876 THE NERVOUS SYSTEM 



VIII. CONDUCTION PATHS OF OLFACTORY APPARATUS 



1 Bipolar cells of olfactory region of nasal epithelium send short (peripheral) processes 

 towards surface of nasal cavity and centrally directed processes, the olfactory nerve, 

 through lamina cribrosa of ethmoid bone into olfactory bulb (glomerular layer). 



2. 'Mitral cells' of olfactory bulb give fibres which form 



(a) The olfactory tract which divides into 



(6) Medial olfactory stria through which fibres pass (1) into parolfactory area (Broca's 

 area); (2) into subcallosal gyrus; and (3) by way of anterior cerebral commis- 

 sure to olfactory bulb of opposite side. 



(c) Intermediate olfactory stria to anterior perforated substance. 



(d) Lateral olfactory stria, which terminates to some extent in anterior perforated 

 substance, but, chiefly in uncus, hippocampal gyrus, and gyrus cinguli (olfactory 

 area). 



3. Cells of uncus and hippocampal gyrus give fibres which form 



(a) The cingulum (in part), by which they are associated with the cortex of the gyrus 

 cinguli, the subcallosal gyrus, and the anterior perforated substance. 



(6) The hippocampal commissure (in part), by which they are connected with the grey 

 substance of the opposite side. 



(c) The fornix, which, interrupted in part in the nuclei of the corpus mammillare, 

 conveys impulses (1) to the anterior nucleus of thalamusof the same and opposite 

 sides (thalamo-mammillary fasciculus), and (2) into the cerebral peduncle and 

 substantia nigra (pedunculo-mammillary fasciculus), and by way of this peduncle 

 probably to the nuclei of the mesencephalon and medulla oblongata. 



THE RELATIONS OF THE BRAIN TO THE WALLS OF THE CRANIAL CAVITY 



The precise methods by which the exact positions of the most important fissures, sulci, 

 gyri, and areas can be ascertained and mapped out on the surface of the head in the living subject 

 are fully described in Section XIII. Here, only a very general survey of the relations of the 

 brain to the cranial bones is given and from a purely anatomical standpoint. 



The parts of the brain which lie in closest relation with the walls of the cranial cavity are 

 the olfactory bulb and tract, the basal and lateral surfaces of the cerebral hemispheres, the inferior 

 surfaces of the lateral lobes of the cerebellum, the ventral surfaces of the medulla and pons, and 

 the hypophysis. 



Certain of these portions of the brain lie in relation with the basi-cramal axis, that is, with 

 the basi-occipital, the basi-sphenoid, and the ethmoid bones, while others are associated with the 

 sides and vault of the cranial cavity. Considering the former portions first, the ventral surface 

 of the medulla oblongata, which is formed by the pyramids, lies upon the upper surface 

 of the basi-occipital bone. More superiorly the ventral surface of the pons rests upon the basi- 

 sphenoid, from which it is partly separated by the basilar artery and the sixth pair of cranial 

 nerves. In front of the dorsum sella? the hypophysis (pituitary body) is lodged in the pituitary 

 fossa. Still further forwards the olfactory tracts lie in grooves on the upper surface of the pre- 

 sphenoid section of the sphenoid bone; and in front of the sphenoid the olfactory bulbs rest upon 

 the cribriform plates of the ethmoid. 



Behind and lateral to the posterior part of the foramen magnum the lateral lobes of the cerebel- 

 lum are in relation with the cranial wall, resting upon the lower parts of the supra-occipital and 

 the posterior parts of the ex-occipital portions of the occipital bone, while anteriorly each lobe is 

 in relation with the inner surface of the mastoid process and the posterior surface of the petrous 

 portion of the temporal bone. The area of the skull wall which is in close relationship with the 

 cerebellar hemispheres may be indicated, on the external surface of the skull, by a line which com- 

 mences at the lower part of the external occipital protuberance and thence runs upwards and 

 outwards. It crosses the superior curved line a little beyond its centre, and, continuing in the 

 same direction, crosses the lower part of the lambdoid suture and reaches a point directly above 

 the asterion (the meeting-point of the occipital, temporal, and parietal bones) ; thence it descends, 

 just in front of the occipito-mastoid suture, to the tip of the mastoid process, and there turns 

 inwards to its termination at the margin of the foramen magnum, immediately behind the 

 posterior end of the occipital condyle. 



The basal surface of each cerebral hemisphere may be said to consist of two parts, an anterior 

 and a posterior, separated by the stem of the lateral fissure. The anterior part, formed by the orbi- 

 tal surface of the frontal lobe, rests upon the upper surfaces of the orbital plate of the frontal 

 bone and the lesser wing of the sphenoid. It is, therefore, in close relat ion with the upper wall of 

 the orbital cavity. The posterior part, behind the stem of the lateral fissure, is formed by the 

 anterior portion of the temporal lobe, including its apex. The apex itself projects against the orbital 

 plate of the great wing of the sphenoid bone, and it is in relationship with the posterior part of the 

 outer wall of the orbit. The basal surface of the hemisphere, behind the apex of the temporal 

 lobe, is in contact with the upper surfaces of the great wing of the sphenoid and the petrous part 

 of the temporal bone. 



The convex surfaces of the cerebral hemispheres have the most extensive relationships with 

 the cranial wall, and it is more especially to these surfaces that the practical surgeon turns his 

 attention. The general area in which the convex surface of each cerebral hemisphere is in relation 

 with the skull bones is readily indicated by a series of lines which correspond with the positions 

 of its superciliary, infero-lateral, and supero-mesial borders. 



The line marking the superciliary margin of the hemisphere commences at the nasion (the 

 mid-point of the fronto-nasal suture); it passes outwards above the superciliary ridge, crosses 

 the temporal ridge, then, turning backwards in the temporal fossa, it reaches the parieto-sphe- 

 noidal suture, and continues backwards along it to its posterior extremity. 



