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  Section: Anatomy of Vertebrate Animals » The Classification and Organization of the Mammalia
 
 
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The Insectivora

 
     
 

It is exceedingly diflficult to give an absolute definition of this group of Mammals. But all the Insectivora possess more than two incisors in the mandible; and their molar teeth, which are always coated with enamel have tuberculated crowns, and form roots.

The fore-limbs have the structure usual among unguiculate Mammals; and, in both limbs, the digits are provided with claws. The hallux is not opposable, and, like the other digits, it is provided with a claw.

In addition to these distinctive characters there are others which are met with in all members of the group.

The Insectivora are, almost all, either plantigrade or semiplantigrade. The clavicles are completely developed in all, except Potamogale. The stomach is simple. The testes of the male are either inguinal or abdominal, and do not descend into a scrotum. The female has a two-horned uterus.

The cerebral hemispheres leave the cerebellum uncovered, in the upper view of the brain; and are almost, or wholly, devoid of sulci and gyri. The corpus callosum is sometimes exceedingly short.

No Insectivore attains a large size, and some, such as the Shrew Mice, are the smallest of the Mammalia.

The Insectivora present a great diversity of organization, the common Hedgehog being an almost central form. The Shrews tend toward the Rodentia, the Tupayae toward the Lemurs; while the Moles, on the one hand, and the Galeopitheci on the other, are aberrant modifications. Relations of a more general character connect them with the Carnivora and the Ungulata.

The Hedgehog (Erinaceus Europaeus) is pentadactyle and plantigrade. It has a long flexible snout. The eyes are small; the pinnae of the ears are rounded, and the integument lining the concha is produced into a transverse, shelf-like fold. The under surface of the body bears hairs of the ordinary kind; but, on the dorsal aspect of the head and trunk, the hairs are converted into strong fluted spines. There are twenty-one dorso-lumbar vertebrae (of which fifteen are dorsal, and six lumbar), three or four sacral, and twelve to fourteen caudal. Accessory processes, or metapophyses, are developed on several of the dorso-lumbar vertebrae. The sternebrae are laterally compressed, except the manubrium, which is broad; and eight of the fifteen pair of ribs are connected with the sternum.

The occipital foramen is placed completely at the hinder extremity of the skull, in the lower part of the perpendicular occipital face of the cranium, and looks backward. There are large paramastoid processes. The glenoidal surface for the mandible is flattened. The zygoma is stout, and the jugal bone is, as it were, applied upon the outer side of it. The orbit has no posterior osseous boundary. The lachrymal foramen lies upon the face. There are unossified spaces in the bony palate, and the posterior margins of the palate are thickened, as in the Lemurs. The large and bullate tympanic bone does not anohylose with the squamosal, or the periotic, and is readily lost from the dry skull. The alisphenoid contributes largely to the formation of the front wall of the tympanum; and a large portion of the inner wall of the tympanic cavity is formed by a broad process of the basisphenoid, the outer and lower edge of which joins, by a sort of harmonia, with the inner and lower edge of the tympanic.

The ascending portion of the ramus of the mandible is short, and the angle is slightly inflected. The two rami are not anchylosed at the symphysis. The supra-scapular fossa is wider than the infra-scapular. The spine is strong, and the acromion bifurcates, sending a prolongation backward. The clavicles are long and convex forward. The humerus has an intercondyloid foramen; but there is no foramen above the inner condyle, and this circumstance is unusual among the Insectivora. The bones of the antibrachium are fixed in the prone position. There is an os centrale in the carpus, so that it has nine bones. The scaphoid and lunare are anchylosed, as in the Carnivora, and the pisiform bone is much elongated. The pollex and the fifth digit are the shortest.

The pelvis is remarkably spacious. The symphysial union of the pubes is always small, and, sometimes, the bones remain separate. The subpubic arch is much rounded. The ilium is narrow, and a mere ridge separates the iliac fossa from the gluteal surface. The femur has a round ligament, and a prominent ridge represents a third trochanter. The distal ends of the tibia and fibula are anchylosed together.

