum ; ia, iliac artery ; k, kidney ; o, ovary ; oa, ovarian artery ; od, oviduct ; p, penis ; r, rectum ; ra, renal artery ; sa, spermatic cord ; ss, scrotal sac ; t, testis ; u, ureter ; ut, uterus ; v, vesti- bule ; va, vagina ; vet, vena cava inferior ; vd, vas deferens.


vas deferens. The epididymis is connected with the scrotal sac by an elastic cord, the gubernaculum, which in early stages grows down into the scrotal sac and guides the testis thither in its descent.


It may be mentioned that the ovary in mammals is peculiar in possessing Graafian follicles (see p. 227).


Ductless Glands

(see Chapter XXXIII)

The spleen is situated in the mesentery near the stomach. It is related to the lymphatic glands, and its function is to act as a filter or purifier of the blood. This it does by destroying worn-out blood-corpuscles, and foreign bodies which may have got into the blood.


The thyroid is two-lobed, and lies across the ventral side of the larynx. It is associated with the parathyroids.


The thymus lies close in front of the heart, and is smaller in older than in younger animals. The adrenals are small compact bodies lying anterior to the kidney on each side. Each consists of a cortex (corresponding to the inter-renal of Scyllium) and a central medulla (supra- renal). The pituitary lies in a depression in the floor of the skull, called the sella turcica. The gland is composed of four parts : anterior, intermedia, tuberalis, and nervosa. The pineal gland is on the roof of the between-brain and between the cerebral hemi- spheres. The pancreas has already been noticed on account of its external secretion into the duodenum, but it also has a very important internal secretion formed by the islets of Langerhans. The gonads produce an internal secretion which is responsible for the differentiation of the sexual characters of their particular sex, but it is not yet clear which tissue is responsible for this effect. In the pregnant female, the follicle from which the egg was liberated becomes a corpus luteum, the internal secretion of which plays an important part in the development of the embryo in the uterus.


Nervous System

The most important characteristics of the mammalian nervous system are to be found in the brain.


The medulla oblongata or myelencephalon is not very different from that of lower forms, but in the metencephalon the cerebellum is much enlarged and divisible into a number of lobes. Its surface is thrown into a number of folds, which increases the quantity of superficial grey matter or cortex which it contains. There is also a band of nerve-fibres which join the two sides of the cerebellum to one another passing ventral to the rest of the hindbrain ; this is the pons Varolii, peculiar to mammals. The cavity of the 4th ventricle does not extend into the cerebellum, which is solid. The roof of the midbrain bears, not two, but four prominences. That is to say, that in place of the two optic lobes of lower vertebrates, there are now four corpora quadrigemina.

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Fig. 66. — Lepus : the brain, seen, A, from the inner side of a longitudinal vertical section ; B, from the ventral side. ac, anterior commissure ; c, cerebellum ; cc, corpus callosum ; ch, cerebral hemisphere ; cq, corpora quadrigemina ; cr, crura cerebri ; fM, foramen of Monro (shown by an arrow) ; h, hippocampal commissure ; he, habenular commissure ; It, lamina termihalis ; Iv, lateral ventricle (cavity of cerebral hemisphere) ; mo, medulla oblongata ; oc, optic chiasma ; ol, olfactory lobe ; p, pineal body ; pb, pituitary body ; pc, posterior com- missure ; pi, pyriform lobe ; pV, pons Varolii ; sc, soft commissure. The roman figures indicate the roots of I, olfactory ; II, optic ; III, oculomotor ; IV, trochlear ; V, trigeminal ; VI, abducens ; VII, facial ; VIII, auditory ; IX, glossopharyngeal ; X, vagus, and XII, hypoglossal nerves.


The sides of the between-brain are thickened to form the optic thalami, so much so indeed that the two sides touch one another across the constricted 3rd ventricle, forming the " soft commissure." The roof of the between-brain bears the pineal stalk, the floor is depressed to form the infundibulum to which the pituitary body is attached. Posterior to the pituitary, the corpora mammillaria form two prominences depending from the floor. The main bundles of fibres which pass up and down from the brain and spinal cord run in the ventral portion of the hindbrain, dorsal to the pons Varolii, and diverge right and left in the region of the infundibulum forming the crura cerebri.


The cerebral hemispheres, or roofs of the lateral ventricles forming the end-brain, are enormous and extend backwards covering over the between-brain and midbrain. The super- ficial layer of nerve-cells or grey matter forming the cerebral cortex, which was slightly developed in reptiles, is in the mammals thick and well formed. The surface is thrown into a few folds, forming sulci and gyri ; but these are not so numerous in the brain of the rabbit as in higher mammals. The body of the hemispheres is marked out into a number of lobes by fissures (frontal, parietal, occipital, and temporal lobes). The two hemispheres are separated by a deep cleft or median fissure, but the cortex of each side is connected with that of the opposite side by a broad band of transverse fibres forming the corpus callosum, likewise peculiar to mammals. The cavities of the hemispheres are the lateral ventricles, which communicate with the 3rd ventricle by the foramina of Monro.


