AECHITECTUKE OF THE BONES OF THE SKELETON. 271 



though it should be noted that the two protuberances do not necessarily coincide, the internal 

 being, as a rule, placed at a higher level than the external. If the bone be held up to the light 

 it will be at once apparent that it is much thinner where it forms the floor of the inferior fossae 

 than in the upper part The basilar portion consists of a spongy core surrounded by a more 

 compact outer envelope, thickest on its lower surface. In the condyles the spongy tissue is 

 arranged radially to their convex articular surfaces, the hypoglossal canal being surrounded by 

 particularly dense and compact bone, which assists in strengthening this naturally weak part 

 of the bone. 



The Temporal Bone. The temporal bone is remarkable for the hardness and density of its 

 petrous part, wherein, is lodged the osseous labyrinth, which contains the delicate organs associated 

 with the senses of hearing and equilibration. The middle ear or tympanum is a cavity which 

 contains the small auditory ossicles, and is separated from the external acoustic meatus by the 

 membrana tympani. In front it communicates with the pharynx by the auditory tube ; behind, 

 it opens into the tympanic antrum and mastoid air-cells by the aditus ad antrum. Superiorly, it is 

 separated from the middle cranial fossa by a thin plate of bone called the tegmen tympani. 

 Inferior ly, its floor is formed in part by the roof of the jugular fossa and the carotid canal. 

 Medially, it is related to the structures which form the inner ear, notably the cochlea and 

 vestibule, in front of which it is separated by a thin plate of bone from the carotid canal. 

 Owing to the disposition of the internal and external acoustic meatus the weakest part of the 

 bone corresponds to a line connecting these two channels, the only parts intervening being the 

 cochlea and tympanum. It is usually in this position that fracture of the bone occurs. 

 Curving over the cavity of the tympanum is the canalis facialis, the thin walls of which are 

 occasionally deficient. These details, together with an account of the tympanic ossicles, will 

 be further dealt with in the section devoted to the Organs of Sense. ' 



The Sphenoid Bone. In the adult the body of the bone is hollow and encloses the sphenoidal 

 air-sinuses, usually two in number, separated by a septum. The arrangement and extent of these 

 air- sinuses vary ; sometimes they are multilocular, at other times simple, while occasionally they 

 extend backwards into the basi-occipital and laterally and downwards into the roots of the great 

 wings and pterygoid processes. Cases are on record in which in the adult the body of the bone 

 was not pneumatic. 



The Lacrimal Bone. The bone consists of a thin papery translucent lamina, somewhat 

 strengthened by the addition of the vertical crest. 



The Vomer. The bone is composed of two compact layers fused below, but separated above 

 by the groove for the lodgment of the rostrum of the sphenoid behind, and the septal cartilage in 

 front. The lamellae are also separated from each other by a canal which runs horizontally from be- 

 hind forwards in the substance of the bone, and which transmits the nutrient vessel of the bone. 



The Nasal Bone. Formed of dense and compact bone ; the strength of the nasal bones is 

 increased by their mode of union and the formation of a median crest posteriorly. 



The Maxilla. The disposition of the maxillary sinus within the body of the bone has 

 been already referred to. In union with its fellow, the vaulted arrangement of the hard palate 

 is well displayed, and the arched outline of the alveolar processes is obvious. It is in 

 these latter processes around the sockets for the reception of the teeth that the spongy tissue 

 of the bone is seen ; elsewhere its walls are formed by thin and dense bone. 



The Zygomatic Bone. In structure the bone is compact, with little spongy tissue. 

 Together with the zygomatic process of the temporal bone it forms the buttress which supports 

 the maxilla and the lateral wall of the orbit. Additional strength is imparted to the bone by 

 the angular mode of union of its orbital and facial parts. 



The Mandible. The mandible is remarkable for the density and thickness of its medial and 

 lateral walls. Where these coalesce below at the base of the body, the bone is particularly stout. 

 Superiorly, where they form the walls of the alveoli, they gradually thin, being thicker, 

 however, on the medial than the lateral side, except in the region of the last molar tooth, 

 where the medial wall is the thinner. The spongy substance is open-meshed below, finer 

 and more condensed where it surrounds the alveoli. The mandibular canal is large and 

 has no very definite wall ; it is prolonged beyond the mental foramen to reach the incisor 

 teeth. From it numerous channels pass upwards to the sockets of the teeth, and it com- 

 municates freely with the surrounding spongy tissue. Above the canal the substance of the 

 bone is broken up by the alveoli for the reception of the roots of the teeth. In the substance 

 rf the condyle the spongy tissue is more compact, with a general striation vertical to the 

 irticular surface. 



The mental protuberance is an essentially human characteristic ; by some it is associated with 

 :he development of speech in man, others regard it as due to the reduction in the size of the teeth. 



The Clavicle. The body consists of an outer layer of compact bone, thickest towards the 

 niddle of the shaft, but gradually thinning towards the extremities, the investing envelope of 

 ^hich consists merely of a thin shell. Within the body the spongy tissue displays a longitudinal 

 itriation, which internally assumes a more cellular appearance. At the acromial end the general 

 irrangement of the fibres resembles the appearance of the sides of a Gothic arch. The curves 

 >f the bone impart an elasticity to it, which is of much service in reducing the effects of the 

 hocks to which it is so frequently subjected. 



The Scapula. For so light and thin a bone, the scapula possesses a remarkable rigidity. This 

 .3 owing to the arrangement of its parts. Stout and thick where it supports the glenoid cavity 



