22 TOPOGRAPHIC AND APPLIED ANATOMY. 



PLATE 1. 



The base of the skull seen from within. The various bones are indicated by different colors. Upon the left side 

 the places of exit of the cranial nerves are colored yellow, the places of entrance of the most important arteries red, 

 and the foramina for the chief veins blue. Upon the right side the weak areas at the base are outlined in red. 



cavity through the posterior compartment of the jugular foramen (designated in the illustration 

 by blue, 12). 



Our attention should also be directed to the emissary foramina (Santorini), which are aper- 

 tures in the cranium giving passage to venous connections between the external veins of the head 

 and the cerebral sinuses. The blood in the emissary veins passes from within outward, so that 

 these veins relieve the cerebral sinuses, particularly when the intracranial blood-pressure is in- 

 creased. This explains the rationale of the withdrawal of blood from the scalp for the purpose 

 of relieving intracranial congestion (for example, behind the ear in the region of the mastoid 

 emissary). [Through them infection is sometimes carried from the surface as in the case of 

 erysipelas and other inflammatory diseases of the scalp to the sinuses, causing thrombosis 

 or meningitis. ED.] The emissary foramina are subject to very great variations. We dis- 

 tinguish: (i) The parietal foramen to either side of the sagittal suture in a line connecting the 

 parietal eminences. It leads to the superior longitudinal sinus and may be absent upon one 

 or both sides. (2) The mastoid foramen for the vein connecting the occipital veins with the lateral 

 sinus (it also transmits a meningeal branch of the occipital artery to the dura mater)-. This 

 foramen is above the base of the mastoid process, two finger-breadths behind the external 

 auditory meatus, and is the largest and most constant of all the emissary foramina (see Plate i, 

 14). When the jugular foramen, the normal point of exit for the venous blood, is narrowed (as 

 in a rachitic closure of this foramen, for example) the mastoid foramen is so much enlarged that 

 it is sufficient to allow the venous blood to escape from the cranial cavity. (3) The posterior 

 condyloid foramen, in the condyloid portion of the occipital bone, connects the deep veins of the 

 neck with the lateral sinus (see Plate i, 13). (4) The occipital foramen in the external occipital 

 protuberance. This is insignificant and frequently replaced by numerous small foramina which 

 transmit connections between the occipital veins and the torcular Herophili. 



The practical study of the base of the skull with the foramina for the passage of the nerves 

 and vessels is important not only on account of the neuralgia and paralyses following bone-disease, 

 but also in the diagnosis of basal fractures, intracranial tumors, and the extension of disease of the 

 middle ear to the cranial cavity. In view of the fact that the modern surgeon operates upon the 

 base of the skull from within [as well as without. ED.] the cranial cavity, an exact knowledge 

 of the topography of this region is most essential. 



Those portions of the base which are thin or relatively weak, on account of the presence of 

 numerous foramina, are particularly predisposed to fractures and injuries in general. They are 

 indicated in Plate i by dotted lines and are found in the following situations. In the anterior 

 cranial fossa : The cribriform plate of the ethmoid bone and the orbital plate of the frontal bone 

 (a), particularly in the depressions which lodge the frontal convolutions. In the middle cranial 

 fossa: The base of the great wing of the sphenoid with the foramina rotundum, ovale, and 

 spinosum ; over the glenoid fossa of the temporal bone (c~) ; the tegmen tympani (d ) or roof of 

 the middle ear (see page 59) and (very important from a practical standpoint) the squamous por- 