One of the most notable peculiarities of the Hedgehog is its power of rolling itself up into a ball, from all sides of which the spines protrude. This is effected, for the most part, by the contraction of the greatly-developed cutaneous muscle, the chief fibres of which are disposed as follows: A very broad band, the orbicularis panniculi, encircles the body laterally. In front, it partly arises from the nasal and frontal bones, and partly is the continuation of a thick mass of fibres which pass over the occiput. Posteriorly, each lateral division of the muscle spreads out into a very broad band, which is thick ventrally and thin dorsally, and adheres closely to the skin, from the line at which the hairy and spinigerous surfaces join, to near the median line of the back. Posteriorly, the two lateral halves of the orbicular muscle pass into one another upon the distal half of the short tail.

The action of this muscle will depend upon the attitude of the animal when it contracts. If the head and tail are fully extended, the orbicularis can only diminish the dimensions of the spinigerous region of the skin and erect the spines. But if the head and tail be more or less flexed, as they always are in the ordinary attitude of the Hedgehog, the orbicularis will play the part of a powerful sphincter, approximating the edges of the spinigerous area toward the centre of the ventral side of the body, and forcibly enfolding the trunk and limbs within the bag thus formed. It is, in fact, the chief agent in coiling the body up, and keeping it so coiled.

Numerous muscular bundles take a radiating direction on the dorsal aspect of the body, and antagonize the orbiculari:
1. A pair of slender occipito-frontales arise from the occipital crest, and are inserted into the integument over the frontal and nasal bones. 2. A pair of occipito-orbiculares arise from the same crest, and pass into the anterior part of the orbicularis. 3. A pair of broader cervico-orbiculares arise from Ihe fascia of the neck, and pass to the dorsal part of the anterior fourth of the orbicularis. 4. Slender dorso-orbiculures arise close to the hinder ends of the trapezii and spread out above the foregoing. 5. Two stout muscles, coccygeo-orbiculare arise from the middle caudal vertebrae, and, after receiving fibres from the ventral region, end in the dorsal margins of the orbicularis. 6. Two muscles attached to the pinnae of the ears (auriculo-orbiculares) pass backward to the orbicularis on each side.

On the ventral aspect are certain muscles which assist the orbicularis: 1. Two broad muscles (sterno-faciales) arise in the middle line, over the anterior part of the sternum, and pass outward and forward to the sides of the lower jaw and the integument of the face and ears. Muscular slips from these are sent up over each shoulder to the orbicularis. 2. A humero-abdominalis arises from each humerus beneath the insertion of the pectoralis major, and, passing backward over the sides of the abdomen, these become connected with the ventral edges of the orbicularis. The external fibres of these muscles are continued round the ischial regions to the coccygeo-orbicularis; the internal fibres pass to the prepuce, and over the middle line of the abdomen, in front of it. 3. A humero-dorsalis arises from the humerus close to the foregoing, and, passing upward and backward through the axilla, spreads out in the mid-dorsal integument and the orbicularis.

The contraction of all these muscles must tend to bring together the edges of the integumentary bag, and to tuck the head, tail, and limbs into it.

In the myology of the limbs the following points are noteworthy: The supinator longus, pronator teres, and palmaris longus, are absent. The palmaris brevis is present. A single muscle takes the place of the extensor secundi internodii pollicis and extensor indicis, and sends a third tendon to the middle digit. The extensor minimi digiti supplies the other two digits. The flexor perforans and flexor pollicis longus are represented by five distinct muscular heads, each with a tendon of its own; but all the tendons unite in the middle of the forearm, and the common tendon again subdivides into only four slips, the pollex receiving no tendon. There are no lumbricales. The pollex has only a rudimentary flexor brevis and an abductor. The other digits have each two interossei, or flexores breves, inserted into the metacarpo-phalangeal sesamoids.

In the leg, the soleus has only a fibular head, and the flexor brevis digitorum arises wholly from the calcaneum. The flexor hallucis and flexor perforans have a common tendon, which, in the sole, divides into five tendons, one for each digit. There are no lumbricales, nor flexor accessorius. The tibialis posticus seems to be represented by twc small muscular beles one of which arises from the prominent end of the tibia, and the other from that of the fibula. The tendons of both pass behind the inner malleolus, and that of the former muscle goes to the tibial and plantar surface of the hallucal metatarsal, while the latter is inserted into the ento-cuneiform bone. The interossei pedis are represented by a pair of flexores breves for each digit except the hallux.

