THE TEMPORAL BONES.

Borders.-The superior border is thin, bevelled at the expense of the internal surface, so as .to overlap the lower border of the parietal bone, forming the squam¬ous suture. The anterior inferior border is thick, serrated, and bevelled, alter¬nately at the expense of the inner and outer surfaces, for articulation with the great wing of the sphenoid.

. The Mastoid Portion  is situated at the posterior part of the bone; its outer surface is rough, and gives attachment to the Occipito-frontalis and Retrahens aurem muscles. It is perforated by numerous foramina; one of these, of large size, situated at the posterior border of the bone, is termed the mastoid foramen; it transmits a vein to the lateral sinus and a small artery from the occipital to supply the dura mater. The position and size of this foramen are very variable. It is not always present; sometimes it is situated in the occipital bone or in the suture between the temporal and the occipital. The mastoid portion is continued below into a conical projection, the mastoid process, the size and form of which vary somewhat. This process serves for the attachment of the Sterno-mastoid, ~Jllenius capitis, and 'rl'achelo-mastoid muscles. On the inner side of the mastoid process is a deep groove, the digastric fossa, for the attachment of the Diga"tric muscle; and, running parallel with it, but more in¬ternal, the occipital gronee. which lodges the occipital artery. The internal surface of the mastoid portion presents a deep, curved groove

. The groove for the lateral sinus is separated from the innermost of the mastoid air-cells by only a thin lamina of bone, and even this may be partly deficient. A section of the mastoid process shows it to be hollowed out into a number of cellular spaces, communicating with each other. called the mastoid cells, which exhibit the greatest possible variety us to their size and number.

THE INTERNAL EAR.

The bony canal of the cochlea  takes two turns and three-quarters round the modiolus. It is a little over an inch in length (about 30 mm.), and
diminishes gradually in size from the base to the summit, where it terminates in a cul-de-eac, the cupola, which forms the apex of the cochlea. The commence¬ment of this canal is about the tenth of an inch in diameter: it diverges from the modiolus toward the tympanum and vestibule, and presents three openings. One, the fenestra rotunda, communicates with the tympanum; in the recent state this aperture is closed by a membrane, the membrana tympani secundaria. Another aperture, of an elliptical form, enters the vestibule. The third is the aperture of the aqueeductus cochle~, leading to a minute funnel-shaped canal, which opens on the basilar surface of the petrous bone and transmits a small vein, and also forms a communication between the subarachnoidean space of the skull and the perilymph contained in the scala tympani.


The lamina spiralis ossea is a bony shelf or ledge which projects outward from
the modiolus into the interior of the spiral canal, and, like the canal, takes two and three-quarter turns round the modiolus. It reaches about half-way toward the outer wall of the spiral tube, and partially di "ides its cavity into two passages or scalee, of which the upper is named the scala vestibuli, while the lower is termed the scala tlj1nlJani. Near the summi t of the cochlea the lamina terminates in a hook-shaped •process, the hamulus, which assists to form the boundary of a small opening, the helicotrema, by which the two scalse communicate with each other. From the canalis spiralis modioli numerous foramina pass outward through the osseous spiral lamina as far as its outer or free edge. In the lower part of the first

THE SACRAL PLEXUS.

The femoral cutaneous branches (descending) are numerous filaments, derived from both sides of the nerves, which are distributed to the back, inner, and outer sides of the thigh, to the skin covering the popliteal space, and to the upper part
of the leg.
The Perforating Cutaneous Nerve usually arises from the second and third sacral
nerves, and is of small size. It is continued backward through the great sacro¬sciatic ligament, and, winding round the lower border of the Gluteus maximus, supplies the integument covering the inner and lower part of that muscle.
The Pudic Nerve is the direct continuation of the lower cord of the sacral plexus, and derives its fibres from the third and fourth sacral nerves,and frequently from the second also. It leaves the pelvis through the great sacro-sciatic foramen, below the Pyriformis. It then crosses the spine of the ischium, and re-enters the pelvis through the lesser sacro-sciatic foramen. It accompanies the pudic vessels upward and forward along the outer wall of the ischio-rectal fossa, being contained in a sheath of the obturator fascia, termed Alcoclc's canal, and divides into two terminal branches. the perineal nerve and the dorsal nerve of the penis or clitoris. Before its division it gi ves off the inferior hemorrhoidal nerve.
The inferior hemorrhoidal nerve is occnsionally derived separately from the sacral plexus. It passes across the iscbio-rectal fossa, with its accompanying vessels, toward the lower end of the rectum, and is distributed to the Sphincter ani externus and to the integument round the anus. Branches of this nerve com¬municate with the inferior pudendal and superficial perineal nerves at the fore part
of the perineum.
The perineal nerve, the inferior and larger of the two terminal branches of the
pudic, is situated below the pudic artery. It accompanies the superficial perineal artery in the perineum, dividing into cutaneous and muscular branches.

THE DORSAL NERVES.

They pass forward in the intercostal spaces with the intercostal vessels, being situated below them. At the back of the chest they lie between the pleura and the External intercostal muscle, but are soon placed between the two planes of Intercostal muscles as far as the middle of the rib. They then enter the substance of the Internal intercostal muscles, and, running amidst their fibres as far as the costal cartilages, they gain the inner surface of the muscles and lie between them and the pleura. Near the sternum. they cross in front of the internal mam¬mary artery and Triangularis sterni muscle. pierce the Internal intercostal muscles, the anterior intercostal membrane. and Pectoralis major muscle, and supply the integument of the front of the chest and over the mammary gland, forming the anterior cutaneous nerves of the thorax; the branch from the second nerve is joined with the eupraclavicular nerves of the cervical plexus.

