821 



SKELETON. 



SKELETON. 



623 



Behind the meatus auditorius is the Mastoid portion. It is pro- 

 longed downwards in a strong conical projection, the mastoid process 

 (7) giving insertion to muscles upon and just above it, and of which 

 the interior is occupied by mynerous cells communicating with the 

 cavity of the tympanum. Behind and within the maatoid process is 

 the digastric groove, to which the muscle of the same name is attached ; 

 and farther back another more shallow groove for the trachelo-mastoid 

 muscle. 



The cerebral surface of the Squamous Portion has a very obliquely 

 cut and grooved upper border, which articulates with the lower border 

 of the parietal bone. On the same surface of the mastoid portion is 

 a deep fossa, which lodges part of the lateral sinus. Both are marked 

 by the impressions of the brain. 



The Petrous Process or portion of the temporal bone (fig. 11, c) has 

 received its namo from the peculiar hardness of its tissue. It has the 

 form of an irregular three-sided pyramid, directed from either side 

 forwards and inwards, and fitting, at the base of the skull, into the 

 angle left between the sphenoid and the occipital bones (e and d). Its 

 base is affixed to the interior and lower part of the squamous bone; 

 its summit fits in the apes of the angle just mentioned. On its 

 posterior surface the most prominent object is the oval aperture of 

 the meatus auditorius intemus, the passage leading to the internal 

 ear, and traversed by the auditory and the facial nerves. On the 

 anterior surface there are a shallow groove leading to a small hole, 

 through which the Vidian nerve and bloodvessels pass, a slight hollow 

 on which the Gasserian ganglion of the fifth pair of nerves lies, and a 

 prominence which indicates the position of the superior semicircular 

 canal of the ear. On the inferior surface, which is placed outaide the 

 skull, there are seen, at the posterior and outer border, a deep fossa 

 (the Jugular), in which the upper part of the internal jugular vein is 

 lodged ; before and on the inner side of this, and separated from it 

 by a prominent ridge, a large oval aperture, through which the 

 internal carotoid artery passes into a tortuous canal, whose other 

 extremity is at the very apex of the bone ; between the jugular fossa 

 and the mastoid process a hole, the Stylo-Mastoid Foramen, through 

 which the facial nerve passes on its way to the face, after penetrating 

 the bottom of the meatus auditorius internus ; and just anterior to 

 this, a long pointed process, the Styloid (8), to which several muscles 

 and ligaments are attached, and whose base is surrounded by an 

 irregular sharp-edged elevation, the Vaginal Process (9). 



The anterior border of the Petrous Bone is articulated with the 

 posterior part of the ala of the sphenoid, leaving an intermediate 

 space, named Foramen Lacerum Medium ; the posterior border is 

 similarly united with the side of the basilar process of the occipital 

 bone, leaving another space, the Foramen Lacerum Posterius, through 

 which the internal jugular vein and the nerves of the eighth pair 

 pass. Near the angle where the anterior border joins the squamous 

 bone is an irregularly-shaped aperture, to which the cartilaginous part 

 of the Kustachian tube is affixed. 



The small bonea of the internal ears, and all the other parts of the 

 organ of hearing, which lie within and near the petrous bone, are 

 described in the article EAR. 