The adult Hedgehog has thirty-six teeth, of which twenty are in the upper, and sixteen in the lower jaw. The dental formula is i. 3.3/3.3 c. 0-0/0-0 p.m. 4-4/2-2 m. 3.3/3.3=36.

The grinding surface of the crowns of the first and second upper molars exhibits a pattern fundamentally similar to that of the corresponding teeth in Man, the Anthropomorpha, and the majority of the Lemurs; that is to say, there are four cusps, and the antero-internal is connected with the posteroexternal cusp by an oblique ridge. The cusps are remarkably sharp and pointed, and the outer surface of the postero-external one alone is somewhat inflected.

In the lower jaw, the corresponding molars are each marked, as in most Lemurs, by two transverse ridges. In front of tbe anterior ridge is a basal prolongation of the tooth, on to which a curved ridge is continued inward and forward from the anterior principal ridge, giving rise to an imperfect crescent with its convexity outward.

According to Rousseau there are twenty-four milk-teeth, i. 3.3/4.4 d.m. 4.4/1.1, which fall out seven weeks after birth.

The brain of the Hedgehog is remarkable for its low organization. The olfactory lobes are singularly large, and are wholly uncovered by the cerebral hemispheres; which, on the other hand, do not extend back sufficiently far to hide any part of the cerebellum. Indeed, they hardly cover the corpora quadrigemina. Only a single shallow longitudinal sulcus marks the upper and outer surface of each hemisphere. On the under surface, a rounded elevation corresponds with the base of each corpus striatum. Behind this, another elevation represents the end of the uncinate gyrus and the termination of the hippocampus major; and therefore answers, in a manner, to the temporal lobe. The inner face of the hemisphere presents neither convolution nor sulcus, except behind and below, where a very broad depression follows the contour of the fissure of Biohat and the fornix, and represents the dentate sulcus. Above, this sulcus ends behind the posterior margin of the corpus callosum. The latter is remarkably short, and directed obliquely backward and upward. It has no genu, and the pre-commissural fibres of the ventricular wall spread out, beneath its anterior end, upon the face of the hemisphere. The part of the corpus callosum which answers to the lyra is very thick in proportion, and is inclined at an acute angle to the rest.

In a transverse section, the corpus callosum is seen to be verv thin, and to curve upward and outward into the roof of the ventricular cavity. The inner walls of the lateral ventricles, which answer to the septum lucidum, are thick, while the fornix is comparatively thin and slender. The anterior commissure is very stout. In this circumstance, as in the small corpus callosum, the brain of the Hedgehog closely approaches that of the Didelphia and Ornithodelphia. There is no trace of a posterior cornu, or calcarine fissure, and the lateral ventricle extends forward into the olfactory lobe. The optic nerves are very slender; the corpora geniculata externa are large and prominent; the nates are smaller than the testes, and transversely elongated. The cerebellum has a large vermis and small lateral lobes; the flocculi are prominent and are lodged in fossae of the periotic bones. The pons Varolii is very small; the corpora trapezoidea proportionally large.

The spinal cord is remarkable for its thickness, and, at the same time, for its brevity, as it ends in the middle of the dorsal region. As a consequence of this arrangement, the cauda equina is particularly large and long.

The stomach is simple, but the mucous membrane of the considerable cardiac dilatation is thrown into numerous, and very strong, longitudinal rugae. The intestine is about six times as long as the body, and presents no distinction into small and large; nor is there any caecum. The liver is divided by deep fissures into six lobes; a cential one which bears the gall-bladder, a bifid spigelian lobe, and, on each side of these, two other lobes. The pancreas is a large and irregularly-ramified gland; and the spleen is elongated and trihedral.

