Part 38

_Origin_ on the medial surface of the concha, just caudad of the proximal end of the tragus. The fibres pass distad to be inserted on the auricular cartilage at the caudal angle of the notch which lies beneath the cranial edge of the auricle, along with the caudal fibres of the adductor auris superior (1).

_Relations._--Lateral surface with the integument; medial surface with the tragus.

_Action._--Draws proximad the cranial margin of the auricle.

=M. antitragicus= (Fig. 169, 6; Fig. 64, _n_).--A small muscle about one centimeter in length and three millimeters in thickness, attached at one end to the caudal border of the antitragus. The fibres pass mediodistad and are inserted on the tragus, in common with the fibres of the tragicus medialis (Fig. 169, 5, 5′).

_Relations._--Outer surface with the adductor auris inferior and the integument; inner surface with the cartilage.

_Action._--Constricts the external auditory opening.

=M. tragicus medialis= (Fig. 169, 5, 5′).--A small, flat muscle, in two parts, on the medial surface of the concha.

_Origin_ on the ventral end of the tragus. The cranial fibres form a thick nearly cylindrical bundle which extends proximad about one centimeter, and is inserted on the cranial surface of the concha just proximad of a deep fold in the cartilage. The caudal fibres form a thin sheet which spreads on the medial surface of the concha and is inserted on an obliquely proximodistal line on the medial surface of the concha, less than one centimeter from the free distal edge.

These two bundles are distinct except at their origin, and are sometimes considered as separate muscles.

_Relations._--Outer surface with the tragicus lateralis (4) and the fat about the concha. Inner surface with the concha.

_Action._--Flexes the concha.

Strauss-Durckheim describes under the name =cornetoconchæus= a small muscle bridging the deep groove caused by the folding of the medial surface of the concha, its proximal end being close to the insertion of the abductor auris brevis (page 100). This muscle was absent in all of the cases examined for its presence.

=M. conchæus externus= (Fig. 169, 9).--A quadrilateral muscle on the lateral surface of the concha. The cranial end of this muscle is attached to the concha a short distance distad of the antitragus along a dorsoventral line six to eight millimeters in length; the fibres pass thence caudodorsad a distance of about one centimeter, where they are again attached to the concha. The muscle thus bridges a shallow depression on the lateral surface of the concha.

_Action._--Constricts the concha.

=The Middle Ear.=

The middle ear is enclosed in a cavity within the tympanic bulla. Its outer boundary, as well as the inner boundary of the external ear, is formed by the =tympanic membrane= (Fig. 170, _c._) This is a thin, semitransparent membrane attached to the bony ring (_a_) surrounding the inner end of the external auditory meatus. It is oval in outline. Across the dorsal third of its inner surface runs the handle of the malleus (_b_), to which the membrane is firmly attached and which by pulling on the membrane gives it the form of a cone instead of a flat surface. The apex of the cone is directed mediad.

The middle ear, or =cavum tympani=, is an ellipsoidal cavity situated in the tympanic bulla. On removing the medial wall of the bulla, a bony plate is seen separating its cavity into two chambers. This plate of bone extends in from the lateral wall of the bulla, forming a complete partition except at the dorsocaudal part, where a notch in the bone forms a free communication between the two chambers. The lateral chamber, or tympanum proper, contains the bones of the ear and is limited externally by the tympanic membrane. At its cranial end is seen the opening of the =tuba auditiva=, or =Eustachian tube=.

The tuba auditiva, or Eustachian tube, is a cartilaginous tube about one and a half to two centimeters long, passing from the nasopharynx to the middle ear and placing these two cavities in communication. The pharyngeal opening is at about the middle of the length of the nasopharynx, near the dorsal border of the lateral wall; it is a narrow slit two or three millimeters long. Thence the tube passes caudolaterad on the ventral surface of the sphenoid bone, and enters the tympanic cavity through the opening just laterad of the styliform process of the tympanic bulla. The tube has a thick, cartilaginous medial wall, while the lateral wall is thin and formed of connective tissue. The lumen of the tube is a curved slit in cross-section.