Branches.-Numerous slender muscular filaments supply the Intercostals, the Infracostales, the Levatores costarum, Serratus posticus superior, and Triangularis sterni muscles. Some of these branches, at the front of the chest, cross the costal cartilages from one to another intercostal space.

Lateral Cutaneous .cVerves.-These are derived from the intercostal nerves, midway between the vertebras and sternum; they pierce the External intercostal and Serratus magnus muscles, and divide into two branches, anterior and posterior.

The anterior branches are reflected forward to the side and the fore part of the chest, supplying the integument of the chest and mamma; those of the fifth and sixth nerves supply the upper digitations of the External oblique.

The posterior branches are reflected backward to supply the integument over the scapula and over the Latissimus dorsi.

The lateral cutaneous branch of the second intercostal nerve is of large size, and does not divide, like the other nerves, into an anterior and posterior branch. It is named, from its origin and distribution, the intercosto-humeral nerve (Fig. 413). It pierces the External intercostal muscle, crosses the axilla to the inner side of the arm, and joins with a filament from the nerve of Wrisberg. It then pierces the fascia, and supplies the skin of the upper half of the inner and back part of the arm, communicating with the internal cutaneous branch of the musculo-spiral nerve. The size of this nerve is in inverse proportion to the size of the other cutaneous nerves, especially the nerve of W risberg. A second intercosto-humeral nerve is frequently given off

THE SPHENOID BONE.

The Lesser Wings (proces8es of Ingrassias) are two thin, triangular plates of bone which arise from the upper and lateral parts of the body of the sphenoid, and, projecting transversely outward, terminate in a sharp point (Fig. 37). The superior surface of each is smooth, flat, broader internally than externally, and supports part of the frontal lobe of the brain. The inferior surface forms the¬back part of the roof of the orbit and the upper boundary of the sphenoidal fissure or foramen lacerum anterius. This fissure is of a triangular form, and leads from the cavity of the cr&nium into the orbit; it is bounded internally by the body of the sphenoid-above, by the lesser wing; below, by the internal margin of the orbital surface of the great wing-and is converted into a foramen by the articu¬lation of this bone with the frontal. It transmits the third, the fourth, the three branches of the ophthalmic division of the fifth, the sixth nerve, some filaments from the cavernous plexus of the sympathetic, the orbital branch of the middle meningeal artery, a recurrent branch from the lachrymal artery to the dura mater, and the ophthalmic vein. The anterior border of the lesser wing is ser¬rated for articulation with the frontal bone; the posterior, smooth and rounded, is received into the fissure of Sylvius of the brain. The inner extremity of this border forms the anterior clinoid process.• The lesser wing is connected to the side of the body by two roots, the upper thin and flat, the lower thicker, obliquely directed, and presenting on its outer side, near its junction with the body, a smail tubercle, for the attachment of the common tendon of origin of three of the muscles of the eye. Between the two roots is the optic foramen, for the transmission of

The Pterygoid Processes (rrripu;, a wing; 0100" likeness), one on each side, descend perpendicularly from the point where the body and greater wing unite (Fig. 39). Each process consists of an external and an internal plate, which are joined together by their anterior borders above, but are separated below, leaving an angular cleft, the pterygoid notch, in which the pterygoid process or tuberosity of the palate bone is received. 'I'he two plates diverge from each other from their line of connection in front, so as to form a V-shaped fossa, the pterygoid f08sa. The external pterygoid plate is broad and thin, turned a little outward, and, by its outer surface, forms part of the inner wall of the zygomatic fossa, giving attachment to the External pterygoid; its inner surface forms part 

 

THE SPINE IN GENERAL.

The lateral surfaces are separated from the posterior by the articular processes in the cervical and lumbar regions, and by the transverse processes in the dorsal. These surfaces present in front the sides of the bodies of the vertebral, marked in the dorsal region by the facets for articulation with the heads of the .ribs. More posteriorly are the intervertebral foramina, formed by the juxtaposition of the intervertebral notches, oval in shape, smallest in the cervical and upper part of the dorsal regions, and gradually increasing in size to the last lumbar.

 They arc situated between the transverse processes in the neck, and in front of them in' the back and loins, and transmit the spinal nerves.
The base of that portion of the vertebral column formed by the twenty-four movable vertebral is formed by the under surface of the body of the fifth lumbar vertebra; and the summit by the upper surface of the atlas.
The vertebral or spinal ca.nal follows the different curves of the spine; it is largest in those regions in which the spine enjoys the greatest freedom of move¬ment, as in the neck and loins, where it is wide and triangular; and narrow and rounded in the back, where motion is more limited.

THE •PERICARDIUM.

The serOU8 layer invests the heart, and is then reflected on the inner surface of the pericardium. It consists, therefore, of a visceral and parietal portion. The former invests the surface of the heart, and the commencement of the great vessels,
. to the extent of an inch and a half from their origin; from these it is reflected upon the inner surface of the fibrous layer, lining, below, the upper surface of the central tendon of the Diaphragm. The serous membrane encloses the aorta and pulmonary artery in a single tube, so that a passage, termed the transverse sinus of the pericardium, exists between these vessels in front and the auricles behind.

The membrane only partially covers the superior vena cava and the four pulmonary veins, and scarcely covers the inferior cava, as this vessel enters the heart almost directly after it has passed through the Diaphragm. Its inner surface is smooth and glistening, and secretes a serous fluid, which serves to facilitate the movements
of the heart.
Arteries of the Pericardium.-These are derived from the internal mammary
and its musculo-phrenic branch, and from the descending thoracic aorta.
Nerves of the Perieardium.-These are branches from the vagus, the phrenic,
and the sympathetic.