THE SPINAL MEDULLA. 



517 



excrescences the cerebral hemispheres are budded off from it ; and they become 

 the dominant part of the nervous system (Fig. 458). 



Each hemisphere is formed, however, from a relatively small part of the 

 side wall of the prosencephalon, the rest of which goes to form the optic 



I diverticula, the thalamus, and the hypothalamus, among other structures. The 

 cerebral hemisphere is at first pre-eminently olfactory in function, the nerves 

 of smell being inserted directly into it. But impressions of the associated sense 



: of taste make their way into the cerebral hemisphere in the most primitive 

 vertebrates : the gustatory nerves are inserted into the medulla oblongata, but 

 fibre-paths are laid down to establish connexions with the hypothalamus, which in 

 turn emits fibres to the cerebral hemisphere (Fig. 454). The thalamus is a greatly 

 swollen part of the prosencephalic wall adjoining the mesencephalon. Its main 

 part receives sensory impressions brought up from the spinal medulla and the 

 terminal nuclei of the sensory cerebral nerves and transmits them to the cerebral 

 hemisphere. Its caudal portion becomes specialised as a special receptive nucleus 

 for visual and acoustic impressions for transmission to the cerebral hemisphere. 

 It is called the metathalamus or corpora geniculata. Thus the cerebral hemi- 

 sphere from being essentially a receptive organ for smell impressions ultimately 

 becomes the terminus of all the sensory paths, and the structure that is concerned 

 with the consciousness of all kinds of sensations. It also controls the voluntary 

 movements of one-half of the body and emits a great strand of fibres pedunculus 

 cerebri to establish relations with the cerebellum and all the motor nuclei on 

 the other side of the encephalon and spinal medulla (Fig. 454, p. 513). 



MEDULLA SPINALIS. 



The spinal medulla is that part of the central nervous system which 

 occupies the upper two-thirds of the vertebral canal. It is an elongated 

 cylindrical structure, slightly flattened in front 

 and behind, which extends from the margin of 

 the foramen magnum to the level of the inferior 

 .border of the body of the first lumbar vertebra or 

 to the superior border of the body of the second 

 lumbar vertebra. Its average length in the male 

 is 45 cm. and in the female 43 cm. 



Lumbar swellipg 

 of the spinal 

 medulla 



A considerable amount of variation within certain 

 'limits (viz., the mid-point of the body of the last 



thoracic vertebra and the superior border of the body 

 : )f the third lumbar vertebra) is observed in different 



individuals as to the precise level at which the spinal 

 :oied ulla ends inferiorly, and in the female there 

 ,^ould appear to be a tendency for the medulla to 

 i reach a slightly lower point in the canal than in the 

 tmle. Further, the relation presented by the spinal 



medulla to the vertebral column differs in a marked 

 , legree in the foetus and infant at different periods of 



ievelopment. Up to the third month of intra-uterine 

 ( .ife the spinal medulla occupies the entire length of 



:he vertebral canal ; it extends downwards to the 



Lowest limit of the vertebral canal. But from this time 



>n wards, as growth proceeds, the vertebral column 



.engthens at a more rapid rate than the medulla. The 



spinal medulla, therefore, has the appearance of shrink- 

 ; in an upward direction within its canal, and at 

 rth its inferior end is usually found to be opposite FIG. 459. HUMAN F(ETUS IN THE THIRD 



:he body of the third lumbar vertebra. MONTH OF DEVELOPMENT, WITH THE 



The attitude assumed by the individual affects to BRAIN AND SPINAL ME DULLA EXPOSED 

 small degree the position of the inferior end of the 



spinal medulla. Thus, when the trunk is bent well forwards, the terminal part of the 



pmal medulla rises slightly within its bony canal. 



Cerebral 

 hemisphere 



Mesencephalon 



Cerebellum 

 Fourth ventricle 



Medulla 

 oblongata 



Cervical swelling 

 of the spinal 

 medulla 