=Bones of the Ear= (ossicula auditus).--There are three small bones in the middle ear, the malleus (hammer) (Fig. 171, 1), incus (anvil) (Fig. 171, 2), and stapes (stirrup) (Fig. 172).

The malleus (Fig. 171, 1) consists of a slender handle (_a_) with a small neck (_b_) attached at an angle to the caudal end of the handle. At the end of the neck is a rounded head (_c_) which articulates with the incus (2), and together with one part of the incus lies in a small fossa dorsad of the fenestra vestibuli. From the medial aspect of the neck extends a short process of bone (_d_) to which is attached the tendon of the tensor tympani muscle. One side of the neck is produced into a thin wing-like plate, bearing on its edge a pointed process (_e_).

The =incus= (Fig. 171, 2) bears much resemblance to a two-fanged molar tooth. It lies in the fossa with the head of the malleus. Its head or crown is directed craniad and has a concave surface which articulates with the head of the malleus. One of the fangs (_f_) extends caudad in the fossa above mentioned and is held in position by a ligament. The other fang (_g_) extends ventromediad and articulates with the head of the stapes. This fang terminates in a minute rounded tubercle known as the =lenticular process=; this in early stages of development is a separate bone, the =os lenticulare=.

The =stapes= (Fig. 172) has the form of a stirrup, and is the last in the chain of bones. The small end is termed the head (_a_) and the broader part the base (_b_). The head of the stapes articulates with one of the processes of the incus. The base of the stapes fits into the fenestra vestibuli of the petrous bone and completely closes it. The stapes is held in position by the stapedius muscle, which is attached to a minute process (_c_) near the head.

Thus the three bones of the middle ear form a chain which connects the medial surface of the tympanic membrane with the fenestra vestibuli. As the fenestra vestibuli forms a portion of the wall of the vestibule, the vibrations of the tympanic membrane are transmitted through the chain of bones to the fluid within the vestibule.

=Muscles of the Middle Ear.=

=M. tensor tympani.=--The tensor tympani muscle is a short conical muscle having origin in a small fossa (Fig. 25, _d_, p. 34) in the petrous bone dorsocraniad of the fenestra vestibuli. Its very short tendon passes laterad and is inserted into a projection (Fig. 171, _d_) on the neck of the malleus.

=M. stapedius.=--The stapedius muscle is a short muscle taking origin from a fossa (Fig. 25, _f_) in the lateral surface of the petrous bone caudad of that for the incus. It is inserted into the head of the stapes.

=The Internal Ear= (Fig. 173).

The internal ear or =labyrinth= is completely enclosed in the petrous bone. In the dry bone the internal ear communicates with the middle ear by the fenestra cochleæ and the fenestra vestibuli, which have been described in the account of the petrous bone (page 33). In the recent state the fenestra vestibuli is closed by the base of the stapes, while the fenestra cochleæ is covered by a delicate membrane,--so that the internal ear forms a closed cavity. It is possible to distinguish a =bony= labyrinth and a =membranous= labyrinth (Fig. 173), the former being the cavity in the petrous bone, while the latter is the real auditory organ, situated within the cavity. The membranous labyrinth repeats in general the form of the bony labyrinth, so that it will not be necessary to distinguish the two in our account of the main features of the structure. The auditory nerve is distributed to the membranous labyrinth. For an account of the minute structure of this organ reference should be made to a text-book of general histology or physiology.

The labyrinth (Fig. 173) consists of a cavity, the =vestibule= (_a_), and connected with this a coiled tube, the =cochlea= (_b_), and the three =semicircular canals= (_c_, _d_, and _e_).

