Even in the infant, however, the urethra is more capacious than is generally imagined, and will admit a much larger sound than we should ri priori suppose ; a fact of no small im portance in sounding at this early period of life. When the penis is erect, the urethra is diminished in diameter, from the pressure of the turgid veins of the spongy body, and from the increased distension of its own blood vessels.
To ascertain the diameter of the urethra, and to compare it in persons of different ages, Sir E. Home examined the canal in two persons—one of the age of 80, and the other 30.
Briggs directed his attention to the dia meter of the urethra in the various parts of its course, and he found the dimensions ma terially altered if the urethra be injected with wax or any other substance ; and the result of his examination throws considerable doubt on the conclusions of Sir Everard Home. From the casts which he made he failed to discover any sudden narrowing or constric tion at the termination of the membranous part of the urethra, or any resemblance in the shape of the curve as represented in Home's plates.f Briggs remarks, " The portion of the urethra which extends from the apex of the prostate forwards to a short distance beyond the arch of the pubis, and in the natural state is the narrowest part of it, when distended, greatly exceeds the rest of the canal in its dimensions, and forms a large oblong sinus from II to 1,4 of an inch in length ; and in its transverse dia meter at its broadest part, from to I-z-ths of an inch, the part of the urethra anterior to it not exceeding of an inch. The broadest part of this sinus lies directly under the arch of the symphysis pubis. The narrow part of the canal, as seen in these injections, is at the point of union between the prostatic and membranous portions." The irregularities in the form of the urethra here noticed do not appear to exist at the earlier periods of life. In a cast of the urethra of a boy 11 years old, made by injecting wax, no inequalities such as those mentioned above in the adult were observable throughout its course, the diameter of the cast, which is nearly cylindrical, measuring pretty uniformly *th of an inch.
Briggs found the curve of the urethra to commence 1 inch anterior to the bulb ; and from this point to its termination in the bladder, to form an arc of a circle of 31 inches in diameter, the chord of the arc being 21 inches, or rather less than one third of the circumference. In another cast the chord of the segment was found to measure inches of a circle of 38 inches in diameter, the inclination of the internal orifice, or en trance into the bladder, forming an obtuse angle with the general course of the urethra.
The same surgeon remarks, that in young subjects the posterior portion rises nearly at right angles from the rest of the canal, and consequently makes a much sharper bend ; and that this ascending portion is com paratively longer than in the adult, as was observed by Camper, who justly attributes the circumstances to the higher position of the bladder in early age. A similar remark was made by Bichat ; and the fact is well known to the lithotomist, who, in directing the cutting instrument into the bladder of the child, raises the point by depressing the handle at this important stage of the operation.
According to Briggs, the most depending part of the curve is at the point where the membranous portion would be intersected by a line drawn through the longitudinal axis of the symphysis pubis to the anus : this would divide the membranous part into two equal parts, and pass through the most dilated part of this portion of the canal.* I shall now pursue the description of the canal, commencing at that part which leads immediately from the bladder, namely the pro static portion, observing that this, together with the membranous part, is contained almost wholly within the pelvis, and the two con stitute therefore the pelvic portion of the urethra, and form the true representative of the female urethra.
The prostatic portion traverses the prostate gland at the distance of about two lines from the anterior, four from the posterior, and seven from the lateral surface of the gland. In this respect, however, it varies considerably: thus, in some cases, it is very close to the posterior surface, especially where the isthmus of the prostate is imperfectly developed.* The prostatic portion is from 12 to 15 lines in length, and it commences at the neck of the bladder by a round opening, which is slightly raised ; it then expands to the width of 4 or 5 lines, and gradually contracts itself into the membranous portion. It varies in length and direction in different subjects, and differs materially in these respects ac cording to age. Lisfranc examined it in eight healthy subjects, and found the diameter of the anterior and posterior portions to vary from 3 to 4 lines, whilst the middle portion measured from 4 to 51 lines.