The pericardium is extremely thin. The arteries arise from the arch of the aorta, as in Man, by an anonyma, a left carotid and left subclavian. The course of the internal sarotid is remarkable. When it reaches the base of the skull it enters the tympanum and there divides into two branches, of which one traverses the stapes, and, passing forward in a groove of the roof of the tympanum, enters the skull and gives rise to the middle meningeal and ophthalmic arteries. The other branch passes over the cochlea, enters the skull by a narrow canal near the sella turcica, and unites with the circle of Willis.

The external jugular vein is very much more capacious than the internal, the latter being very small and hardly traceable to the internal jugular foramen. It is by the external jugular vein, in fact, that the great mass of the blood within the skull is carried away, a foramen in the squamosal bone allowing of a free communication between the external jugular vein and the lateral sinus. There is a left superior vena cava, which winds round the base of the left auricle, receives the coronary vein, and opens into the right auricle. The vascular system thus retains many embryonic characters.

The right lung is four-lobed; the left may possess from one to three lobes.

Two ossifications, one on each side of the opening for the aorta, occur in the diaphragm.

The testes of the male do not leave the cavity of the abdomen, but they descend as far as the inner side of the inguinal ring, to which they are connected by a short gubernaculum and cremaster. The vasa deferentia descend to the base of the bladder and then enter a hollow muscular sheath on their way to a "chamber," which is lodged in the distal end of that sheath. This "chamber" passes into the penial urethra; the cystic urethra opens into it by a narrow slit in its front wall; and it receives the ducts of three pair of appendages. The proximal pair consist of a multitude of ramified tubuli, which have been found to contain spermatozoa, and are usually regarded as vesiculae seminales. The middle pair (the so-called "prostatic glands") have a similar structure and have also been observed to contain spermatozoa. The lowermost pair are Cowper's glands. The "chamber" appears to represent the urogenital sinus of the embryo, which has not become differentiated into prostatic and bulbous urethra.

The ovaries are enclosed in wide-mouthed peritoneal sacs, and a ligamentous band, the diaphragmatic ligament, extends from the ovary to the posterior surface of the diaphragm. The cornua uteri are large and long. There are five pair of teats; the anterior pair being axillary and the posterior inguinaL The other three pair are equidistant, and lie along the ventral surface, internal to the edge of the orbicularis panniculi.

Like the Rodentia, the Insectivora have a great diversity of habit; some Galeopitheci flitting through the air after the fashion of the flying Squirrels; some arboreal, as the Tupayae; some terrestrial and cursorial, like the majority of the order. A few are swimmers; and some, like the Mole, are the most completely fossorial of Mammals.

The most aberrant form of the Insectivora is the genus Galeopithecus, essentially an Insectivore of arboreal and frugivorous habit, with very long and slender limbs. These are connected with one another, with the sides of the neck and body, and with the tail, by a great fold of the integument, which is called patagium; and, unlike the web of the Bat's wing, is hairy on both sides, and extends between the digits of the pes. By the help of this great parachute-like expansion, the Galeopitheous is enabled to make floating leaps, from tree to tree, through great distances. When at rest, the Galeopitheci suspend themselves by their fore-and hindfeet, the body and the head hanging downward; a position which is sometimes assumed by the Marmosets among the Primates.

The fore-limbs are slightly larger than the hind-limbs. There are four axillary teats. The male has a pendent penis and inguinal scrotal pouches. The pollex and the hallux are short, and capable of considerable movement in adduction and abduction, but they are not opposable; and their claws are like those of the other digits.

The occipital foramen is in the posterior face of the skull. The orbit is nearly, but not quite, encircled by bone. The lachrymal foramen is in the orbit. The bony roof of the palate is wide and its posterior margin is thickened. There is a strong curved post-glenoidal process of the squamosal, which unites with the mastoid, beneath the auditory meatus, and restricts the movement of the mandible to the vertical plane. A longitudinal section of the skull shows a large olfactory chamber projecting beyond that for the cerebral lobes, and two longitudinal ridges, upon the inner face of the latter, prove that these lobes must have possessed corresponding sulci. The tentorial plane is nearly vertical and the floccular fossae are very deep.