The =cochlea= (_b_) is situated in the promontory,--the rounded elevation of the petrous bone extending craniad from the fenestra cochleæ. It consists of a bony tube coiled spirally about a central column known as the =modiolus=. The coil has nearly the form of a snail’s shell, and is somewhat more lengthened in the cat than in man. A plate of bone, the =lamina spiralis=, incompletely divides the cavity of the canal lengthwise into two parts, known as =scalæ= (_f_ and _g_). One of these opens at the fenestra cochleæ (_h_) into the tympanic cavity (or would so open but for the thin membrane covering the fenestra); this is known as the =scala tympani= (_f_). The other scala is connected with the vestibule (_a_) and is known as the =scala vestibuli= (_g_).

The =vestibule= (_a_) is a pyramidal cavity situated mediad of the caudal end of the cochlea. It is connected with the scala vestibuli of the cochlea, and with the three semicircular canals. The fenestra vestibuli (_i_) is an opening in its bony wall, filled in the recent state by the stapes. On the medial surface of the vestibule are a number of minute orifices through which the auditory nerve enters.

The =semicircular canals= (_c_, _d_, and _e_) are three curved tubes, each forming about two-thirds of a circle, and opening at both ends into the vestibule. They are imbedded in the petrous bone. The planes of the three tubes intersect nearly at right angles; from their position in man they have been named =external= (_c_), =superior= (_d_), and =posterior= (_e_). The external one (_c_) is nearly horizontal in position, and surrounds the curved fossa (Fig. 25, _f_) for the stapedius muscle, caudad of the fenestra vestibuli. The superior (Fig. 173, _d_) and posterior (_e_) are in nearly dorsoventral planes, but at right angles to each other. The superior (_d_) lies in the ridge forming the caudal boundary of the appendicular fossa. The posterior (_e_) is situated in a continuation caudad of the ridge which forms the dorsal boundary of the internal auditory meatus. One end of each canal is slightly enlarged to form an =ampulla=. The superior (_d_) and posterior (_e_) canals join at one end and enter the vestibule by a common opening. One end of the external canal (_c_) opens into the vestibule so close to one end of the posterior canal (_e_) that they appear to have a common opening, and this is commonly stated to be the case; according to Jayne, however, the openings are really separate. The two remaining ends (of the superior (_d_) and external (_c_) canals) also enter the vestibule separately, so that the three canals enter the vestibule by five openings.

III. THE OLFACTORY ORGAN. ORGANON OLFACTUS.

The olfactory organ consists of the olfactory mucous membrane, in the dorsocaudal part of the nasal cavity. The nasal cavity has been described in connection with the respiratory system.

IV. THE ORGAN OF TASTE. ORGANON GUSTUS.

The sense of taste is located in the mouth-cavity, especially in the mucosa of the tongue and the soft palate. The proper organs of taste are the so-called taste-buds, found on the papillæ of the tongue, the soft palate, etc. Their exact distribution in the cat has apparently not been worked out.

V. THE INTEGUMENT. INTEGUMENTUM COMMUNE.

The skin of the cat consists of the usual two layers, the outer =epidermis= and the inner fibrous =corium=. The skin is particularly thick and tough about the sides of the neck and face.

The skin is covered almost completely with fine soft hairs. Only the end of the nose, the pads on the soles of the feet, and the nipples are without hair. A number of long stiff sensory hairs (vibrissæ) are found on the upper lip, the cheek, and above the upper eyelid.

The pads on the soles of the feet are cushion-like projections of skin, composed largely of connective and elastic tissue and fat. There is one large pad for each foot, and in addition a small pad for each digit. The fore limb has besides a small conical pad which is situated almost exactly over the pisiform bone.

The =nails= are horny outgrowths of the epidermis, covering the distal phalanges.

The =muscles= of the skin are the cutaneus maximus and platysma; these have been described (page 93). The superficial facial muscles are differentiations of the skin-muscles.

APPENDIX.

PRACTICAL DIRECTIONS.