Beneath the temporal lobes are the pyriform lobes which correspond to part of the roof of the end-brain of reptiles, and which communicate with the olfactory lobes in front. The floor of the end-brain is marked by the optic chiasma and the corpus striatum.


The various regions and centres of the brain in mammals are extensively connected with one another by tracts of fibres. Most of these connexions can only be made out by detailed study, but the transverse tracts or commissures are easily seen in a longitudinal section of the brain. Of these, the corpus callosum (connecting cerebral cortex) and the pons Varolii (connecting cerebellar cortex) have already been mentioned. In addition there are the following : the hippocampal com- missure, which connects the two hippocampal lobes, running ventral and posterior to the corpus callosum and dorsal to the 3rd ventricle ; the anterior commissure, connecting the two halves of the corpus striatum, and running in the anterior wall of the 3rd ventricle or lamina terminalis ; the habenular commissure, connecting the optic thalami, running across the roof of the 3rd ventricle just beneath the pineal body ; the posterior commissure, in the roof of the midbrain ; the inferior commissure, crossing the floor of the 3rd ventricle close to the optic chiasma. The " soft commissure " is not really a com- missure, since it does not transmit a transverse tract of fibres.


Meningeal Membranes

The brain is surrounded by the vascular pia mater, which projects into the lateral ventricles, the 3rd and the 4th ventricles, forming in each a choroid plexus. Outside the pia mater is the arachnoid membrane, and outside this again is the protective and hard dura mater. The cerebro- spinal fluid which fills the canal of the spinal cord and the ventricles of the brain communicates with the space contained by these meningeal membranes through an opening in the roof of the 4th ventricle, the foramen of Magendie.


Nerves

The distribution of the peripheral nerves in the head is not dissimilar from that in lower forms, but attention may be paid to the conditions in the region of the neck. On each side of the neck, just lateral to the trachea, there are a number of nerves running parallel with the carotid artery and jugular vein. The vagus is one of these : it emerges from the skull (through the foramen lacerum posterius) and swells into a ganglion from which a nerve runs backwards. It soon gives off an anterior laryngeal nerve which runs to the larynx, and a small depressor nerve which accompanies the vagus in its course backwards to the heart. The vagus passes ventral to the aortic arch on the left, and ventral to the innominate artery (which corresponds to the aortic arch) on the right. Immediately after passing the artery, the vagus gives off a posterior or recurrent laryngeal nerve which loops round the artery, passes dorsal to it, and runs forwards again along the side of the trachea. On the left side the loop of the recurrent laryngeal passes behind the ductus arteriosus. This peculiar course of the recurrent laryngeal nerve is easily understood on referring to the nervous system of Scyllium. The anterior laryngeal nerve corresponds to part of the first branch of the vagus which runs in the 4th visceral (2nd branchial) arch. The posterior or recurrent laryngeal nerve corresponds to part of the 4th branch of the vagus which runs in the 7th visceral (5th branchial) arch. Now the aortic arch, and its representative on the right the innominate artery, are the blood-vessels of the 4th visceral (2nd branchial) arch ; and the ductus arteriosus is the vessel of the 6th visceral (4th branchial) arch. In development these arches are displaced backwards to a considerable extent. But this backward movement of these arteries necessarily pulls back the nerves of the next posterior visceral arch, and this is why the recurrent laryngeal nerves have to loop round the arteries before they can reach their destination. The main branch of the vagus continues backwards to the heart, stomach, and intestine and corresponds to the visceral branch of the vagus of Scyllium. It transmits fibres which belong to the parasympathetic (autonomic) nervous system.


Parallel with the vagus in the neck is the trunk of the sympathetic nervous system. It swells into the anterior cervical ganglion, on a level with the ganglion of the vagus, and continues forwards into the head accompanying the internal carotid artery. Farther back, the sympathetic trunks have a posterior cervical ganglion, and run backwards accompanying the aorta, swelling into ganglia in most of the segments of the thorax and abdomen. From some of these ganglia, fibres run to the anterior mesenteric ganglion on the root of the anterior mesenteric artery, and to the posterior mesenteric ganglion, which is situated near the root of the posterior mesenteric artery. From these ganglia, fibres are distributed to the smooth muscles of the gut, bladder, and other viscera. It may be repeated that the feature which distinguishes the autonomic (sympathetic and parasympathetic) nerves from the remainder, is the fact that in the autonomic system the muscles and glands are not connected directly with the brain or spinal cord by a single nerve-cell, but by two, one reaching from the brain or cord to the sympathetic (or parasympathetic) ganglion, and the other continuing from this ganglion to the muscle or gland in question. Such muscles are always smooth and involuntary. Striped (voluntary) muscles are innervated direct from the brain or cord by nerve-cells which go all the way without interruption.


