452 MR. youatt’s veterinary lectures. 
tending over and attached by cellular substance to a considerable 
portion of the side of the thyroid cartilage, and inserted into the 
tubercle at the postero-inferior portion of that cartilage. Its 
office is to raise the thyroid cartilage, and with it the larynx ; or 
to depress the os hyoides, accordingly as we consider the one or 
the other the fixed point. The hyo-epiglottideus I also described 
in that lecture, as being the fraenum, or stay of the epiglottis ; 
extending from the back of the epiglottis to the base of the spur 
of the os hyoides ; sufficiently yielding to suffer the cover of the 
aperture into the larynx to be depressed when the pellet of food 
passes, assisting the inherent elasticity of the cartilage in raising 
it again immediately for the purpose of respiration, and, with 
this exception, keeping the aperture always open that the animal 
may breathe freely. This muscle, you will observe, is bifurcated 
in the ox, and the two hyoideal origins are found near, or 
generally upon, the internal face of the joint uniting the smaller 
cornua: it is also bifurcated in the dog. 
There are, therefore, nine pairs of muscles proper to the larynx, 
and one single muscle, and one pair and a single muscle, common 
to the larynx and the os hyoides. 
The Ne?'ves of the Larynx. —These muscles are abundantly 
supplied with nervous energy from the respiratory system , the 
par vagum, or eighth pair. If they could not be traced by 
anatomical demonstration, their mode of action would abun¬ 
dantly prove their origin; for they are partly under the control of 
the will, and partly involuntary. I am not assured that they 
are not indebted for their power of voluntary motion to their 
frequent anastomoses with the motor nerves of the spinal chord. 
However this may be, while the whole process of respiration is 
partly under the control of the will, the muscles of the larynx 
concerned in one stage of it are likewise so ; but then they act 
independently of the will, for during sleep and unconsciousness 
the machine continues to work. 
Derived from different Sources. —We have seen that, although 
very unequally divided, the muscles of the larynx may be classed 
under two heads, the dilators and the constrictors. They are 
both supplied by the par vagum, but, singularly enough, by 
different branches of it. A branch of the recurrent nerve gives 
some fibres to the external muscles of the thyroid cartilage, if I 
may so term them, and then passes through the foramen, which 
you will observe at the base of the superior cornu of this carti¬ 
lage to be distributed over all the other dilator muscles; but the 
arytenoid muscle, and also the thyro-epiglottideus, are supplied 
by the laryngeal branch of the eighth pair, which passes through 
the same foramen and is distributed over the constrictor muscles. 