The following appendix on the dissection and study of the cat is not intended as a general treatise on methods of preservation and dissection, but attempts only to give the application of well-known methods to the dissection and study of the particular animal with which the book deals. General methods which are referred to but not described must be looked up, if unfamiliar, in the manuals of general methods.

_Aids to Dissection and Study of the Cat._--A copy of Jayne’s “Mammalian Anatomy” should be in the laboratory for reference. Only the volume on the Bones of the Cat has been issued thus far.

Wilder and Gage’s “Anatomical Technology” will be found very useful for methods of work.

The plates of Strauss-Durckheim’s “Anatomie du Chat” are exceedingly valuable for the bones and muscles, and should be in the laboratory if possible. Outline reproductions of these plates have been published by H. S. Williams (G. P. Putnam’s Sons, New York, 1875). The use of both the original plates and the reproductions is made difficult by the antiquated nomenclature used for the structures figured.

_Method of Use of the Present Directions._--Except in the case of the Bones, the order of study of the structures is usually not the same as the order in which they are described. This is of course due to practical difficulties in dissection. The present directions give the _order of study_, or at least the order of dissection; of course after dissection the structures can be reviewed in the logical order given in the descriptions. But in the first study and dissection, the student is to use these directions for the _order of work_, while using the body of the book for the descriptions. It will be necessary therefore to keep the book opened in two places,--one in the appendix, for the directions; one in the body of the book, for descriptions. In studying the muscles, for example, the student will follow the order of dissection given in the appendix, but as every muscle is dissected he will find, confirm, and study the description of that muscle given in the body of the book. Other organs are studied in a similar manner.

Before commencing the study, read that portion of the preface which explains the use of _terms of direction_ (page VII).

Note that the page on which any figure is found may be determined by consulting the list of figures given in the Table of Contents.

THE BONES.

The bones will of course be studied from dried specimens. These may be prepared once for all and kept in the laboratory from year to year. A number of mounted skeletons of the cat should be at hand; these must be prepared by some one who has skill in such work.

_Preparation of Separate Bones._--For preparing the separate bones the liquid-soap process recommended by Wilder and Gage (“Anatomical Technology,” p. 107) will be found most convenient. This is as follows: The skin and soft parts are removed as far as possible without injury to the bones.

Make the following “liquid-soap” mixture:

Soft water 2000 cc. Strong ammonia 150 cc. Nitrate of potash (saltpetre) 12 grams. Hard soap 75 grams.

Heat these together till a homogeneous mixture is formed. Now immerse the bone or bones to be cleaned in a liquid composed of four parts water to one part of the above liquid-soap mixture. Boil forty minutes; pour off the liquid and renew it. Boil about half an hour longer; then remove soft parts with fingers, scalpel, and brush. The bones may be boiled as much longer in the mixture as is necessary to make the soft parts come away easily. If they are boiled too long, however, the epiphyses will separate from the bones. This is especially likely to occur in young skeletons, which must therefore not be boiled so long as the bones of an old cat.

Skeletons prepared in the following rough way are useful for many purposes. The entire body of the cat, or a single limb, is placed in the liquid-soap mixture (after partial removal of soft parts), and boiled till the muscles may be removed, but the ligaments, holding the bones together, remain. Clean thoroughly without removing the ligaments, and allow to dry. The bones are thus held together in their natural positions. Skeletons so prepared are not elegant, but are frequently useful.

_Disarticulation of Skulls._--The bones of the skull may be separated as follows: Clean the skull in the liquid-soap mixture, as above described. Then fill the cranial cavity with dry rice, beans, corn, or some other seeds that swell much on imbibition of water. Cork up the foramen magnum, and place the skull for twelve to twenty-four hours in water. The swelling of the grain will partially separate the bones at the sutures. The bones may then usually be separated completely by hand.

Entire skulls, and longitudinal sections, as represented in Fig. 43, p. 60, should be at hand.