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Fig. 67. — Diagram showing the structure of the ear in mammals. ac, auditory capsule ; c, cochlea ; de, ductus endolymphaticus ; Et, Eustachian tube ; jo, fenestra ovalis ; fr, fenestra, rotunda ; i, incus (quad- rate) ; m, malleus (articular) ; oe, external auditory meatus (outer ear) ; s, stapes (columella auris, hyomandibula) ; sa, saccule ; sc, semicircular canal ; tc, tympanic cavity (middle ear) ; tm, tympanic membrane (ear- drum) ; «, utricle.


As in lower forms, the ganglia of the sympathetic trunk are connected with the spinal ganglia by rami communicantes.


The diaphragm contains muscles of somatic origin which are innervated by the phrenic nerves. These nerves are formed from the 4th and 5th cervical spinal nerves, and run back to the diaphragm on each side of the heart. The length of their course shows the amount which the diaphragm, together with the heart and aortic arches, have moved back- wards during development ; a movement which has already been noticed in connexion with the recurrent laryngeal nerves.


Sense-organs

The sense-organs of the mammal show certain peculiarities. The sensory surface of the olfactory organs is increased by the formation of folds supported by the turbinal bones. Jacobson's organ opens into the mouth in some forms, but it disappears in others. The ear is remarkable for the external pinna, and the inclusion of the articular and quadrate as the malleus and incus, in the chain of bones which together with the stapes (columella auris) connect the tympanic membrane with the fenestra ovalis. The projection from the saccule which forms the ductus cochlearis in lower forms, and is responsible for hearing as apart from appreciating balance (the function of the rest of the ear), is in the mammals very highly developed. It is much elongated, and is coiled in a spiral which enables it to be accommodated in the compara- tively small cochlear part of the auditory capsule.


The eyelids are movable and muscular, and well supplied with glands ; lachrymal and Harderian glands are present, and a naso-lachrymal duct.


Characteristics of Lepus, typical of Mammals : Hair ; Bones with diaphysis and epiphyses ; Two condyles to the skull ; Loss of coracoid ; Tympanic bulla ; Lower jaw composed of dentary only ; Teeth heterodont and diphyodont ; Articulation of dentary with squamosal ; Conversion of articular and quadrate into malleus and incus ; Diaphragm ; Single left aortic arch ; Non-nucleated red blood-corpuscles ; Great expansion of cortex in cerebral hemispheres ; Corpus callosum ; Pons varolii ; Turbinals ; Cochlea spirally wound ; Descent of testes into scrotal sac ; Mammary glands ; Uterus and placenta (allantoic) ; Graafian follicles.


Literature

Bradley, O. C. A Guide to the Dissection of the Dog. Longmans, Green, London, 1912.

Howell, A. B. Anatomy of the Woodrat. Bailliere, Tindall and Cox, 1926.

Marshall, A. M., and Hurst, C. A Junior Course in Practical Zoology. John Murray, London, 1920.


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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
Vertebrate Zoology 1928: PART I 1. The Vertebrate Type as contrasted with the Invertebrate | 2. Amphioxus, a primitive Chordate | 3. Petromyzon, a Chordate with a skull, heart, and kidney | 4. Scyllium, a Chordate with jaws, stomach, and fins | 5. Gadus, a Chordate with bone | 6. Ceratodus, a Chordate with a lung | 7. Triton, a Chordate with 5-toed limbs | 8. Lacerta, a Chordate living entirely on land | 9. Columba, a Chordate with wings | 10. Lepus, a warm-blooded, viviparous Chordate PART II 11. The development of Amphioxus | 12. The development of Rana (the Frog) | 13. The development of Gallus (the Chick) | 14. The development of Lepus (the Rabbit) PART III 15. The Blastopore | 16. The Embryonic Membranes | 17. The Skin and its derivatives | 18. The Teeth | 19. The Coelom and Mesoderm | 20. The Skull | 21. The Vertebral Column, Ribs, and Sternum | 22. Fins and Limbs | 23. The Tail | 24. The Vascular System | 25. The Respiratory system | 26. The Alimentary system | 27. The Excretory and Reproductive systems | 28. The Head and Neck | 29. The functional divisions of the Nervous system | 30. The Brain and comparative Behaviour | 31. The Autonomic Nervous system | 32. The Sense-organs | 33. The Ductless glands | 34. Regulatory mechanisms | 35. Blood-relationships among the Chordates PART IV 36. The bearing of Physical and Climatic factors on Chordates | 37. The origin of Chordates, and their radiation as aquatic animals | 38. The evolution of the Amphibia : the first land-Chordates | 39. The evolution of the Reptiles | 40. The evolution of the Birds | 41. The evolution of the Mammalia | 42. The evolution of the Primates and Man | 43. Conclusions | Figures | Historic Embryology



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