The ulna is very slender inferiorly, where it becomes anchylosed to the distal end of the radius, which bears the carpus. When the ilia are horizontal, the acetabula look a little upward and backward as well as outward. The fibula is complete. As in the Sloths and most Primates, the navicular and cuboid readily rotate upon the astragalus and calcaneum, so that the planta pedis is habitually turned inward.

The dental formula is i. 2.2/3.3 c. 1-1/1-1 p.m.m. 5-5/5-5=34.

The outer incisor, in the upper jaw, has two roots, a peculiarity which is not known to occur elsewhere. The canines of both jaws also have two roots, as in some other Insectivora. The lower incisors are single-fanged; and their crowns are broad, flat, and divided by numerous deep longitudinal fissures, or "pectinated."

The length of the whole alimentary canal from mouth to anus is not more than six times that of the body. The sacculated caecum is as long as the stomach, and its capacity must be greater than that of the latter organ.

Galeopithecus has, at one time, been placed among the Lemurs, and at another, among the Bats. But the resemblances with the former are general and superficial, and the differences in the form of the brain, the dentition, the structure of the limbs and of the skull, exclude it from the order of the Primates.

Galeopithecus agrees with the Bats in the disposition of the tail, and in the existence of a patagium provided with special muscles. Further, in a slight obliquity of the acetabula, such as is seen in its extreme development in the Bats; in the imperfect condition of the ulnae; and in the pectoral position of the teats and the pendent penis. Both of these last, however, it must be recollected, are also Primatic characters. Finally, the somewhat similarly pectinated lower incisor teeth are found in the Cheiropteran genera, Diphylla and Desmodus.

But Galeopithecus differs from the Bats completely in the structure of the fore-limbs; in the position of the hind-limbs and the absence of a calcar; in the two-fanged outer incisors and canines; and in the presence of a caecum.

On the other hand, the peculiarities of the skull and brain are mainly insectivorous, as is the two-fanged canine; and I see no reason for dissenting from Prof. Peters's view that Galeopithecus belongs neither to the Primates, nor to the Cheiroptera, but that it is an aberrant Insectivore.

With respect to other Insectivora, it is worthy of note, that Macroscelides has the radius and the ulna ancbylosed. The Tupayae possess a large caecum. Chrysochloris has pectoral mammary glands; Centetes and the Moles have the penis pendent.

The Tupayae are soft-furred, long-tailed, tree-loving animals, with complete bony orbits and a large cacum, and are those Insectivora which most nearly approach the Lemurs.

The Shrews (Sorices) most nearly resemble Rodents outwardly, being very like small mice. The zygoma is imperfect, the tibia and fibula are anchylosed, and the pubic bones do not meet in the symphysis. There are sixteen to twenty teeth in the upper jaw and twelve in the mandible. Canines are absent, and there are six incisors above and four below. The inner lower incisors are greatly elongated and proclivous, and some of the teeth not unfrequently become anchylosed with the jaws. There is no caecum, and peculiar musk-glands are sometimes developed at the sides of the body.

The Moles (Talpinae) have no external ears, and the eyes are rudimentary. The fore-limbs are much larger than the hind, and are inclosed within the integument up to ihe carpus. The palmar surface of the broad manus is turned outward and backward.

The skeleton of a Flying-Fox. (Pteropus)
Fig. 199. - The skeleton of a Flying-Fox. (Pteropus).
The manubrium of the sternum is very broad, and its ventral surface gives rise to a strong median crest. The scapula is as long as the humerus and the radius together. It is triquetral and possesses an acromial process, but no distinct corracoid. The clavicle, which is verv strong, is perforated by a great foramen, and at the middle of its posterior margin sends off a truncated reentering process. Proximally, it furnishes an articular surface for the humerus. In the carpus there is a distinct centrale, and a large accessory C-shaped bone lies on its radial side. The pubes are separate at the symphysis, and an accessory styloid bone is connected with the naviculare of the foot.

The distribution of the Insectivora is singular in this respect, that, although they are met with, under very various climatal conditions, throughout the Old World and North America, there are none in South America or Australia.

In the fossil condition they are not certainly known to occur in strata older than the tertiary.


 
     
 
 
     



     
 
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