EESTEVEN ON THE ANATOMY OF THE SPINAL COBD. 387 



like the vagus, with the anterior grey substance of the medulla. 

 These extensive and intimate connexions seem to afford an explana- 

 tion of the mechanism by which impressions made on the vagus and 

 on the incident fibres of the trifacial and spinal nerves, may call 

 into action the whole class of respiratory muscles ; and if the tract 

 just described in the upper part of the cervical region be continu- 

 ous, as it probably is, with the tractus intermedio- lateralis, which 

 is reached by the dorsal nerves supplying the intercostal and other 

 respiratory muscles of the trunk, the explanation in question wiU 

 be stni more complete. 



The tractus intermedio-lateralis is larger at the upper part than 

 in the middle of the dorsal region. On the one hand it projects 

 further into the lateral columns, and on the other tapers inwards, 

 across the grey substance, to near the front of the vesicidar cylinder. 

 As it ascends through the cervical enlargement it gradually disap- 

 pears. In the region of the upper cervical nerves there appears a 

 vesicular tract in the same position as the tractus intermedio-lateralis, 

 composed of the same kind of cells, which send their processes out- 

 wards through the lateral column, and inwards to join the fibres of 

 the transverse commissure behind the central canal. It is traversed 

 by the fibres of the spinal accessory nerve as they bend forwards on 

 their way to the anterior cornu. (fig. 3.) 



To insure accuracy of description Mr. Clarke has further divided 

 each posterior cornu into its caput and its cervix. Tlie capict is its 

 broad expanded extremity, the cervix is the remainder of the cornu 

 as far forwards as the imaginary line above described. This dis- 

 tinction is founded on the facts : 1. That the caput differs in struc- 

 ture from the cervix ; and 2. that in the medulla oblongata it is 

 thrown aside from the cervix and after being traversed in succession 

 by the roots of the vagus and glossopharyngeal nerves, becomes the 

 principal nucleus of the trifacial. 



The caput cornu posterioris consists of, 1. an outer and compara- 

 tively transparent portion — the gelatinous substance, and 2, the inner, 

 more opaque portion, or base. 



The gelatinous substance (fig. 4. g.) consists of, A. Nerve fibres. 

 B. Nerve cells. G. Blood-vessels. D. Connective tissue. 



A. The nerve fibres are transverse, longitudinal and oblique. Tlie 

 transverse fibres run out through the posterior border of the gela- 

 tinous substance through the posterior columns towards the 

 posterior fissure, to form the posterior roots of nerves. The 

 primitive fibres composing these bundles are not grey fibres, but 

 tubules of small average size, the larger possessing double con- 

 tours. They vary from y^^^o to -sToTo^^^ ^^ ^^ mch. in diameter, 

 intermixed with some of ^i^q-q of an inch within the posterior 

 border of the gelatinous substance. The oblique fibres are inter- 

 mediate between the transverse and horizontal, of which they are 

 continuations, as well as with some fibres of the posterior roots. 



B. Nerve cells — these vary much both in shape and size. They are 