The Occipital Bone (fig. 15 is a view of the internal surface) forms 

 the posterior and lower part of the middle of the skull, a portion 

 being at the outer wall and a portion at the base. Its lower and 



4 



4 



anterior part is narrow, and has a rough surface (1) in front, which is 

 united with the body of the sphenoid bone. Viewing it at its internal 

 surface, it presents, as one proceeds from this surface backwards and 

 upwards, a smooth hollow surface, which gradually widens, and is 

 limited behind by a large oval opening. The surface 2 is that of the 

 Basilar Process, in which the medulla oblongata and pons varolii 

 [URAIJI] rest; the aperture 3 is the Foramen Magnum, through which 

 the medulla passes into the spinal canal, where it is continued into 

 the spinal cord. By the sides of this foramen, near where the basilar 

 procera joins the buck and expanded part of the bone, there are four 

 foramina, two on either side the anterior and posterior Condyloid 



Foramina, of which the anterior transmit the hypoglossal nerves, on 

 which the motions of the tongue depend, and the latter give passage 

 to veins communicating with the vertebral veins. Opposite the fore 

 part of the foramen magnum the basilar process suddenly widens into 

 the greater portion of the occipital boue, which forma the back of the 

 head. In this part are four large hollows (4, 4, 5, 5), of which the 

 two upper lodge the surfaces of the posterior lobes of the cerebrum, 

 the two lower those of the lobes of the cerebellum. They are 

 separated by two ridges, which bisect each other at nearly right 

 angles. The upper part of that which runs vertically has attached 

 to it a portion of the falx major, and to its lower part is affixed the 

 falx cerebelli ; that which runs transversely gives insertion to the 

 back part of the tentorium cerebelli, whose anterior borders are fixed 

 to the upper angles of the petrous bone. By these ridges are broad 

 shallow grooves, which lodge parts of the sinuses of the brain. By 

 the upper half of the vertical ridge is the extremity of the longitudinal 

 sinus, the grooves for which, in the frontal and parietal bones, have been 

 already mentioned, and which, at the internal occipital spine, where 

 the ridges bisect each other, meets the inferior longitudinal and other 

 sinuses, to form what is named the Torcular Herophili, their common 

 point of meeting. From this there proceed the two lateral sinuses, 

 which run above the transverse ridge on either side, then cross over 

 the posterior inferior angle of each of the parietal bonea, then lie for 

 a short distance on the inside of the mastoid portion of the temporal, 

 from which they pass through the foramen lacerum posterius by a 

 special aperture, marked by a deep notch in the border of the occi- 

 pital bone, near the angle (6), which separates the baailar from the 

 other portion. 



The inferior and outer surface presents on the Basilar Process 

 numerous irregularities, from which the back part of the pharynx ia 

 suspended, and into which certain muscles and ligaments of the front 

 of the apine are inaerted. The Foramen Magnum has here an even 

 and grounded border ; and by its sides two elevations, each with a 

 smooth convex oval surface, whose larger axis is directed forwards, 

 inwards, and downwards ; these are the Condylea, by which the 

 occipital bone articulates moveably with the first vertebra of the 

 spine. Near these also are the outer orifices of the anterior and 

 posterior condyloid foramina, and around them very rough surfaces 

 for the insertion of ligamenta and muscles. On the outer surface of 

 the expanded posterior portion of the bone are three ridges, one of 

 which passes from the border of the foramen magnum backwards and 

 upwards in correspondence with the internal vertical ridge, and is 

 crossed on its way by two transverse arched ridges. At the crossing 

 of the upper of these two ia a sharp prominence, the occipital spine or 

 protuberance. The two transverse ridges and the spaces below them 

 give attachment to muscles; the spine, to the ligamentuin nuchje. 

 Above the upper ridge the surface ia smooth. 



The upper and lateral borders (7) of the Occipital Bone are deeply 

 toothed, and form the Lambdoidal Suture, with the parietal bones 

 above and the mastoid below. In the course of this suture there 

 occur, more often than in that of any other, insulated portions of bone, 

 of various size and form, called Ossa Wormiana, surrounded by margins 

 toothed as in the regular line of suture. 



The Sphenoid Bone (fig. 11, e) is placed in the middle of the base of 

 the skull, and has a very complicate form. Fig. 16, a, gives a front, 



Fig. 16. 



and 5, a back and upper view of it. Its principal parts are described 

 as a body (1, 1), two Greater Ala; (2, 2), two Lesser Alic (b, 3, 3), and, 

 on each side, two Pterygoid Processes (a, 4, 4). The body is the 

 central part, and has somewhat the form of a hollow cube. The 

 chief part of its upper or cerebral surface is hollowed, forming what 

 is called the Sella Tureica (i, 4), and lodging the pituitary gland. 

 [BRAIN.] It is bounded at its four corners by bluntly pointed pro- 

 minences called Clinoid Processes (see Jiff. 11), to which prolongations 

 of dura mater are attached. Between, and a little in front of the two 

 anterior of these, ia a level surface (b, 5) on which the commissure of 

 the optio nerves rests, and which has behind a slight elevation, the 