_Study of the Bones._--The bones should be studied in the order in which they are described. No further special directions for their study are necessary.

_Study of Ligaments._--It will usually not be practicable for each student to make the dissections and preparations necessary for studying the ligaments. Rather should these be examined on demonstration preparations preserved in the laboratory. Such preparations may most easily be made by using _fresh_ material, and dissecting away muscles and other tissues, leaving only the ligaments connecting the bones. The preparations are then preserved in one or two per cent. formalin. It is difficult to prepare the ligaments satisfactorily on preserved material, because they do not stand out clearly from other tissues in such material, so that they are not easily distinguishable.

MUSCLES.

_Preparation and Preservation of Material._--Formalin forms by far the most satisfactory preservative for anatomical material. It is much better in almost every respect than alcohol, and has the additional advantage of being much cheaper.

After injecting with formalin, the cat may be preserved either immersed in a weaker solution of the same substance, or may be kept for a long time simply wrapped in a cloth dampened with formalin. The details of preparation differ according to which method is to be used.

In either case, the cat should be killed with chloroform, by placing it in a tight jar or box with a cloth or bit of cotton saturated with chloroform. It is not necessary to bleed the specimen. When the cat is dead, remove it to a tray, place it on its back, and tie the limbs loosely so that they will remain outspread.

Expose the femoral artery in the thigh, as follows: Make a median longitudinal incision through the skin of the abdomen, from a point an inch or two caudad of the xiphoid process to the pubis. Make an incision passing from near the middle of this obliquely laterad onto the thigh, for about three inches. Reflect the two flaps in the angles between these cuts, then with forceps and tracer isolate the femoral artery just as it passes into the thigh (Fig. 127, _a_). Make with scissors an oblique incision in one side of the artery, one-half to one inch from the point where it leaves the abdominal cavity. Introduce a canula directed toward the body, and tie it in place.

1. If the cat is to be preserved immersed in a solution of formalin, it should now be injected with a five per cent. solution of commercial formalin in ordinary water (commercial formalin 5 parts; water 95 parts). Into an adult cat 300 to 400 cubic centimeters of the five per cent. formalin should be injected.

The cat may now be preserved in a one per cent. solution of formalin. Before immersing it in the formalin it is well to remove the hair in some way, otherwise this will hold much water and be very inconvenient in dissection. The neatest method is to cut off the hair with scissors; this takes some time, however. If the skin-muscles (p. 93) are not to be dissected, the skin may be removed. It is well in this case to leave the skin on the sides of the head and on the feet.

It is not necessary to open the abdominal cavity, after injecting with the five per cent. formalin. If there is at any time reason to suspect that the viscera are not keeping well, the abdominal and thoracic cavities should be filled with five per cent. formalin by making a small hole through the body wall into the cavity, introducing a canula, and injecting till the cavity is full.

The specimen should be kept in a jar or other vessel and should be completely immersed beneath the one per cent. formalin. Any parts projecting above the fluid may be attacked by mould. If a part is found to be moulded, it should be submerged, when the mould will be killed. The mould injures the specimen if allowed to grow for a long time, but if discovered soon, no very serious harm results.

During dissection, exposed parts of the body should be kept from drying by covering with a damp cloth. Specimens preserved in formalin dry out much less quickly than those preserved in alcohol.

2. For some purposes it is much more convenient to preserve the material without immersing it in a vessel of formalin. Specimens injected with five per cent. formalin, as above directed, would usually keep perfectly without further treatment, but for two difficulties: (1) the specimen may _dry_; (2) it may be attacked by mould.

The drying may be prevented as follows: Use for injecting a mixture of the five per cent. formalin with one-sixth its volume of glycerine. This will dry only very slowly, and if pains are taken to keep exposed parts covered with cloths dampened with the mixture of formalin and glycerine, there will be no trouble from this source. The skin should not be removed from such specimens except as necessary in the course of dissection. The hair will give little trouble, because dry.